Method and apparatus for time companding an analog signal
TECHNICAL FIELD 
     This invention relates generally to time compression and the corresponding expansion (companding) of analog signals, and in particular to companding voice signals to achieve spectral efficiency. 
     BACKGROUND ART 
     Contemporary communication system designers are continuously challenged with transmitting more and more information down a given communication channel. For example, the current standard for a land mobile communication channel comprises a radio frequency channel having a bandwidth of 25 kHz for one message. Conventionally, two such channels are required for a full duplex conversation: one to transmit, and one to receive. As available frequency spectrum diminishes, the need to transmit more information down a communication channel becomes paramount. This need is experienced today in urban areas where a limited amount of frequency spectrum must be shared by a large number of transceivers in a given geographic area. 
     One technique to send more information down a communication channel is time compression multiplexing (TCM). A TCM communication system comprises an analog system wherein analog signals are sampled and stored in a storage means. The samples are extracted, in turn, and transmitted at a high rate of speed. In this way, several signals may be sent over the same communication channel in a time division multiplex (TDM) fashion. In a TDM system, a communication channel is divided into a plurality of slots. Each transceiving device may transmit or receive information in one or more of the slots. 
     TCM advantageously exploits the fact that time compression merely scales the occupied spectrum in relation to a time scaling factor. Accordingly, two voices compressed by approximately 2:1 may be transmitted over a single conventional 25 kHz radio channel by slightly reducing the deviation, and improving filtering to reduce adjacent channel &#34;splatter&#34;. TCM stands in contrast to digital coding techniques that require considerably more complexity and processing for a speech signal to occupy a given bandwidth. 
     The TCM slot duration (i.e., the duration of the speech burst), is selected by balancing several considerations, including audio delay (preferably small), and the limitations of adapting a transceiving device from the transmit to the receive mode, or vice versa (i.e., synthesizer and antenna switch settling times). A typical value for a 2:1 compression system comprises a 60 ms slot. A full duplex system is synchronized such that immediately after transmitting information in a slot, the transceiver adapts to receive information from a subsequent slot. This alternating transceiving procedure operates to allow continuous communication between two parties simultaneously. 
     TCM communication systems, however, suffer a detriment stemming from the fact that the recovered analog signal is discontinuous at the slot boundaries. In FIG. 1, a typical TCM signal recovery process 100 is shown. A time compressed signal is initially received in a series of time slots (102, 104, and 106). The information in each slot is expanded by the inverse of the compression ratio, and the signal is recovered by concatenating the information in a contiguous manner as shown (102&#39;, 104&#39;, and 106&#39;). However, at the slot boundaries, discontinuities generally appear due to the limited bandwidth communication channel and imperfections in timing recovery at the receiver. These discontinuities cause distortion in the recovered signal As used herein, the boundary areas of the reconstructed slots is referred to as the &#34;splice-zone&#34;. The &#34;splice-zone distortion&#34; 108 generally manifests itself in the reconstructed waveform as &#34;pops&#34; and &#34;clicks&#34; occurring at the TDM frame (slot) rate. To solve this problem, some designers have installed gain compression devices to lower the gain in the splice-zone to avoid amplifying the clicks and pops resulting from the distortion. Such gain compressors are both expensive and complicated since they must effectively reduce the gain in the splice-zone and restore it in a controlled fashion to properly amplify the intelligible information. 
     Generally, transceiving devices may be mobile units, portable units, or base stations. Generally, a portable unit is defined as a communication unit typically designed to be carried on or about the person. A mobile unit is a transceiving device designed to be installed in vehicles. A base station is contemplated to be a transceiving device permanently or semipermanently installed at a fixed location. As used herein, all of these devices are collectively referred to herein as transceiving devices. 
     Accordingly, a need exists for a spectrally efficient communication system capable of supplying increased information content down a limited bandwidth channel without introducing splice-zone distortion as in prior systems. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a spectrally efficient communication system. 
     It is another object of the present invention to provide a spectrally efficient communication system that minimizes splice-zone distortion. 
     It is a further object of the present invention to provide a time compander that eliminates the need for complicated gain compressors. 
     It is yet another object of the present invention to provide a transceiving device having reduced circuitry while providing the capability of a full duplex communication using only one 25 kHz land mobile channel. 
     Accordingly, these and other objects are achieved in using the method and apparatus for time companding an analog signal as taught by the present invention. 
     Briefly, according to the invention, an analog message such as a voice message is digitized in a continuously variable slope delta-modulator (CVSD) and stored in a digital storage means at a first rate. The digitized message is removed from the storage means and converted back into an analog message using a continuously variable slope delta-demodulator at a second rate being at least twice as fast as the first rate. The analog signal is then transmitted during at least one time slot of a communication channel By combining the advantages of CVSD digitization, digital storage, and analog transmission, the present invention overcomes the detriments of the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may be understood by reference to the following description, taken in conjunction with the accompanying drawings, and the several figures of which like reference numerals identify like elements, and in which: 
     FIG. 1 is an illustration of a received TDM voice signal and the associated splice-zone distortion; 
     FIG. 2 is a block diagram of a transceiver according to the present invention; 
     FIG. 3 is a block diagram of a typical continuously variable slope delta-modulator; 
     FIG. 4 is an illustration of the data flow to the CVSD device 228 of FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 2, there is shown a block diagram of a full duplex transceiving device in accordance with the present invention. To transmit, a voice signal may be first filtered by filter 202, which provides an appropriately band-limited audio signal to a CVSD encoder 204. Preferably, the CVSD encoder 204 is an MC3518 manufactured by Motorola, Inc., or its functional equivalent. The CVSD encoder 204 continuously digitizes the audio signal at a first rate defined by a clock signal 206. Preferably, this clock rate is 64 kHz although other rates may be used. The digitized audio signal 208 is coupled to a Synchronous Serial Data Adaptor (SSDA) 210. The SSDA 210 receives the digitized signal and, under control of a microprocessor 212, transmits the digitized information to a digital storage device such as random access memory (RAM) 214. Preferably, the SSDA is an MC6852, and the microprocessor is an MC6803, both manufactured by Motorola, Inc., although their functional equivalents may be substituted. Of course, the microprocessor 212 receives its instructions from a read only memory (ROM) 216 and each of the SSDAs, microprocessor(s), RAM(s), and ROM(s) are interconnected with an address bus 218 and a data bus 220. The stored digitized voice is extracted from the RAM 214 and routed to an SSDA 222 for transmission on to at least one time slot of a communication channel. To transmit, the microprocessor 212 places the appropriate logic signal on the R/T line 224. The data exits the SSDA 222 and is presented to an AND gate 224. As a second input, the AND gate 224 receives an inverted (242) not-clear-to-send (CTS) signal, which is locally generated in the receiver 250 by any suitable synchronization recovery technique known in the art. Of course, the microprocessor 212 also receives the CTS signal, which operates as a slot (frame) marker facilitating proper control of the data flow within the transceiver 200. 
     When transmitting a message, the CTS signal is asserted (logic 0), the inverted (242) form of which enables the CVSD message to pass through the AND gate 224, to the OR gate 226, and finally to the CVSD device 228. The CVSD device 228 operates in the decode mode transmission due to the logical state of the R/T line 224. The R/T line 224 also enables the transmission gate 230 and disables the transmission gate 232. Thus, the stored CVSD signal is routed from the SSDA to the CVSD device 228, which converts the message back into an analog signal at a rate defined by a high speed clock signal 246. Preferably, the frequency of the high speed clock is 153.6 MHz, although other frequencies may be used so long as the the frequency of the high speed clock is slightly greater than twice that of the low speed clock signal 206. The now analog message travels from the CVSD device 228 through the transmission gate 230 to the filter 232, which removes any remaining quantization noise in the converted signal. The filtered signal is then coupled to the transmitter 234. An antenna switch 236 couples the antenna 238 to the transmitter 234 and the voice message is transmitted in at least one time slot of the channel. 
     Those skilled in the art will appreciate that a small &#34;guard band&#34; is placed between the slots of a TDM communication system to accommodate timing imperfections, propagation delays, or to provide additional signalling, synchronization, or supervisory data. After the message has been transmitted, the CTS signal 240 will negate (logic 1), which disables the AND gate 224 and enables the AND gate 244. During this time, the high speed clock signal 246, divided by two (248), is routed via the AND gate 244 and the OR gate 226 to the CVSD device 228. Since the CVSD device is clocked at twice the speed of the signal at its input, an alternating logic 1 - logic 0 pattern will be transmitted in the guard band slot between the slots filled with the voice message. This alternating logic 1 - logic 0 pattern is commonly referred to as the &#34;dotting&#34; pattern. 
     When receiving a message from the channel, the microprocessor 212 places the appropriate signal on the R/T line 242. This causes the antenna 238 to be coupled to a receiver 250 via the antenna switch 236. The received signal is routed via the transmission gate 233 to the filter 232 and into the CVSD device 228. In the receive mode, the same CVSD device used as a decoder in the transmit mode (228), operates as an encoder to digitize the received analog signal. The received signal is also coupled to the transmitter 234, however, this presents no problem since the transmitter is no longer coupled to the antenna. The CVSD device 228 accepts the analog signal and provides a digitized equivalent signal to the SSDA 222 at the rate provided by the high speed clock 246. The SSDA 222, under control of the microprocessor 212, routes the high speed digitized data to the RAM 214. This signal is extracted from the RAM 214 and routed to the SSDA 210, which continuously couples it to a CVSD decoder 252 at the lower data rate (206). The CVSD decoder 252 reconstructs the message as an analog signal and presents it to a filter 254. Optionally, the decoded signal may be amplified in an amplifier 256 prior to filtering. The filter 254 appropriately band-limits the received message and also removes any remaining quantization noise from the message. The recovered signal is then provided to any subsequent circuitry such as an audio power amplifier and speaker. Accordingly, since the transmitter and receiver operate at greater than twice the speed of the CVSD encoder 204 and decoder 252 a full duplex conversation may be had using a single communication channel. This provides a two to one improvement in spectral efficiency. 
     The conversion of the digitized message back into an analog message affords the present invention the advantages of both digital storage and analog transmission. The selection, by the present invention, of CVSD A/D devices, in combination with the dotting pattern guard band, minimizes the splice-zone distortion experienced by TCM communication systems. 
     In FIG. 3 there is shown a block diagram of a typical Continuously Variable Slope Delta-Modulator (CVSD) 300. To digitize an analog signal, a comparator 302 compares the analog signal with a second signal 304 provided by an integrator (generally leaky) 306. The output of the comparator 302 reflects the sign difference between the audio signal and the feedback signal 304. This signal is sampled in the sampler 308 at a given clock rate 310. The sampler outputs a digital signal 312, which represents a digitized analog signal that may be stored or transmitted. The CVSD signal 312 is received by the integrator 306 and a coincidence detector 314. The purpose of the coincidence detector 314 is to look for a predetermined number (typically three or four) of digital output bits that all have the same level (i.e., either logic 1 or logic 0. Should the coincidence detector detect four consecutive identical logic levels, a signal to the gain control 316 would increase the gain of the integrator 306. Conversely, the absence of four consecutive levels causes the gain of the integrator 306 to be reduced. In this way, the dynamic range of the CVSD device 300 is improved as is known in the art. 
     In the decode mode, a received CVSD signal is applied (312) to the coincidence detector 314 and the integrator 306. The coincidence detector 314 controls the gain control device 316, which, in turn, controls the gain of the integrator 306. The integrator output (304) is provided as the decoded output signal to any subsequent circuitry as is known in the art. 
     Referring now to FIG. 4, the data flow 400 of the CVSD device 228 is shown As previously mentioned, the present invention contemplates filling a time frame corresponding to the channel guard band with an alternating logic 1 - logic 0 (dotting) pattern. The dotting pattern assures the absence of coincidence in the CVSD device 228. Thus, as described in conjunction with FIG. 3, the gain of the CVSD device is minimized between slots thereby minimizing splice-zone distortion. The use of the dotting pattern also causes the integrator output signal 304 (see FIG. 3) to decay to a center value 110 (see FIG. 1) in a consistent fashion. This gradual decay and center starting point also aids in reducing splice-zone distortion. This eliminates the need for post-reconstruction gain compressors required by other techniques implementing TCM. Thus, the data flow 400 of the CVSD device 228 comprises at least two time slots 402 and 406 (corresponding to the communication channel slots) alternately interleaved with a predetermined pattern 404, which preferably comprises a dotting pattern. Accordingly, the CVSD device 228 (see FIG. 2) operates in the decode mode during data portions 402 and 404, and operates in the encode mode only during portion 406. 
     While a particular embodiment of the invention has been described and shown, it should be understood that the invention is not limited thereto since many modifications may be made. It is therefore contemplated to cover by the present application any and all such modifications that may fall within the true spirit and scope of the basic underlying principles disclosed and claimed herein.