Circuits for modifying the dynamic range of an input signal
[ CIRCUITS FOR MODIFYING THE DYNAMIC RANGE OF AN INPUT SIGNAL Apr. 1, 1975 OTHER PUBLICATIONS Goodell et al., Auditory Perception, Electronics, July [75] Inventor: Ray Milton Dolby, London, England I946 pp 142448 [73] Assignee: Dolby Laboratories, Inc., New York,  
  N.Y. Primary E.raminer-Paul L. Gensler [22 Filed: May 13 1974 Attorney, Agent, 0! FirmRobert F. OConnell; Dike, {211 App NO 469 085 Bronstein, Roberts, Cushman &amp; Pfund Related US. Application Data 57 ABSTRACT [62] Division of Ser. No. 356.l26. May I, 1973, Pat. No.  
  1828180. The invention provides signal compressors, expanders and noise reduction systems. In the compressor, a lin- [30] Foreign Application Priority Data eagytreatediignal comlponerlit thaztcfiim binaeld there- WI 1n opposi I011 21 nonineary re e sign compo- May 1972 Ummd Kmgdom 20406, nent which is however linear with respect to dynamic range above a threshold. Below the threshold the non- 5,5 3; 1 fi8 i i5: linearly treated component has a gain which falls as [58} i 333/14 R 28 the signal level falls. The expander is complementary e 0 &#34;g f b 2 to the compressor; the two signal components combine additively. The compressors and expanders are useful in video, audio and other circuits for effecting [56] References Cited noise reduction UNITED STATES PATENTS 3,8l5,039 6/1974 Fujisawa et al. 333/14 x 6 36 Drawmg figures CON  l l/VPUT OUTPUT l r- 1/!1 J Oo o lA/PUT our/=07 FIG 7(0) INPUT INPUT 40:15 -30d5 W I R7 W PRIOR ART our ur oda OdB  
 OUTPUT OUTPUT lrgPur ggrPuT 3w 5 UP 8 FIG. 13.  
 lNPUT OUTPUT CONTROL OUTPUT F/LTER //VPUT I I 28 29 CONTROL D5 D6 T FIG. I5.  
  38 &#34;34 IX. CONTROL CONTROL &#34;37 33 C0 CON 35&#34; ll Q M/PUT 17 OUTPUT CIRCUITS FOR MODIFYING THE DYNAMIC RANGE OF AN INPUT SIGNAL This is a division of application Ser. No. 356,126, filed on May 1, I973 now U.S. Pat. No. 3,828,280  
  This invention relates to circuits which modify the dynamic range of an input signal that is to say, signal compressors which compress the dynamic range and signal expanders which expand the dynamic range. Compressors and expanders are sometimes required to work independently of each other; more often, however, the compressor compresses the dynamic range of an input signal before the signal is transmitted or recorded. The complementary expander expands the dynamic range of the received signal or the signal played back from the recording i.e., the expander restores the linearity of the dynamic range relative to the input signal. Noise introduced during transmission or the record/replay process is substantially reduced and the compressor-expander combination therefore acts as a noise reduction system.  
  A problem which exists with many dynamic range modification circuits for use in noise reduction systems is that they tend to distort or introduce level errors into high level signals; in a noise reduction system there is no need to modify high level signals, since noise usually has a low value relative to the maximum signal level. Thus compressors and expanders for such systems should be designed in such a way that manipulation of signal dynamics are eliminated at high levels and are confined only to low levels. This can be achieved by the use of a general class of circuits, for producing an output signal in a specified frequency band in response to an input signal in this band and having, at any given frequency in the band, an input-output transfer characteristic which is divided into two regions comprising low and high levels, in which at least the high level region has transfer characteristics determined only by fixed circuit elements providing substantially linear transfer characteristics which on a decibel plot are parallel to but displaced from those of the low level region, the transition from the low level region to the high level region being effected by variable circuit means, the parameters of which are variable in response to the levels of one or more signals in the circuit, the said parameters passing to an extreme condition is effecting the transition from the low level region to the high level region, whereby in the high level region any variations and imperfections in the parameters have an insignificant influence on the transfer characteristic and the output signal.  
  Circuits which comply with this general statement have been described in the specifications of British Pat. Nos. l,l 20,54] and 1,253,031 in the name of Ray Milton Dolby, two general classes of circuits being assigned the designations Type 1 and Type 2 in specification l,253,03 l these designations being used hereinafter.  
  As will be explained below the present invention provides two further general classes of circuits, which will be called Type 3 and Type 4.  
  The background to the present invention, and the invention itself, will be further explained with reference to the accompanying drawings, in which:  
  FIGS. Ha). 1(h) and 2(a), 2(1)) are general block diagrams of Type 1 and Type 2 circuits respectively,  
  FIGS. 3(a) and 3(b) represent the characteristics of a limiter,  
  FIGS. 4(a) and 4(b) represent the characteristics of a circuit referred to as a conveyor in this specification.  
  FIGS. 5(a), 5(b) and 6(0), 6(b) are general block diagrams of Type 3 and Type 4 circuits respectively,  
  FIGS. 7(a) and 7(b) represent the transfer characteristics of Type 3 and 4 circuits,  
  FIGS. 8 and 8(a) show a known circuit acting as a Type 3 expander,  
  FIGS. 9 to 12 show Type 3 and 4 circuits switchable between compressor and expander configurations,  
 FIG. 13 shows a syllabic (non-distorting) type of conveyor,  
  FIG. 14 is a circuit diagram of a Type 3 compressor employing a syllabic conveyor,  
  FIG. 15 is a circuit diagram of a Type 3 compressor providing independent action in two frequency bands,  
  FIG. 16 is a circuit diagram of a Type 3 compressor providing compressor action in a band which narrows to exclude high level signals from the compressor actron,  
  FIGS. 16(a) and 16(1)) show explanatory frequency response curves relating to FIG. 16,  
  FIGS. 17 and 18 show conveyors utilized to construct limiters,  
 FIGS. 19 and 20 show limiters utilized to construct conveyors,  
  FIG. 21 shows a Type 1 compressor utilizing a limiter constructed in accordance with FIG. 17,  
  FIG. 22 shows a circuit operative either as a conveyor or a limiter,  
  FIGS. 23(a) and (b) show two complementary networks,  
  FIG. 24 shows the known practical realization of the network of FIG. 23(b), and  
  FIG. 25 shows a modified form of FIG. 23(a) enabling true complementarity to be more readily achieved.  
  To simplify presentation of the invention, the convention is adopted in all block diagrams that, wherever signals are combined they are combined, (i.e., mixed) additively (by blocks denoted and inverters are shown (by blocks denoted when subtractive combination is required. It will be appreciated that the same overall result may be achieved in various ways with inverters in places other than those shown and/or with the use of combining circuits such as differential amplifiers which actually do subtract one signal from the other. It is merely necessary that closed loops illustrated as inverting should, overall, remain inverting; that non-inverting loops should, overall, remain noninverting; and that the results of combining signals should remain additive or subtractive, as the case may be.  
  It should be understood that amplifiers and/or attenuators, generally not shown, may be used wherever necessary to establish suitable signal levels or impedance matching conditions. It is necessary, however, that suitable relative signal levels are established at the combining means in the main path to create the required compressor or expander action.  
  In each of FIGS. 1, 2, 5 and 6 a compressor is shown at (a) and an expander at (b), the compressor feeding the expander via an information channel represented by a broken connection with the symbol N which signifies the noise introduced in the information channel and reduced by the action of the expander. The term information channel&#34; is used to denote either a transmission channel feeding the encoded signal from the compressor to the expander in real time, or a record/- playback system.  
  In the Type 1 compressor 1C of FIG. 1(a) the main path is constituted by a linear network followed by a combining means 11. The linear network may introduce gain, (i.e,, either amplification or attenuation corresponding to gain less than unity), or frequency response or phase changes (i.e., filters and phase shifters may be included), although the main path possesses at least dynamic range linearity and can be completely linear; in the latter case, the output signal component contributed thereby is proportional on an instantaneous basis to the input signal.  
  The main path signal component is boosted by the signal component from a limiting further path 12 which may contribute gain or attenuation and may be frequency selective but has the essential characteristic of limiting the further path signal component.  
  A limiter may be defined for the purposes of this application as a circuit which, below a threshold, passes a signal with dynamic range linearity and, above the threshold, passes the signal with a gain which diminishes as the input signal level rises at such a rate that the output level is prevented from rising materially above a maximum level, referred to as the limiting level. The characteristics of a limiter are illustrated in FIG. 3(a) of the accompanying drawings in which output level is plotted against input level, on a linear plot. The threshold and limiting level are indicated at T and LL respectively. Curve 13 shows one possibility in which the output level is held at the limiting level above the threshold; FIG. 3(b) shows the corresponding plot of gain versus input level. Other possibilities exist within the above definition. Thus, in FIG. 3(a) curve 14 shows the output level falling from the limiting level above the threshold and curve 15 shows the output level rising slightly above the limiting level.  
  In the Type I expander 1E of FIG. Itb) the main path is constituted by a combining means 16 followed by a linear network 17 whose gain and phase characteristics are complementary to those of the network 10 in the compressor. The main path signal component is now bucked by the further path signal component by virtue of an inverter 18. The limiting further paths I2 in the compressor 1C and expander 1E are identical.  
  The Type 2 circuits of FIGS. 2(a) and (b) differ primarily in the points from which the further path takes its input, these being as follows:  
 Circuit Type Further Path Input taken from Main path input Main path output Main path output Main path output Type I compressor IC Type expander IE Type 2 compressor 2C Type 2 expander 2E iter is such that, at high signal levels, the output of the further path makes a negligible contribution to the overall output of the compressor or expander which effectively appears, at such levels, as only the main path; this, as stated above, has dynamic range linearity.  
  The main path can, in fact, consist of nothing more than a direct connection through the combining means although it may comprise an amplifier or an attenuator. as noted above. The further path can include an amplifier and/or attenuator preceding and/or succeeding the limiter. The paths may also include frequency response or phase equalizers. In all circumstances the considerations discussed in the preceding paragraph have to be interpreted at the point where the main path and further path components are actually combined. If a frequency selective network is utilized in the main path it can be employed to effect equalization, e.g., in an audio application.  
  The present invention is based upon the recognition that it is possible to construct further paths whose char acteristics complement those of limiting further paths and which can be used to construct compressors and expanders complying with the aforesaid general statement. It is necessary to provide a name for a circuit complementary to a limiter and it will be called a conveyor in this specification. A conveyor is defined herein&#39; as a circuit which, above a threshold, passes a signal with dynamic range linearity and, below the threshold, passes the signal with a gain which diminishes as the input signal level falls. Such circuits have been described in the art, but their significance and utility have heretofore evidently not been recognized, especially in the context of the present invention. The characteristics ofa conveyor are illustrated in FIGS. 4(a) and 4(b) of the accompanying drawings, these figures corresponding to the limiter FIGS. 3(a) and 3(b) respectively and again being linear plots. The name conveyor has been chosen in that, as a limiter limits signals above a threshold, a conveyor conveys signals above a threshold. Just as a further path including a limiter may be referred to as a limiting further path or a further path having the characteristics ofa limiter, a further path including a conveyor may be referred to as a conveying further path or a further path having the characteristics of a conveyor.  
  Given the existence of a conveying further path, whose practical realizations are discussed below, it becomes possible to construct further compressors, expanders, and noise reduction systems of two types which will be called Type 3 and Type 4. Type 3 devices are related to Type 1 devices and Type 4 devices are related to Type 2 devices, but in each case the limiter in the further path is replaced by a conveyor, and the further path component is subtracted from the main path component in the case of a compressor and is added to the main path component in the case of an expander.  
  The essential features of these new devices are illustrated in FIGS. 5 and 6 of the accompanying drawings as follows:  
 FIG. 5(a) Type 3 compressor, denoted 3C FIG. 5(b) Type 3 expander, denoted 3E FIG. 6(a) Type 4 compressor, denoted 4C FIG. 6(b) Type 4 expander, denoted 4E In these Figures the block 19 is the conveying further path. Other circuit components are referenced in the same way as in FIGS. 1 and 2.  
  The characteristic of the conveying further path 19 is illustrated in FIG. 7(a) with the threshold thereof denoted T. The characteristic of the main path is represented by line 21 in FIG. 7(b). The effect of subtracting characteristic 20 from characteristic 21 is to create the compressor characteristic 22 in which point T corresponds to the threshold T of FIG. 7(a) and the compressor threshold TT corresponds to the point at which the output of the conveying means has fallen to a negligible value, e.g., 65 dB. Likewise, the effect of adding characteristic 20 to characteristic 21 is to create the expander characteristic 22.  
  It is convenient to explain here that a circuit is described in the aforementioned U.S. Pat. No. l,253,03l which is illustrated in FIG. 8 of the accompanying drawings. This circuit was regarded (and so described in the said specification) as a simplified Type 2 expander; with the benefit of hindsight it is now recognized that the circuit is actually a Type 4 expander in which the main path is through the resistor R1 (FIG. 8). The further path comprises a conveyor through the diodes DI and D2 and a low pass filter, which is constituted by the resistors R] and R2, capacitor C1 and the diodes D1 and D2. The adding means is provided by the junction between R1 and R2.  
  The circuit of FIG. 8 is easier to understand if it is redrawn as in FIG. 8(a) with the resistors R1 and R2 replaced by a single resistor R1, 2. The main path is represented by a direct connection 17, corresponding to the network 17 in FIG. 6(b), with the combining means 16 which adds the input signal and the conveying means output signal at the top end of C1. The main and further paths are thus readily identifiable in FIG. 8(a). The circuit simplification to FIG. 8 is possible because the effect of the combining circuit 16 can be achieved by splitting R1, 2 into two resistors R1 and R2 and connecting the output to the junction of the resistors. The sum of the values of the resistors is equal to the value RI, 2 required to establish correctly the cutoff frequency of the low-pass filter. The ratio of R1 and R2 determines the ratio in which the main path and further path components combine.  
  The action of the circuit can be understood most readily from FIG. 8(a). The main path component is boosted by the further path component only in the pass band of the low-pass filter. Within the pass band, such boosting is independent of signal level. Thus, there is no expansion of dynamic range within the pass band of the filter. It will be noted that the filter is in parallel with the conveying diodes. The reason for this is more fully explained below.  
  Consider a frequency somewhat above the normal or quiescent cut-off frequency of the filter. At this frequency there is no boost at low levels. At high levels however. the diodes D1 and D2 conduct and the reduced series resistance raises the cut-off frequency of the filter to include the said frequency in the pass band. Thus, the boosting action takes place at this frequency. Because the boosting action is absent at low levels but present at high levels, expander action is created in accordance with FIG. 7(b).  
  The filtering and conveying means of FIG. 8 can also be used in Type 4 compressors and Type 3 compressors and expanders. as well as in complete noise reduction systems. Video applications are particularly relevant for such circuits. To accommodate colour sub-carriers, Cl may be replaced by a parallel resonant network; for  
 composite signals such a network may be placed in series with C 1.  
  The conveyor in the circuit just discussed consists simply of a pair of back-to-back diodes and is thus an instantaneous conveyor. This conveyor can also be regarded as a variable coupling means, as described in British Patent Application No. 7958/72. Conveyors can however, take various other forms including circuits in which an impedance element is so controlled in response to the level of a signal in the compressor or expander as to create the conveyor action. Such circuits can be quite complex and can comprise a plurality of signal paths in parallel; so long as the overall action of the circuit is in accordance with the foregoing definition the circuit is, for the purposes of this application, a conveyor. The circuits described in British Patent applications 7958/72 (variable coupling means) and 7959/72 (variable combining means) are circuits which can be arranged to act as conveyors. The circuits of these applications have first and second paths and, as described in the aforementioned applications, the two paths are used in combination to establish a compressor or expander action. However, by modifying the relative actions of the two paths, quantitatively rather than qualitatively, the combined action of the paths can be used to create a conveyor action such that, above a low threshold, the circuit has a linear dynamic characteristic. Therefore, a circuit as described in either of the two aforesaid applications can be used as a conveyor in the further path of a Type 3 or Type 4 compressor or expander, both the first and second paths of the circuit being within the further path of the compressor or expander. The use of variable combining means will be further described below in relation to FIG. 22.  
  Variable combining means can be used to provide selective connections to various signal points in the further path or paths. For example, the variable combining means may provide an automatic variable selection of the input to or the output from a filter in the further path.  
  Variable coupling means are used in a similar way, except that the variable coupling provides a frequency selective action which results from the variable coupling and not only from fixed filters. The variable coupling means may, for example, comprise an automatically variable impedance across a filter in the further path; the input to and the output from the filter are thus variably coupled, which results in overall alterations of the frequency response characteristic.  
  FIGS. 9 to 12 show schemes for switching Type 3 and Type 4 circuits between compressor and expander configurations. In each case the switching is effected by a changeover switch 25 whose two settings are labelled 3C and 3E or 4C and 4E to denote the type of compressor or expander action created. FIGS. 9 and 11 show switching on the input side of the further path 19 for Type 3 and Type 4 circuits respectively. FIGS. 10 and 12 show switching on the output side of the further path 19 for Type 3 and Type 4 circuits respectively.  
  These switchable circuits are important for effecting noise reduction in a recording/playback procedure, the compressor configuration being employed to encode the signal prior to recording and the expander configuration being employed to decode the signal recovered on playback.  
  In general, a complete noise reduction system comprises the combination of a Type 3 compressor and a Type 3 expander or the combination of a Type 4 com pressor and a Type 4 expander. Provided that the characteristics of the main and further paths are the same in the compressor and expander. the expander action is inherently complementary to the compressor action, whereby the original information signal is recovered unchanged after encoding by the compressor and then decoding by the expander. If the information channel is a record/playback system. the compressor and expander can comprise the same processing circuitry with switching arrangements as in FIGS. 9 to I2.  
  There are, nevertheless, circumstances in which it is desired merely to compress or expand the dynamic range of a signal; Type 3 and 4 compressors and expanders can be utilized independently of each other for such purposes.  
  In Type 3 and Type 4 devices, the difference between or the sum ofthe main path component and the further path component will be seen to create an overall compressor or expander action. Above the threshold, the output of the compressor or expander consists of the difference or sum respectively of two components,  
 both of which possess dynamic range linearity. lt follows that the output of the compressor or expander is linear above the threshold.  
  In noise reduction systems it is usually sufficient to treat only the low level portion of the dynamic range e.g., levels less than dB, 40 dB, or even 60 dB with respect to the nominal maximum operating level (one. two or three orders of magnitude less). Any distortions introduced by operation of the conveying means in the region between T and TT in FIG. 7(b) are therefore confined to comparatively low levels, at which they are unobtrusive.  
  lf compressors and complementary expanders are to be used in noise reduction systems it is important that signal modulated noise effects should be avoided. This is best achieved by ensuring that the various portions of the frequency spectrum are compressed or expanded as independently of each other as possible. Thus, the degree of compression or expansion (i.e., the noise reduction) obtained at the extreme high audio frequencies, for example, should be influenced as little as possible by the signal levels at low and mid frequencies.  
  To this end the further path can include a filter, to restrict the signal component passed by the further path to a particular part of the overall frequency band (referred to in the foregoing general statement as the specified frequency band).  
  For example, in an audio application, the turnover frequency at low signal levels can be placed at around 3 KHz and the boost can be [0 dB (at --40 dB or less). Such a compressor used in conjunction with a complementary expander can then provide a high frequency noise reduction of l0 dB. Also, as explained in the specifications mentioned above, a plurality of further paths in parallel can be used.  
  If the signal to be handled is a carrier frequency, then the compressor is suitably adapted to deal with the carrier and its sidebands. This will usually involve an automatic narrowing and widening of the frequency band on a symmetrical basis, although it is possible for the bandwidth control to be asymmetrical to suit single sideband or vestigial side band carrier signals. Trap circuits may be employed to exclude carrier frequencies which would otherwise choke the action of the compressor or expander.  
  The aforementioned specifications also described the use of frequency selective circuits which restrict the compressor or expander action to restricted portions of the overall band. When a high level component appears at any frequency within the restricted band, the circuit adapts itself and causes the restricted band to narrow to preclude compressor or expander action on the said frequency, at which frequency the normal characteris tic provided by the main path thereby obtains. The modified (i.e., compressed or expanded) characteristic still applies for the low level signals within the narrowed restricted band, whereby compressor or expander action, and hence noise reduction, is still effected within this narrowed band. This may be referred to as the narrowing band principle since the restricted band undergoes a narrowing action to confine compression, expansion and noise reduction to frequencies where only low level signal components are present. By this method a high degree of compression and expansion can be maintained at frequencies removed from the high-level signal frequency, with consequent good noise reduction and avoidance of signal modulated noise effects.  
  In the application of this principle to Type 1 and Type 2 devices the pass band of the filter creates the limiter characteristic below the threshold, FIG. 3(a), whereas the stop band of the filter creates the characteristic above the threshold and the pass band therefore has to narrow if the signal at a frequency within the pass band exceeds the threshold. A complementary principle can be applied to Type 3 and Type 4 devices but it will be appreciated, in particular from the description of the operation of FIG. 8(a), that it is the stop band of the filter which must create the conveyor characteristic below the threshold, FIG. 4(a), whereas the pass band of the filter must create the characteristic above the threshold. Therefore in the case of Type 3 and 4 devices, although very similar to those described in the aforementioned specifications can be employed, it will be understood that it is now the stop band which must be narrowed to put a frequency at which the threshold is exceeded into the pass band; in other words the Type 3 and 4 devices require the pass band to be broadened where the Type I and 2 devices require the pass band to be narrowed.  
  As will be apparent from FIG. 8, this invention can be embodied in various types of instantaneous, or at least non-linear, compressors and expanders which compress and expand the dynamic range of individual waveforms of the signal being processed. Such devices are useful in processing video and other signals in which phase is preserved. However, the invention can also be embodied in linear devices (referred to as syllabic devices in telecommunications) which do not distort individual waveforms. The word linear in this context refers only to the linear treatment of individual waveforms. On a long time scale the further path possesses the non-linearity illustrated in FIG. 4(a) or FIG. 7(a). To this end the conveying means is arranged to respond with a suitable time constant to the level of a signal (or differences of levels) in the compressor or expander. One very simple conveyor circuit for achiev ing this result is illustrated in FIG. 13 of the accompanying drawings.  
  A resistor R3 is connected between an input terminal and an output terminal which is also connected to ground through an FET F1. A control circuit 26 derives a control signal from the input of the conveyor by rectifying, amplifying if need be, and smoothing the input signal with the required time constant. The polarity of the control signal and the type of FET are so chosen that, as the input signal rises, the PET is rendered pro gressively less conductive, being completely cut off once the threshold T of FIG. 4(a) is reached. Above the threshold the conveyor is therefore linear. Below the threshold the circuit acts as an attenuator with a degree of attenuation which increases as the signal level falls. This circuit may therefore be used as the conveying further path 19 in any of FIGS. 5, 6, 9, 10, 11 and 12.  
  FIG. 14 of the accompanying drawings shows another possibility, this being the complete circuit of a Type 3 compressor. The main path is constituted by a resistor R4. The further path includes a filter 28 the pass band of which defines the band within which the compressor operates. The filter is in series with the conveyor. The further path component is subtracted from the main path component by a transistor T1 and its collector load resistor R5. The transistor T1 with its emitter load circuit acts as the conveyor; the emitter load circuit is constituted by a transistor T2 with emitter resistor R6 and a fixed base bias and by the parallel connection therewith, via a coupling capacitor C2, of a resistor R7 and a field effect transistor F2. The conduction of this FET is controlled by a control control 29 having the functions of the circuit 26 of FIG. 13 except that it is now arranged that, as the signal level at the output of the filter increases, the FET becomes more conductive. being fully conductive once the threshold is reached.  
  Consider firstly the situation at very low levels and within the pass band of the filter 28. The FEt F2 presents a very high impedance and the emitter current of T1 is determined entirely by T2. This current is arranged to be substantially constant by virtue of the high collector impedance of T2, whereby Tl provides little or no gain and the further path component is attenuated. Consider now the situation when the threshold has been reached. F2 is fully conductive and presents an impedance of around lOO ohms. R and R7 may be some tens of thousands of ohms but are still small compared with the effective impedance of T2 and R6. The gain of the further path component is now substantially higher and is determined effectively by the ratio of R5 and R7; the impedance of F2 can be ignored in relation to R7. Therefore, above the threshold, the conveyor acts with dynamic range linearity.  
  Within the pass band of the filter the further path component, which is subtracted from the main path component, is attenuated at low levels but not at high levels. Therefore, the dynamic range of the output signal is compressed. Within the stop band of the filter the further path component is not attenuated at low levels and no compressor action is created. However, in the configuration of this example, in which the filter and conveyor are in series, the overall effect is to superimpose an equalization characteristic on the compressor output; the output at high levels will be greater in the pass band frequency range.  
  The high level characteristics of the circuit of FIG. 14 are not dependent upon the precise characteristics of F2. This illustrates an important property ofType 3 and 4 compressors and expanders in that it is possible with solid state techniques, particularly employing FETs or integrated circuits, now available to construct conveyors, and also subtracting and adding networks, whose characteristics are determined outside transition regions by fixed circuit components. It is therefore possible to achieve stability and reproducibility of performance independently of variations in the parameters of semiconductor devices occurring from batch to batch or with temperature or time.  
  On the question of stability, the gain of the further path in Type 2 devices must be less than unity; this applies also to Type 3 devices. Given this simple requirement these compressor or expanders are inherently stable.  
  The input to the control circuit 26 or 29 can be de&#39; rived from a number of places in the device. The points chosen in FIGS. 13 and 14 are desirable in order to achieve stable operation. The smoothing can be effected by a two-stage integration network, making it possible to keep the attack time of the system short while at the same time keeping signal distortion and generation of modulation products to a minimum. The first stage should have a short time constant. The second stage, having a longer time constant. is coupled to the first stage in a non-linear fashion, such as by a diode-resistor combination, whereby under relatively uniform signal conditions the second stage is able to provide additional smoothing. However, for large, abrupt changes in signal amplitude the nonlinear network conducts and causes the time constant of the second network to be reduced.  
  During the attack period overshoots or undershoots may be produced. It is possible to limit these to a low amplitude by the use of appropriately connected nonlinear elements such as diodes. The use of such diodes is illustrated in FIGS. 13 and 14. In FIG. 13 diodes D3 and D4 conduct when the signal level rises abruptly and so avoid the undershoot in the signal passed by the conveyor which would otherwise occur in the finite time taken from the conduction of the FET F1 to drop. in FIG. 14 diodes D5 and D6 conduct when the signal level rises abruptly and so prevent overshoot arising in the finite time taken for FET F2 to become conductive.  
  Both compressors and expanders of the invention have been separately described herein, but it is alsc possible to effect a change of mode by the use of nega tive feedback amplifiers, a compressor or expander being put into the feedback loop to produce expander or compressor action respectively.  
  In Type 3 and 4 devices, close attention must be given to the effects of filters in either path since the output at high levels is formed by the difference between or sum of two filter outputs. Phase shift networks placed in either or both paths are sometimes useful particularly for optimising the overall response charac teristics of the system at various levels.  
  As was seen in FIG. 14, one technique is to use a fil ter in series with the conveyor, the pass band of the fil ter determining the band in which compression or ex pansion takes place. Several parallel bands and path: may be used. It is then possible to achieve compressior or expansion independently in different frequenc bands.  
  A further frequency selective technique utilizes se ries connected filters to which are connected variable combining or coupling means utilized as conveying means. for example. in the manner illustrated in FIG. 15. The conveying means eliminates the compression or expansion action by by-passing the filter or changing its characteristics so as to transmit the signal at high levels.  
  In FIG. 15 the main path is constituted by a direct connection 17 and the combining means 11. A first further path section comprises a controlled conveying means 31 connected to a band stop filter 32. The signal developed across the band stop filter is detected by a differential amplifier 33 and applied to a control circuit 34, which rectifies and smooths the signal to derive the control signal which causes the conveying means 31 to convey the signal in the stop band as the signal level in the stop band rises.  
  The first further path section is followed by a second section comprising a controlled conveying means 35, band stop filter 36 and control circuit 37. As an alternative to the differential amplifier 33, the second section has a band pass filter 38 which selects the frequency region excluded by the band stop filter 36.  
  Similar considerations apply to the narrowing band Type 3 compressor shown in FIG. 16. The conveying means here is a variable coupling means which consists of an FET F3 connected across a tuned band pass filter formed by a series resistance R8 and a shunt arm comprising an inductor L1 and a capacitor C3 in parallel. R8 is also shunted by back to back diodes D7 and D8 for eliminating overshoots. The signal in the stop band of the filter is detected by a differential amplifier 40 connected across R8 and is rectified and smoothed by a control circuit 41 to derive a control signal which increases the conduction of F3 as the level of the signal in the stop bands increases.  
  When F3 has a high impedance, R2, L1 and C3 cre ate a relatively narrow pass band shown at response curve 42 in FIG. 16(u). This pass band can be centred on a carrier frequency fc whereby the carrier signal and its inner side-bands are permanently excluded from compressor action because they are always passed by the filter to the inverter 18 and combining means 11. In the stop bands 43 and 44 however. low-level signals are prevented from passing through the further path. If the level of outer side-bands of the carrier signal increases. the control signal reduces the resistance of F3, thereby decreasing the series resistance ofthe filter and broadening the pass band, e.g., as shown at 45 in FIG. 16(0). Signals within the broadened pass band 45 are now excluded from the compressor action. However. the different treatment of signals within the frequency regions 46 when the low level characteristic 42 applies and when the high level characteristic 45 applies has the effect of establish dynamic range compression for such signals.  
  Ifthe inductor L1 is eliminated. the tuned pass band 42 of FIG. 16(0) becomes the low pass band 47 of FIG. 16th) which remains as such so long as there are no high level, high frequency components in the stop band 48. If such components appear, the pass band broadens to characteristic 49. Compression is confined to the high frequency stop band of the filter, because it is only in this band that low level components do not buck the main path component at the combining means 11. The converse situation in which compression is confined to a low frequency stop band can be obtained by eliminating C3 and using only R8 and L1.  
  Although FIGS. 14, I5 and 16 all show only Type 3 compressors, it will be apparent from FIGS. 5 and 6 how the circuits can be re-arranged to form Type 3 ex panders or Type 4 compressors or expanders.  
  The invention is additionally concerned with modified versions of Type 1, 2, 3 and 4 devices in which a conveying means is utilized to construct a limiter or filter/limiter (limiting means) or vice versa.  
  The relevant possibilities are illustrated in FIGS. 17 to 20 of the accompanying drawings, as follows:  
 FIG. 17: conveying means connected in a forward loop to construct limiting means. The gains of the two paths must be substantially the same at high levels so that the output signal is limited, as required. This distinguishes the circuit from FIG. 5(a) in which the gains differ substantially in order to yeild a high level output signal at high input levels.  
 FIG. 18: conveying means connected in a feedback loop to construct limiting means. The loop gain of the negative feedback loop must be high to cause the output to be small, i.e., limited, at high levels. This distinguishes the circuit from FIG. 6(a) in which the gain must be such that the output signal is a high level signal at high input levels.  
 FIG. 19: limiting means connected in a forward loop to construct conveying means. The low level gains must in this case be the same to achieve cancellation at low levels. This distinguishes the circuit from FIG. 2(b) in which the further path signal bucks but certainly must not cancel the main path signal at low levels.  
 FIG. 20: limiting means connected in a feedback loop to construct conveying means. Again a high loop gain is necessary so that, at low levels below the threshold of the limiter, the output shall be negligible. This distinguishes the circuit from FIG. 1th) in which the further path gain is such that the further path signal bucks but by no means cancels the main path signal.  
  In many practical situations, the configuration of FIGS. 18 and 20 are preferred to those of FIGS. 17 and 19 since the use of a negative feedback loop leads to a stable and reproducible circuit without the need for high precision components.  
  Another eight Type 1 and 2 circuit configurations can therefore be generated by replacing the limiting further path 12in any one ofFIGS.1(a),1(b), 2(a) and 2(0) by the circuit of either FIG. 17 or FIG. 18. Likewise, another eight Type 3 and 4 circuit configurations can be generated by replacing the conveying further path 19in any one of FIGS. (5a), 5(b), 6(a) and 6(b) by the circuit of either FIG. 19 or FIG. 20.  
  As one specific example, FIG. 21 of the accompanying drawings illustrates a Type 1 compressor utilizing the limiter circuit of FIG. 17. One reason for choosing this specific example is that it represents, with one fundamental difference, the circuit of a compressor described in Auditory Perception by Goodell and Michel, Electronics July 1946, pages I42 to 148. The fundamental difference is that Goodell and Michel do not use a true conveyor as defined herein. They use a vacuum tube as a variable gain device; apart from the disadvantages which must arise from the lack of stability of the circuit characteristics, a variable gain vacuum tube has no threshold above which its gain is constant. There are three paths in parallel in FIG. 21; in the Goodell and Michel circuit one of these is non-linear at all signal levels, not being a conveyor, and it is therefore impossible to achieve overall dynamic range linearity at high signal levels. Overall dynamic range linearity is achieved in all circuits of this invention at high levels because the output of the conveyor is, like the output of the other paths, linear above the threshold.  
  In FIG. 21 the Type I compressor comprises a linear network It), combining means II and limiting further path 12 as in FIG. 1(a). The limiting further path in cludes a filter 50 which selects the pass band within which compressor action takes place. The rest of the further path is a limiter based upon a conveyor as in FIG. 17.  
  FIG. 22 shows a circuit which relates the concepts of variable combining means, limiters, and conveyors. Depicting a variable combining means, potentiometer 51 couples to an output terminal 52 a proportion of input signals I and 2 at terminals 53a and [2 determined by the setting ofthe tap 54 of the potentiometer. A control unit 55 adjusts the position of the tap in dependence upon the level of a signal derived from a suitable point on the input or output side of the circuit.  
  Limiters and conveyors are formed when signal 2 is zero; terminal 53b may be connected to earth. If. then, the sense of potentiometer adjustment is such that the degree of signal transfer decreases above a threshold as the said signal level rises. the circuit acts as a limiter. If. conversely, the sense of adjustment is such that the degree of signal transfer decreases below a threshold as the signal level falls, the circuit acts as a conveyor.  
  Alternatively, the circuit of FIG. 22 may be operated as a conveyor by connecting the inputs I and 2 to different points, e.g.. the outputs of different filters, the adjustment of the potentiometer causing the circuit to have the characteristics of a conveyor, at least within a particular frequency band.  
  Although a potentiometer with an adjustable tap has been shown, it will be appreciated that the principles illustrated apply to purely electronic circuits as well.  
  In this invention, use has been made of particular compressor/expander configurations which ensure theoretically perfect reciprocity or complementarity of the compressor and expander when used together in a noise reduction system. Such configurations are shown in FIGS. 1, 2, and 6.  
  Various explanations and analyses of the complementarity mechanism have been given in the present and previous patent specifications. A new interpretation and analysis, providing even greater applicability for the concepts involved, is now given.  
  Referring to FIG. 23(a), a network 60 with a transfer function or characteristic B, of gain, frequency response, delay, and dynamic range (linearity vs non linearity), is shown at (u). The network 61 of FIG. 23(b), with reciprocal characteristics. I/B, may precede or follow the network of FIG. 23(a) in a complete transmission channel. Such networks may, for example, be used as complementary equalizers. If non-linear in operation, they may be used as compressor/expanders, whether on an instantaneous or syllabic basis.  
  Given a network with characteristics B, it is often difficult or impractical to construct a network with characteristics l/B. Therefore, a convenient and frequently used technique of generating the required characteristic is to place a network with characteristic B in the negative feedback loop of a high gain amplifier 62, as  
 (b) In FIG. 24 E,,=AE,-,ABE I Rearranging. E,=  
  With (a) and (b) in tandem, say E E (the signal in the transmission channel). Then ThusE, E,ifA m For a reciprocity error not to exceed l7c, the product AB must be at least I00. Unfortunately, if the network [3 is at all complex it may be difficult to make the feedback loop stable. Therefore, it is useful to have a method which reduces the gain requirement and eases the stability problem.  
  An inspection of Equation (1) shows that the high gain requirement would be eliminated if the numerator were to have an additional term I/A to complement that in the denominator. Since the numerator B represents the transfer function of the network 60, the terms l/A represents a compensation or correction compo nent which must be taken from the input E, and transferred to the output E by a transfer function l/A, provided by an amplifier 64. This situation is depicted in FIG. 25(a). IfA is not too low (say 10), then the correction component will be relatively small (say 10% of the signal from the network B). The output of the network thus usually provides the majority of the signal E even at high levels. This is in contrast with previously described, but analogous in configuration Type I devices, using limiting further paths, in which the contribution of the linear path greatly exceeds that of the non-linear path. However, there may be applications in which it is desired to construct a conventional high level expander from a conventional high level compressor, or vice versa, using negative feedback amplifiers. The principles of the reciprocity correction scheme may then be used, with the Type I configuration, but without the usual low level limiting requirement normally associated with Type 1 devices; this is equivalent to the Type 4 configuration, but with compressor and expander reversed.  
  Equivalent reciprocity correction schemes using positive, instead of negative, feedback amplifiers may also be employed, the inverting means 63 being removed from the amplifier loop in FIG. 24 and inverting means being inserted in the correction signal path in FIG. 25. The configuration produced are analogous to the Type 2 and Type 3 devices discussed previously.  
  In these reciprocity correction schemes a correction signal which is not small may change the overall effective characteristic [3 of the network (i.e., of the signal E In such instances it may be necessary to modify B to B, such that B and the correction signal then yield l the desired overall result (as if the characteristic of the network had been B without any correction).  
  One application of such reciprocity correction schemes is to the series mode types of compressors and expanders described in British Patent Application No. 6747/71. Such circuits, in particular the compressors, can be utilized as the network 60 in FIGS. 24 and 25.  
 I claim:  
  I. A circuit for modifying the dynamic range of an input signal. comprising first circuit means responsive to the input signal to provide a first signal component which possesses dynamic range linearity relative to the input signal, second circuit means which include a circuit having the characteristics of a limiter and which provide a second signal component which is non-linear relative to the input signal. and means for combining the first and second signal components to provide an output signal whose dynamic range is modified relative to that of the input signal, wherein the circuit having the characteristics of a limiter comprises a first circuit path and a second circuit path which includes a conveyor and is connected in parallel with the first circuit path as one of a negative feed-forward and a negative feedback loop relative to the first circuit path.  
  2. A circuit according to claim 1, wherein the second circuit path is a negative feed-forward path relative to the first circuit path and the gains of the two paths are substantially the same above the threshold of the conveyor included in the second circuit path.  
  3. A circuit according to claim 1. wherein the second circuit path is a negative feedback path relative to the first circuit path. and the open loop gain of the feedback loop is sufficiently high to cause the output signal of the circuit having the characteristics of a limiter to be limited above the threshold of the conveyor.  
  4. A circuit for modifying the dynamic range of an input signal, comprising first circuit means responsive to the input signal to provide a first signal component which possesses dynamic range linearity relative to the input signal, second circuit means which include a circuit having the characteristics of a conveyor and which provide a second signal component which is non-linear relative to the input signal, and means for combining the first and second signal components to provide an output signal whose dynamic range is modified relative to that of the input signal, wherein the circuit having the characteristics of a conveyor comprises a first circuit path and a second circuit path which includes a limiter and is connected in parallel with the first circuit path as one of a negative feed-forward and a negative feedback loop relative to the first circuit path.  
  5. A circuit according to claim 4, wherein the second circuit path is a negative feed-forward path relative to the first circuit path and the gains of the two paths are substantially the same below the threshold of the limiter included in the second circuit path.  
  6. A circuit according to claim 4, wherein the second circuit path is a negative feedback path relative to the first circuit path. and the open loop gain of the feedback loop is sufficiently high to cause the output signal of the circuit having the characteristics of a conveyor to be strongly limited below the threshold of the lim-