Extended definition widescreen television signal processing system with alternate subcarrier
This is a division of application Ser. No. 139,340, filed December 29, 1987. 
    
    
     FIELD OF THE INVENTION 
     This invention concerns apparatus for processing nonstandard auxiliary television image information such as may be associated with a widescreen extended definition (EDTV) system. 
     BACKGROUND OF THE INVENTION 
     A conventional television receiver, such as a receiver in accordance with NTSC broadcast standards adopted in the United States and elsewhere, has a 4:3 aspect ratio (the ratio of the width to the height of a displayed image). Recently, there has been interest in using higher aspect ratios for television receiver systems, such as 2:1, 16:9 or 5:3, since such higher aspect ratios more nearly approximate or equal the aspect ratio of the human eye than does the 4:3 aspect ratio of a conventional television receiver. Video information signals with a 5:3 aspect ratio have received particular attention since this ratio approximates that of motion picture film, and thus such signals can be transmitted and received without cropping the image information. However, widescreen television systems which simply transmit signals having an increased aspect ratio as compared to conventional systems are incompatible with conventional aspect ratio receivers. This makes widespread adoption of widescreen systems difficult. 
     It is therefore desirable to have a widescreen system which is compatible with conventional television receivers. One such system is disclosed in a copending C. H. Strolle et al., U.S. patent application Ser. No. 078,150 titled &#34;Compatible Widescreen Television System&#34;, filed July 27, 1987. 
     It is even more desirable to have such a compatible widescreen system with provisions for enhancing or extending the definition of the displayed image so as to provide extra image detail. For example, such widescreen EDTV (extended definition television) system may include apparatus for providing a progressively scanned image. 
     In accordance with the principles of the present invention, there are disclosed herein method and apparatus for encoding and decoding a compatible EDTV widescreen video signal representing a high resolution picture with an aspect ratio greater than the standard 4:3 aspect ratio, suitable for transmission via a single standard broadcast channel. 
     SUMMARY OF THE INVENTION 
     In a disclosed preferred embodiment of a compatible widescreen EDTV television system, an original high resolution, progressively scanned widescreen signal is encoded to include four components derived from a composite signal. The four components are processed separately before being recombined in a single signal transmission channel. 
     A first component is a main 2:1 interlaced signal with a standard 4:3 aspect ratio. This component comprises a central portion of the widescreen signal that has been time expanded to occupy nearly the entire 4:3 aspect ratio active line time, and side panel horizontal low frequency information that has been time compressed into the left and right horizontal image overscan regions where such information is hidden from view in a standard television receiver display. 
     A second component is an auxiliary 2:1 interlaced signal comprising left and right side panel high frequency information that have each been time expanded to half the active line time. Thus expanded side panel information occupies substantially the entire active line time. 
     A third component is an auxiliary 2:1 interlaced signal, derived from the widescreen signal source, comprising high frequency horizontal luminance detail information between approximately 5.0 MHz and 6.2 MHz. In accordance with the principles of the present invention, the second and third components modulate an alternate subcarrier which exhibits a frequency other than the frequency of a chrominance subcarrier within a band of frequencies occupied by chrominance image information. 
     A fourth component is an auxiliary 2:1 interlaced &#34;helper&#34; signal comprising vertical-temporal (V-T) luminance detail information that would otherwise be lost in the conversion from progressive scan to interlaced format. This signal component helps to reconstruct missing image information and to reduce or eliminate unwanted flicker and motion artifacts at a widescreen EDTV receiver. 
     At a widescreen EDTV receiver, a composite signal containing the described four components is decoded into the constituent four components. The decoded components are processed separately and used to develop an image representative widescreen signal with enhanced resolution. 
     The disclosed widescreen EDTV system offers several significant improvements over a standard NTSC system. The wider aspect ratio, with the visible impact of motion picture film, is immediately apparent. The widescreen picture is &#34;quieter&#34;, virtually free from the interline flicker so common in standard NTSC receiver displays. The picture is also &#34;cleaner&#34;, virtually free from &#34;crawling dots&#34;, &#34;hanging dots&#34; and disturbing rainbow color effects. The widescreen picture has noticeably increased resolution in both spatial dimensions. Line structure is not visible because of the increased line density. In moving portions of the picture, absent are annoying beats between moving horizontal edges and the scanning structure. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates a general overview of a compatible widescreen EDTV encoder system in accordance with the present invention; 
     FIG. 1a shows a detailed block diagram of the encoder for the disclosed system; 
     FIG. 1b contains diagrams helpful in understanding the operation of the disclosed system; and 
     FIG. 2 shows a block diagram of a portion of a widescreen EDTV receiver including decoder apparatus in accordance with the present invention. 
    
    
     A system intended to transmit wide aspect ratio pictures, e.g., 5:3, through a standard, e.g., NTSC, broadcast channel should achieve a high quality picture display by a widescreen receiver, while greatly reducing or eliminating observable degradations in a standard 4:3 aspect ratio display. The use of signal compression techniques on the side panels of a picture takes advantage of the horizontal overscan region of a standard NTSC television receiver display, but may sacrifice image resolution in the side panel regions of a reconstructed widescreen picture. Since compression in time results in an expansion in the frequency domain, only low frequency components would survive processing in a standard television channel, which exhibits a smaller bandwidth compared with that required for a widescreen signal. Thus, when the compressed side panels of a compatible widescreen signal are expanded in a widescreen receiver, there results a noticeable difference between the resolution or high frequency content of the center portion of a displayed widescreen picture and the side panels, unless steps are taken to avoid this effect. This noticeable difference is due to the fact that low frequency side panel information would be recovered, but high frequency information would be lost due to video channel bandlimiting effects. 
     In the system of FIG. 1, elements which are common to the more detailed system of FIG. 1a are identified by the same reference number. As shown in FIG. 1, an original widescreen progressive-scan signal with left, right and center panel information is processed so as to develop four separate encoding components. These four components were described above, and are illustrated generally in FIG. 1 in the context of an image display. Processing of the first component (containing time expanded center portion information and time compressed side portion low frequency information) is such that the resulting luminance bandwidth does not exceed the NTSC luminance bandwidth of 4.2 MHz in this example. This signal is color encoded in standard NTSC format, and the luminance and chrominance components of this signal are suitably pre-filtered (e.g., using field comb filters) to provide improved luminance-chrominance separation at both standard NTSC and widescreen receivers. 
     The time expansion of the second component (side panel high frequency information) reduces its horizontal bandwidth to about 1.1 MHz. This component is spatially uncorrelated with the main signal (the first component), and special precautions are taken to mask its visibility on standard NTSC receivers, as will be discussed. 
     The 5.0 to 6.2 MHz extended high-frequency luminance information content of the third component is first shifted downward in frequency to a frequency range of 0 to 1.2 MHz before further processing. This component is mapped into the standard 4:3 format, which spatially correlates it with the main signal (the first component) to mask its visibility on standard NTSC receivers. The compressed side panel information of the third component exhibits a bandwidth which is one-sixth that of the center information (0-1.2 MHz). 
     The fourth component (vertical-temporal helper) is mapped into standard 4:3 format to correlate it with the main signal component to thereby mask its visibility on standard NTSC receivers and is horizontally bandwidth limited to 750 KHz. 
     The first, second, and third components are processed by respective intraframe averagers 38, 64, and 76 (a type of vertical-temporal (V-T) filter) to eliminate V-T crosstalk between the main and auxiliary signal components at a widescreen receiver. The first component is intra-frame averaged only above approximately 1.5 MHz. The second and third intraframe averaged components, identified as X and Z, are non-linearly amplitude compressed prior to quadrature modulating a 3.108 MHz alternate subcarrier ASC, having a field alternating phase unlike a chrominance subcarrier, in a block 80. A modulated signal (M) from block 80 is added to the intraframe averaged first component (N) in an adder 40. A resulting output signal is a 4.2 MHz bandwidth baseband signal (NTSCF) that, together with a 750 KHz low pass filtered fourth component (YTN) from a filter 79, quadrature modulates an RF picture carrier in a block 57 to produce an NTSC compatible RF signal which can be transmitted to a standard NTSC receiver or a widescreen progressive scan receiver via a single, standard bandwidth, broadcast channel. 
     As will be seen from the encoder of FIG. 1a, the use of time compression on the first component allows low frequency side panel information to be squeezed entirely into the horizontal overscan region of a standard NTSC signal. The high frequency side panel information is spectrally shared with the standard NTSC signal through the video transmission channel, in a manner transparent to a standard receiver, through the use of an alternate subcarrier quadrature modulation technique involving block 80 as will be discussed. When received by a standard NTSC receiver, only the center panel portion of the main signal (the first component) is seen. The second and third components may create a low amplitude interference pattern that is not perceived at normal viewing distances and at normal picture control settings. The fourth component is removed completely in receivers with synchronous video detectors. In receivers with envelope detectors, the fourth component is processed but not perceived because it is correlated with the main signal. 
     FIG. 1b illustrates the RF spectrum of the disclosed EDTV widescreen system, including the auxiliary information, compared to the RF spectrum of a standard NTSC system. In the spectrum of the disclosed system the side panel highs and the extra high frequency horizontal luminance detail information extend approximately 1.1 MHz on either side of the 3.108 MHz alternate subcarrier (ASC) frequency. The V-T helper signal information (component 4) extends 750 KHz on either side of the main signal picture carrier frequency. 
     A widescreen progressive scan receiver includes apparatus for reconstructing the original widescreen progressive scan signal. Compared to a standard NTSC signal, the reconstructed widescreen signal has left and right side panels with standard NTSC resolution, and a 4:3 aspect ratio center panel with superior horizontal and vertical luminance detail particularly in stationary portions of an image. 
     Two basic considerations govern the signal processing technique associated with the development and processing of the first, second, third, and fourth signal components. These considerations are compatibility with existing receivers, and recoverability at the receiver. 
     Full compatibility implies receiver and transmitter compatibility such that existing standard receivers can receive widescreen EDTV signals and produce a standard display without special adaptors. Compatibility in this sense requires, for example, that the transmitter image scanning format is substantially the same as, or within the tolerance of, the receiver image scanning format. Compatibility also means that extra non-standard components must be physically or perceptually hidden in the main signal when displayed on standard receivers. To achieve compatibility in the latter sense, the disclosed system uses the following techniques to hide the auxiliary components. 
     As discussed above, the side panel lows are physically hidden in the normal horizontal overscan region of a standard receiver. Component 2, which is a low-energy signal compared to the side panel lows component, and component 3, which is a normally low energy high frequency detail signal, are amplitude compressed and quadrature modulated onto an alternate subcarrier at 3.108 MHz, which is an interlaced frequency (an odd multiple of one-half the horizontal line rate). The frequency, phase, and amplitude of the alternate subcarrier are chosen so that the visibility of the modulated alternate subcarrier signal is reduced as much as possible, e.g., by controlling the phase of the alternate subcarrier from field to field so that it alternates 180° from one field to the next, unlike the phase of the chrominance subcarrier from one field to the next. Although the modulated alternate subcarrier components reside entirely within the chrominance passband (2.0-4.2 MHz), the modulated alternate subcarrier components are perceptually hidden because they are displayed as field rate complementary color flicker, which is not perceived by the human eye at normal levels of chrominance saturation. Also, nonlinear amplitude compression of the modulation components prior to amplitude modulation advantageously reduces instantaneous amplitude overshoots to an acceptable lower level. Component 3 is hidden by time expanding the center panel information to match the standard 4:3 format, thereby spatially correlating (and temporally correlating) component 3 with component 1. This is accomplished by means of a format encoder as will be discussed. Such spatial correlation helps to prevent the component 3 information from interfering with the component 1 information after component 3 is quadrature modulated with component 2 on the alternate subcarrier and combined with component 1. 
     Component 4, the &#34;helper&#34; signal, also is hidden by time expanding the center panel information to match the standard 4:3 format, thereby spatially correlating component 4 with the main signal. Component 4 is removed at standard receivers with synchronous detectors, and is perceptually hidden at standard receivers with envelope detectors because it is spatially correlated with the main signal. 
     Recovery of components 1, 2, and 3 at a widescreen progressive scan receiver is accomplished by utilizing a process of intraframe averaging at the transmitter and receiver. This process is associated with elements 38, 64, and 76 in the transmitter system of FIGS. 1 and 1a, and with associated elements at the receiver as will be discussed. Intraframe averaging is one type of signal conditioning technique which prepares two highly visually correlated signals for mutual combining so that they can be recovered efficiently and accurately afterwards, such as by means of a field storage device, free from V-T (vertical-temporal) crosstalk even in the presence of motion in the case of image representative signals. The type of signal conditioning employed for this purpose essentially involves making two signals identical on a field basis, i.e., by obtaining two samples with identical values a field (262H) apart. Intraframe averaging is a convenient technique for achieving this objective, but other techniques can also be used. Intraframe averaging is basically a linear, time varying digital pre-filtering and post-filtering process to ensure the accurate recovering of two highly visually correlated combined signals. Horizontal crosstalk is eliminated by guardbands between horizontal pre-filters at the transmitter encoder and post-filters at the receiver decoder. 
     The process of intraframe averaging in the time domain is a process wherein pairs of fields are made identical by averaging pixels that are 262H apart. The average value replaces the original values in each pair. This V-T averaging occurs within a frame and does not cross frame boundaries. In the case of component 1, intraframe averaging is performed only on information above approximately 1.5 MHz so as not to affect lower frequency vertical detail information. In the case of components 1 and 2, intraframe averaging is performed on a composite signal including luminance (y) and chrominance (c) components throughout the chrominance band. The chrominance component of the composite signal survives intraframe averaging because pixels 262H apart are &#34;in-phase&#34; with respect to the color subcarrier. The phase of the new alternate subcarrier is controlled so that it is exactly out of phase for pixels 262H apart, and is therefore unlike the phase of the chrominance subcarrier. 
     With intraframe averaging V-T crosstalk is virtually eliminated, even in the presence of motion. In this regard, the process of intraframe averaging produces identical samples 262H apart. At the receiver it is a simple matter to recover the information content of these samples exactly, i.e., free from crosstalk, by averaging and differencing pixel samples 262H apart within a frame as will be discussed, thereby recovering main and auxiliary signal information. At a decoder in the receiver, the intraframe averaged original information can be recovered substantially intact via an intraframe averaging and differencing process since the original highly visually correlated information has been made substantially identical field-to-field. 
     Also at the receiver, the RF channel is quadrature demodulated using a synchronous RF detector. Component 4 is thereby separated from the other three components. Intraframe averaging and differencing are used to separate component 1 from modulated components 2 and 3, and quadrature demodulation is used to separate components 2 and 3, as will be discussed with regard to FIG. 2. 
     After the four components have been recovered, the composite signals are NTSC decoded and separated into luminance and chrominance components. Inverse mapping is performed on all components to recover the widescreen aspect ratio, and the side panel highs are combined with the lows to recover full side panel resolution. The extended high frequency luminance detail information is shifted to its original frequency range and added to the luminance signal, which is converted to the progressive scan format using temporal interpolation and the helper signal. The chrominance signal is converted to progressive scan format using unassisted temporal interpolation. Finally, the luminance and chrominance progressive scan signals are converted to analog form and matrixed to produce RGB color image signals for display by a widescreen progressive scan display device. 
     The widescreen EDTV system is shown in greater detail in FIG. 1a. Referring to FIG. 1a, a 525 line, 60 field/sec. widescreen progressive scan camera 10 provides a widescreen color signal with R, G, B components and a wide aspect ratio of 5:3 in this example. An interlaced signal source could also be used, but a progressive scan signal source produces superior results. A widescreen camera has a greater aspect ratio and a greater video bandwidth compared to a standard NTSC camera, the video bandwidth of a widescreen camera being proportional to the product of its aspect ratio and the total number of lines per frame, among other factors. Assuming constant velocity scanning by the widescreen camera, an increase in its aspect ratio causes a corresponding increase in its video bandwidth as well as horizontal compression of picture information when the signal is displayed by a standard television receiver with a 4:3 aspect ratio. For these reasons, it is necessary to modify the widescreen signal for full NTSC compatibility. 
     The color video signal processed by the encoder system of FIG. 1 contains both luminance and chrominance signal components. The luminance and chrominance signals contain both low and high frequency information, which in the following discussion will be referred to as &#34;lows&#34; and &#34;highs&#34;, respectively. 
     The wide bandwidth widescreen progressive scan color video signals from camera 10 are matrixed in a unit 12 to derive luminance component Y and color difference signal components I and Q from the R, G, B color signals. Wideband progressive scan signals Y, I, Q are sampled at an eight-times chrominance subcarrier rate (8×fsc), and are converted from analog to digital (binary) form individually by separate analog-to-digital converters (ADC) in an ADC unit 14 before being filtered individually by separate vertical-temporal (V-T) low pass filters in a filter unit 16 to produce filtered signals YF, IF and QF. The separate filters are 3×3 linear time invariant filters. These filters reduce vertical-temporal resolution slightly, particularly diagonal V-T resolution, to prevent unwanted interlace artifacts (such as flicker, jagged edges, and other aliasing related effects) in the main signal (component 1 in FIG. 1) after progressive scan to interlace conversion. The filters maintain nearly full vertical resolution in stationary portions of the image. 
     The center panel expansion factor (CEF) is a function of the difference between the width of an image displayed by a widescreen receiver and the width of an image displayed by a standard receiver. The image width of a widescreen display with a 5:3 aspect ratio is 1.25 times greater than the image width of a standard display with a 4:3 aspect ratio. This factor of 1.25 is a preliminary center panel expansion factor which must be adjusted to account for the overscan region of a standard receiver, and to account for an intentional slight overlap of the boundary regions between the center and side panels as will be explained. These considerations dictate a CEF of 1.19. 
     The progressive scan signals from filter network 16 exhibit a bandwidth of 0-14.32 MHz and are respectively converted into 2:1 interlaced signals by means of progressive scan (P) to interlace (I) converters 17a, 17b and 17c. The bandwidth of output signals IF&#39;, QF&#39; and YF&#39; from converters 17a-17c exhibit a bandwidth of 0-7.16 MHz since the horizontal scanning rate for interlaced signals is half that of progressive scan signals. In the conversion process, the progressive scan signal is subsampled, taking half the available pixel samples to produce the 2:1 interlaced main signal. Specifically, each progressive scan signal is converted to 2:1 interlaced format by retaining either the odd or even lines in each field and reading out the retained pixels at a 4×fsc rate (14.32 MHz). All subsequent digital processing of the interlaced signals occurs at the 4×fsc rate. 
     Network 17c also includes an error prediction network. One output of network 17c, YF&#39;, is the interlaced subsampled luminance version of the prefiltered progressive scan component. Another output (luminance) signal of network 17c, YT, comprises vertical-temporal information derived from image frame difference information and represents a temporal prediction, or temporal interpolation, error between actual and predicted values of luminance samples &#34;missing&#34; at the receiver, as will be explained. The prediction is based on a temporal average of the amplitudes of &#34;before&#34; and &#34;after&#34; pixels, which are available at the receiver. Signal YT, a luminance &#34;helper&#34; signal that assists to reconstruct the progressive scan signal at the receiver, essentially accounts for an error that the receiver is expected to make with respect to non-stationary image signals and facilitates cancellation of such error at the receiver. In stationary portions of an image the error is zero, and perfect reconstruction is performed at the receiver. It has been found that a chrominance helper signal is not needed as a practical matter, and that a luminance helper signal is sufficient to produce good results since the human eye is less sensitive to a lack of chrominance vertical or temporal detail. 
     In FIG. 1a, interlaced widescreen signals IF&#39;, QF&#39; and YF&#39; from converters 17a-17c are respectively filtered by horizontal lowpass filters 19a, 19b and 19c to produce a signal IF&#34; with a bandwidth of 0-600 KHz, a signal QF&#34; with a bandwidth of 0-600 KHz, and a signal YF&#34; with a bandwidth of 0-5 MHz. These signals are next subjected to a format encoding process which encodes each of these signals into a 4:3 format by means of format encoding apparatus associated with a side-center signal separator and processor unit 18. Briefly, the center portion of each widescreen line is time-expanded and mapped into the displayed portion of the active line time with a 4:3 aspect ratio. Time expansion causes a decrease in bandwidth so that the original widescreen interlaced frequencies are made compatible with the standard NTSC bandwidth. The side panels are split into horizontal frequency bands so that the I and Q color highs component exhibit a bandwidth of 83 KHz-600  KHz and the Y luminance highs component exhibits a bandwidth of 700 KHz-5.0 MHz. The side panel lows, i.e., signals YO, IO and QO, include a DC component and are time-compressed and mapped into the left and right horizontal image overscan regions on each line. The side panel highs are processed separately. Filtered interlaced signals IF&#34;, QF&#34; and YF&#34; are processed by side-center panel signal separator and processor 18 to produce three groups of output signals: YE, IE and QE; YO, IO and QO; and YH, IH and QH. The first two groups of signals (YE, IE, QE and YO, IO, QO) are processed to develop a signal containing a full bandwidth center panel component, and side panel luminance lows compressed into horizontal overscan regions. The third group of signals (YH, IH, QH) is processed to develop a signal containing side panel highs. When these signals are combined, an NTSC compatible widescreen signal with a 4:3 display aspect ratio is produced. 
     Signals YE, IE and QE contain complete center panel information and exhibit the same format. Briefly, signal YE is derived from signal YF&#34; as follows. Widescreen signal YF&#34; contains pixels 1-754 occuring during the active line interval of the widescreen signal, containing side and center panel information. The center panel information (pixels 75-680) is extracted as a center panel luminance signal YC via a time de-multiplexing process. Signal YC is time expanded by the center panel expansion factor of 1.19 (i.e., 5.0 MHz÷4.2 MHz) to produce NTSC compatible center panel signal YE. Signal YE exhibits an NTSC compatible bandwidth (0-4.2 MHz) due to the time expansion by factor 1.19. Signals IE and QE are developed from signals IF&#34; and QF&#34;, respectively, and are similarly processed in the manner of signal YE. 
     Signals YO, IO and QO provide the low frequency side panel information (&#34;lows&#34;) which is inserted into the left and right horizontal overscan regions. Signals YO, IO and QO exhibit the same format. Briefly, signal YO is derived from signal YF&#34; as follows. Widescreen signal YF contains left panel information associated with pixels 1-84 and right panel information associated with pixels 671-754. As will be discussed, signal YF&#34; is low pass filtered to produce a luminance lows signal with a 0-700 KHz bandwidth, from which signal a left and right side panel lows signal is extracted via a time de-multiplexing process. Luminance lows signal YL&#39; is time compressed to produce side panel lows signal YO with compressed low frequency information in the overscan regions associated with pixels 1-14 and 741-754. The compressed side lows signal exhibits an increased BW proportional to the amount of time compression. Signals IO and QO are developed from signals IF&#34; and QF&#34; respectively, and are similarly processed in the manner of signal YO. 
     Signals YE, IE, QE and YO, IO, QO are combined by a side-center signal combiner 28, e.g. a time multiplexer, to produce signals YN, IN and QN with an NTSC compatible bandwidth and a 4:3 aspect ratio. Combiner 28 also includes appropriate signal delays for equalizing the transit times of the signals being combined. Such equalizing signal delays are also included elsewhere in the system as required to equalize signal transit times. 
     A modulator 30, bandpass filter 32, H-V-T bandstop filter 34 and combiner 36 constitute an improved NTSC signal encoder 31. Chrominance signals IN and QN are quadrature modulated on a subcarrier SC at the NTSC chrominance subcarrier frequency, nominally 3.58 MHz, by modulator 30 to produce a modulated signal CN. Modulator 30 is of conventional design. Modulated signal CN is bandpass filtered in the vertical (V) and temporal (T) dimensions by means of two-dimensional (V-T) filter 32, which removes crosstalk artifacts in the interlaced chrominance signal before it is applied to a chrominance signal input of combiner 36 as a signal CP. Luminance signal YN is bandstop filtered in the horizontal (H), vertical (V) and temporal (T) dimensions by means of three-dimensional H-V-T bandstop filter 34 before being applied, as a signal YP, to a luminance input of combiner 36. Filtering luminance signal YN and chrominance color difference signals IN and QN serves to assure that luminance-chrominance crosstalk will be significantly reduced after subsequent NTSC encoding. 
     H-V-T bandstop filter 34 in FIG. 1a removes upwardly moving diagonal frequency components from luminance signal YN. These frequency components are similar in appearance to chrominance subcarrier components and are removed to make a hole in the frequency spectrum into which modulated chrominance will be inserted. The removal of the upwardly moving diagonal frequency components from luminance signal YN does not visibly degrade a displayed picture because it has been determined that the human eye is substantially insensitive to these frequency components. Filter 34 exhibits a cut-off frequency of approximately 1.5 MHZ so as not to impair luminance vertical detail information. 
     V-T bandpass filter 32 reduces the chrominance bandwidth so that modulated chrominance side panel information can be inserted into the hole created in the luminance spectrum by filter 34. Filter 32 reduces the vertical and temporal resolution of chrominance information such that static and moving edges are slightly blurred, but this effect is of little or no consequence due to the insensitivity of the human eye to such effect. 
     An output center/side lows signal C/SL from combiner 36 contains NTSC compatible information to be displayed, as derived from the center panel of the widescreen signal, as well as compressed side panel lows (both luminance and chrominance) derived from the side panels of the widescreen signal and situated in the left and right horizontal overscan regions not seen by a viewer of an NTSC receiver display. The compressed side panel lows in the overscan region represent one constituent part of the side panel information for a widescreen display. The other constituent part, the side panel highs, is developed by processor 18 as will be discussed below. 
     Signal C/SL is processed by an intraframe averager 38 to produce a signal N, which is applied to an input of an adder 40. Intraframe averaged signal N is essentially identical to signal C/SL because of the high visual correlation of intraframe image information of signal C/SL. Averager 38 averages signal C/SL above approximately 1.5 MHz and assists to reduce or eliminate vertical-temporal crosstalk between the main and auxiliary signals. The highpass frequency range of 1.5 MHz and above over which intraframe averager 38 operates was chosen to assure that full intraframe averaging is accomplished for information at 2 MHz and above, to prevent luminance vertical detail information from being degraded by the process of intraframe averaging. Horizontal crosstalk is eliminated by means of a 200 KHz guardband between a filter associated with intraframe averager 38 in encoder 31 and a filter associated with an intraframe averager-differencer unit in the decoder of FIG. 2. 
     Signals IH, QH, and YH are placed in NTSC format by means of an NTSC encoder 60 which is similar to encoder 31. Specifically, encoder 60 includes apparatus for quadrature modulating side panel chrominance highs information onto the side panel luminance highs information at 3.58 MHz, to produce signal NTSCH, the side panel highs information in NTSC format. 
     The use of multi-dimensional bandpass filtering in NTSC encoders 31 and 60 advantageously permits the luminance and chrominance components to be separated virtually free of crosstalk at the receiver when the receiver includes complementary multi-dimensional filtering for separating the luminance and chrominance information. The use of complementary filters for luminance/chrominance encoding and decoding is called cooperative processing and is discussed in detail in an article by C. H. Strolle titled &#34;Cooperative Processing for Improved Chrominance/Luminance Separation&#34;, published in the SMPTE Journal, Vol. 95, No. 8, August 1986, pp. 782-789. Even standard receivers using conventional notch and line-comb filters will benefit from the use of such multi-dimensional pre-filtering at the encoder by exhibiting reduced chrominance/luminance crosstalk. 
     Signal NTSCH is time expanded by a unit 62 to produce an expanded side highs signal ESH. Specifically, the expansion is accomplished by a &#34;mapping&#34; process which maps left side panel pixels 1-84 of signal NTSCH into pixel positions 1-377 of signal ESH, i.e. the left side highs of signal NTSCH are expanded to occupy one half the line time of signal ESH. The right side panel portion (pixels 671-754) of signal NTSCH is similarly processed. The time expansion process reduces the horizontal bandwidth of the information comprising signal ESH (compared to that of signal NTSCH) by a factor of 377/84. Signal ESH is intra-frame averaged by a network 64 to produce a signal X. Intraframe averaged signal X is essentially identical to signal ESH because of the high visual correlation of intraframe image information of signal ESH. Signal X is applied to a signal input of a quadrature modulator 80. 
     Signal YF&#39; is also filtered by a horizontal bandpass filter 70 with a passband of 5 MHz-6.2 MHz. The output signal from filter 70, horizontal luminance highs, is applied to an amplitude modulator 72 where it amplitude modulates a 5 MHz carrier signal f c . Modulator 72 includes an output low pass filter with a cut-off frequency of approximately 1.2 MHz to obtain a signal with a 0-1.2 MHz passband at the output of modulator 72. The upper (aliased) sideband (5.0-6.2 MHz) produced by the modulation process is removed by the 1.2 MHz lowpass filter. Effectively, horizontal luminance highs frequencies in the range 5.0 MHz-6.2 MHz have been shifted to the range 0-1.2 MHz as a result of the amplitude modulation process and subsequent low pass filtering. The carrier amplitude should be large enough so that the original signal amplitudes are retained after filtering by the 1.2 MHz low pass filter. That is, a frequency shift without affecting amplitude is produced. 
     The frequency-shifted horizontal luminance highs signal from unit 72 is encoded by means of a format encoder 74 to spatially correlate this signal with the main signal, C/SL. Encoder 74 is similar to format encoding networks associated with units 18 and 28 for the purpose of expanding the center panel information and compressing the side panel lows information into the horizontal overscan region. That is, encoder 74 encodes the frequency shifted horizontal luminance highs into a standard 4:3 format using techniques that will be discussed in connection with FIGS. 6-8. When the center portion of the input signal to encoder 74 is time expanded, its bandwidth drops to approximately 1.0 MHz from 1.2 MHz, and the output signal from encoder 74 becomes spatially correlated with the main signal. The side panel information is lowpass filtered within unit 72 to 170 KHz before being time-compressed by encoder 74. The signal from encoder 74 is intraframe averaged by means of apparatus 76 before being applied to unit 80 as signal Z. Intraframe averaged signal Z is essentially identical to the signal from encoder 74 because of the high visual correlation of intraframe image information of the signal from encoder 74. Modulating signal X, a composite signal containing luminance and chrominance information, and modulating signal Z exhibit substantially the same bandwidth, approximately 0-1.1 MHz. 
     Unit 80 performs nonlinear gamma function amplitude compression on large amplitude excursions of the two auxiliary signals, X and Z, before these signals quadrature modulate an alternate subcarrier signal ASC. A gamma of 0.7 is used, whereby the absolute value of each sample is raised to the 0.7 power and multiplied by the sign of the original sample value. Gamma compression reduces the visibility of potentially interfering large amplitude excursions of the modulated signals on exisiting receivers, and allows predictable recovery at the widescreen receiver since the inverse of the gamma function employed at the encoder is predictable and can be readily implemented at the receiver decoder. 
     The amplitude compressed signals are then quadrature modulated on a 3.1075 MHz phase-controlled alternate subcarrier ASC, which is an odd multiple of one half the horizontal line frequency (395×H/2). The phase of the alternate subcarrier is caused to alternate 180° from one field to the next, unlike the phase of the chrominance subcarrier. The field alternating phase of the alternate subcarrier permits the auxiliary modulating information of signals X and Z to overlap chrominance information and produces complementary phased auxiliary information components A1, -A1 and A3, -A3 of the modulated auxiliary signal, which facilitates the separation of the auxiliary information using a relatively uncomplicated field storage device at the receiver. The quadrature modulated signal, M, is added to signal N in adder 40. The resulting signal, NTSCF, is a 4.2 MHz NTSC compatible signal. 
     The described non-linear gamma function employed in the encoder for the purpose of large amplitude compression is a constituent part of a non-linear companding (compression-expansion) system which also includes a complementary gamma function in the decoder of a widescreen receiver for the purpose of amplitude expansion, as will be discussed subsequently. The disclosed non-linear companding system has been found to significantly reduce the impact of auxiliary non-standard information upon the standard information, without causing visible degradation of an image due to noise effects. The companding system uses a non-linear gamma function to instantaneously compress large amplitued excursions of auxiliary, non-standard widescreen high frequency information at the encoder, with a complementary non-linear gamma function being used to correspondingly expand such high frequency information at the decoder. The result is a reduction in the amount of interference with existing standard video information caused by large amplitude auxiliary high frequency information, in the disclosed compatible widescreen system wherein non-standard auxiliary widescreen information is split into low and high frequency portions subjected to companding. At the decoder, non-linear amplitude expansion of the compressed high frequency information does not result in excessive perceived noise since large amplitude high frequency information is typically associated with high contrast image edges, and the human eye is insensitive to noise at such edges. The described companding process also advantageously reduces cross-modulation products between the alternate and chrominance subcarriers, with associated reduction in visible beat products. 
     Luminance helper signal YT exhibits a bandwidth of 7.16 MHz and is encoded into the 4:3 format (in the same manner as accomplished by encoder 74 by means of a format encoder 78, and is horizontally lowpass filtered to 750 KHz by a filter 79 to produce a signal YTN. The side portions are lowpass filtered to 125 KHz before time compression by means of an input lowpass filter of format encoder 78 with a cut-off frequency of 125 KHz. The side portion highs are discarded. Thus signal YTN is spatially correlated with main signal C/SL. 
     Signals YTN and NTSCF are converted from digital (binary) to analog form by means of DAC units 53 and 54 respectively, before these signals are applied to an RF quadrature modulator 57 for modulating a TV RF carrier signal. The RF modulated signal is afterwards applied to a transmitter 55 for broadcast via an antenna 56. 
     Alternate subcarrier ASC associated with modulator 80 is horizontally synchronized and has a frequency chosen to insure adequate separation (e.g., 20-30 db.) of side and center information, and to have insignificant impact upon an image displayed by a standard NTSC receiver. The ASC frequency preferably should be an interlace frequency at an odd multiple of one half the horizontal line rate so as not to produce interference which would compromise the quality of a displayed picture. 
     Quadrature modulation such as provided by unit 80 advantageously permits two narrowband signals to be transmitted simultaneously. Time expanding the modulating highs signals results in a bandwidth reduction, consistent with the narrowband requirements of quadrature modulation. The more the bandwidth is reduced, the less likely it is that interference between the carrier and modulating signals will result. Furthermore, the typically high energy DC component of the side panel information is compressed into the overscan region rather than being used as a modulating signal. Thus the energy of the modulating signal, and therefore the potential interference of the modulating signal, are greatly reduced. 
     The encoded NTSC compatible widescreen signal broadcast by antenna 56 is intended to be received by both NTSC receivers and widescreen receivers, as illustrated by FIG. 2. 
     In FIG. 2, a broadcast compatible widescreen EDTV interlaced television signal is received by an antenna 210 and applied to an antenna input of an NTSC receiver 212. Receiver 212 processes the compatible widescreen signal in normal fashion to produce an image display with a 4:3 aspect ratio, with the widescreen side panel information being in part compressed (i.e., &#34;lows&#34;) into the horizontal overscan regions out of sight of the viewer, and being in part (i.e., &#34;highs&#34;) contained in the modulated alternate subcarrier signal which does not disrupt the standard receiver operation. 
     The compatible widescreen EDTV signal received by antenna 210 is also applied to a widescreen progressive scan receiver 220 capable of displaying a video image with a wide aspect ratio of, e.g., 5:3. The received widescreen signal is processed by an input unit 222 including radio frequency (RF) tuner and amplifier circuits, a synchronous video demodulator (a quadrature demodulator) which produces a baseband video signal, and analog-to-digital (ADC) converter circuits for producing a baseband video signal (NTSCF) in binary form. The ADC circuits operate at a sampling rate of four times the chrominance subcarrier frequency (4×fsc). 
     Signal NTSCF is applied to an intraframe averager-differencer unit 224 which averages (additively combines) and differences (subtractively combines) image lines 262H apart within frames, above 1.7 MHz, to recover main signal N and quadrature modulated signal M substantially free from V-T crosstalk. A 200 KHz horizontal crosstalk guardband is provided between the 1.7 MHz lower limit operating frequency of unit 224 and the 1.5 MHz lower limit operating frequency of unit 38 in the encoder of FIG. 1a. Recovered signal N contains information which is essentially visually identical to image information of main signal C/SL, due to the high visual intraframe image correlation of original main signal C/SL as intraframe averaged in the encoder of FIG. 1a. 
     Signal M is coupled to a quadrature demodulator and amplitude expander unit 226 for demodulating auxiliary signals X and Z in response to an alternate subcarrier ASC with a field alternating phase, similar to signal ASC discussed in connection with FIG. 1a. Demodulated signals X and Z contain information which is essentially visually identical to image information of signal ESH and of the output signal from unit 74 in FIG. 1a, due to the high visual intraframe image correlation of these signals as intraframe averaged by the encoder of FIG. 1a. Unit 226 also includes a 1.5 MHz lowpass filter to remove unwanted high frequency demodulation products at twice the alternate subcarrier frequency, and an amplitude expander for expanding the (previously compressed) demodulated signals using an inverse-gamma function (gamma=1/0.7=1.429), i.e., the inverse of the non-linear compression function performed by unit 80 in FIG. 1a. 
     A unit 228 time compresses the color encoded side panel highs so that they occupy their original time slots, thereby recovering signal NTSCH. Unit 228 time compresses signal NTSCH by the same amount that unit 62 of FIG. 1a time expanded signal NTSCH. 
     A luminance (Y) highs decoder 230 decodes luminance horizontal highs signal Z into widescreen format. The sides are time expanded (by the same amount as sides time compression in the encoder of FIG. 1a), and the center is time compressed (by the same amount as sides time expansion in the encoder of FIG. 1a). The panels are spliced together in the 10-pixel overlap region. 
     Modulator 232 amplitude modulates the signal from decoder 230 on a 5.0 MHz carrier f c . The amplitude modulated signal is afterwards high pass filtered by a filter 234 with a 5.0 Mhz cut-off frequency to remove the lower sideband. In the output signal from filter 234, center panel frequencies of 5.0 to 6.2 MHz are recovered, and side panel frequencies of 5.0 to 5.2 MHz are recovered. The signal from filter 234 is applied to an adder 236. 
     Signal NTSCH from compressor 228 is applied to a unit 240 for separating the luminance highs from the chrominance highs to produce signals YH, IH and QH. 
     Signal N from unit 224 is separated into its constituent luminance and chrominance components YN, IN and QN by means of a luminance-chrominance separator 242 which can be similar to separator 240. 
     Signals YH, IH, QH and YN, IN, QN are provided as inputs to a Y-I-Q format decoder 244, which decodes the luminance and chrominance components into widescreen format. The side panel lows are time expanded, the center panel is time compressed, the side panel highs are added to the side panel lows, and the side panels are spliced to the center panel in the 10-pixel overlap region. 
     Signal YF&#39; is coupled to adder 236 where it is summed with the signal from filter 234. By this process recovered extended high frequency horizontal luminance detail information is added to decoded luminance signal YF&#39;. 
     Signals YF&#39;, IF&#39; and QF&#39; are converted from interlaced to progressive scan format by means of converters 250, 252 and 254, respectively. Luminance progressive scan converter 250 also responds to &#34;helper&#34; luminance signal YT from a format decoder 260, which decodes encoded &#34;helper&#34; signal YTN. Decoder 260 decodes signal YTN into widescreen format. 
     I and Q converters 252 and 254 convert interlace to progressive scan signals by temporally averaging lines one frame apart to produce the missing progressive scan line information. 
     Luminance progressive scan converter unit 250 receives signal YT to assist reconstructing a missing progressive scan pixel sample. Full temporal detail is recovered within the band of horizontal frequencies contained in the encoded line difference signal (750 KHz, after encoding). Above this band of horizontal frequencies signal YT is zero, so the missing sample is reconstructed by temporal averaging. 
     Widescreen progressive scan signals YF, IF and QF are converted to analog form by means of a digital-to-analog converter 262 before being applied to a video signal processor and matrix amplifier unit 264. The video signal processor component of unit 264 includes signal amplifying, DC level shifting, peaking, brightness control, contrast control and other conventional video signal processing circuits. Matrix amplifier 264 combines luminance signal YF with color difference signals IF and QF to produce color image representative video signals R, G and B. These color signals are amplified by display driver amplifiers in unit 264 to a level suitable for directly driving a widescreen color image display device 270, e.g. a widescreen kinescope.