Abstract:
Disclosed is a centralized circuit for determining whether input signals from a plurality of channels are speech which includes memory means in which digital amplitude samples from each of the input channels are stored in real time. A control means then periodically retrieves the samples from the memory in a time faster than real time and produces a series of time compressed input signals which are compared with a variable threshold derived from a stored level for each channel. Moreover, means are disclosed for setting a new threshold whenever the time compressed input signals differs from the old level for a predetermined time, the new level thereafter being stored in place of the previously stored old level.

Description:
BACKGROUND OF THE INVENTION 
     This application is a continuation-in-part of Ser. No. 131,159 filed Mar. 17, 1980. 
    
    
     This invention relates to time assignment speech interpolation systems and, more particularly, to a speech detector with associated gain control for such systems. 
     Because of the extremely high cost of communications transmission facilities, e.g., satellite channels and undersea transmission lines, the prior art has sought various means to maximize the efficiency of existing transmission facilities. One such system is known as a time assignment speech interpolation (TASI) system. In a typical TASI system, calls from n callers are transmitted across, for example n/2 transmission facilities to a remote location. At that location, the n/2 facilities are connected to n output channels. TASI systems operate on the assumption, verified as a statistical fact, that at any given time not all callers will wish to talk simultaneously. In fact, as a general rule, callers are actively talking less than half of the time the talker and the listener are interconnected. Accordingly, TASI systems may be defined as switching systems which interconnect talker and listener only when the talker is actively speaking, provided there is a transmission facility available at that time. 
     In a TASI system, an input channel is connected to a transmission facility only when a speech signal is detected on that channel by a speech detector. It is desirable that such a speech detector have a variable threshold so that speech may be distinguished from noise even when speech characteristics and ambient noise vary. 
     In the ideal situation, it is desirable that speech be continuously monitored on each input channel and that the speech detection threshold be frequently updated to reflect changes in the ambient conditions on each input channel. However, in the past, such an arrangement required that each input channel have a separate speech detector associated therewith which greatly increases the cost and complexity of speech detection circuitry. 
     Accordingly, the prior art has employed centralized speech detectors which are time shared among the various input channels. In this regard, see U.S. Pat. No. 3,520,999--May. The use of centralized speech detection eliminates some duplication of circuit components, however, this benefit is achieved at the expense of the ability to continuously monitor each input channel, since while a time shared speech detector is monitoring one channel other channels are not monitored. 
     In a centralized speech detector, such as that disclosed by May, each input channel is provided with a level detector which provides a signal proportional to the amplitude of the input signal amplitude on a particular channel at a given time. This signal is then directed to a centralized comparison means where the signal amplitude is compared against a stored threshold for that channel. 
     It would be desirable to provide a centralized speech detector circuit which includes a centralized amplitude detector as opposed to the distributed level detectors employed by May. More importantly, in the May approach, speech is determined to be present when the amplitude of the input signal exceeds a stored threshold at a particular time. However, at any one time, only one input channel may be monitored. It would be desirable to monitor all input channels over a period of time in order to provide more accurate speech detection and to do so on a centralized basis. Speech is then determined to be present only when the average input signal power over an extended period of time exceeds a stored threshold. 
     In U.S. Pat. No. 4,028,496 to LaMarche, a speech detection circuit is employed in which the threshold against which the instantaneous input signal sample is compared is the short term running average of the previous samples. However, Lamarche, like May, does not observe the input signal over an extended period of time. It would be desirable to compare the input signal power over a period of time, vis-a-vis a stored threshold, rather than to compare the instantaneous input signal sample versus a threshold as shown by LaMarche and May and to do so on a centralized basis. 
     SUMMARY OF THE INVENTION 
     In accordance with Ser. No. 131,159 filed Mar. 17, 1980 in conjunction with a time division switching network, each input channel is sampled and the speech signal samples from all channels are directed to a centralized speech detection circuit including a time speed-up memory. Stated another way, the signal samples from each input channel are obtained over a period of time and are continuously read into a centralized memory at a particular input rate and subsequently read out of that memory at a faster rate. Thus, the centralized time speed-up memory contains the entire recent past signal history for each channel. The time compressed amplitude history for each channel is compared in a centralized comparison means. Since the signal history is time compressed, a single, centralized comparison means may be utilized on a time shared basis and yet each of the input channels may be monitored over an extended time period. 
     In accordance with present invention, speech detection may be further improved by the provision of an automatic gain control circuit responsive to the aforementioned centralized speech detection circuit. Speech detection is enhanced on channels carrying the signals of weak talkers by the gain control circuit by increasing the amplitude of signals on those channels in accordance with the time compressed amplitude history for that channel as stored in the memory of the centralized speech detector. This arrangement enhances not only the transmission of the speech of weak talkers but also the ability of the centralized speech detector to distinguish that speech as such. 
     RELATED PATENTS AND APPLICATIONS 
     U.S. Pat. No. 4,153,816 entitled, &#34;Time Assignment Speech Interpolation Communication System with Variable Delays&#34; to William A. Morgan, and U.S. Pat. No. 4,184,051 entitled, &#34;Digital Memory Providing Fixed and Variable Delays in a TASI system&#34; to Glenn R. Clingenpeel, describe a TASI system upon which the present invention is an improvement and also U.S. Pat. No. 4,165,449 to P. A. Vachon, &#34;Echo Suppressor Circuit&#34;, describes an echo suppressor useful in conjunction with the present invention. The disclosures of these patents are incorporated herein by reference. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the sending side of a TASI system; 
     FIGS. 2A-2B are waveforms depicting the operation of the centralized speech detector of the present invention; 
     FIG. 3 is a block diagram of the centralized speech detector circuit of the present invention; 
     FIG. 4 is a block diagram of an automatic gain control circuit useful with the centralized speech detector of FIG. 3; 
     FIG. 5A is a plot of the desired output of the automatic gain control circuit of FIG. 4 versus the input signal applied thereto; 
     FIG. 5B is a plot of the binary value of the old speaker level generated by the speech detector circuit of FIG. 3 versus old speaker levels in dBm; and 
     FIG. 5C is a plot of the gain applied by the gain control circuit of FIG. 4 in adjusting the input signal amplitude to produce the output of FIG. 5A versus the gain index code identifying that gain, which gain index code is determined in accordance with the binary value of the old speaker level. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Before describing the present invention, the sending side of a TASI system will first be described with reference to FIG. 1. 
     In this example, there are 31 input channels of which only channels 1, 2, 3, and channel 31 have been shown. As is conventional in such systems, signals from the input channels after passing through an echo suppressor such as that described in the aforementioned Vachon patent, are applied to a switching network 10 which selectively connects speech signals on an input channel, 1 . . . 31, to an available transmission facility, 1A . . . 16A. Only transmission facilities 1A, 2A, and 16A have been shown. Typically, there are about half as many transmission facilities as there are input channels. A central speech detector circuit 11 detects the presence of a speech signal on an input channel. In response to the detection of a speech signal on the input channel, a controller 15 produces a signal which operates switching network 10 to connect the input channel to an available transmission facility. FIG. 1 shows input channel 3 connected to transmission facility 16A. 
     A fixed delay buffer or memory means 13 is connected between each of the input channels and the switching network 10. Incoming signals are stored in fixed segmented buffer 13 for a time interval during which a symbol from signalling generator 18 is applied to the transmission facility. This symbol from signalling generator 18 identifies the input channel to which each transmission facility has been assigned at a given time. For example, assume transmission facility 16A has been assigned to input channel 3. Signalling generator 18 generates a symbol representing input channel 3. This symbol is applied to the transmission facility 16A before the speech signal, sometimes referred to as a speech burst. The delay provided by fixed buffer 13 thus provides the time interval required to insert the symbol before the speech signal. At the receiving side, the symbol identifying input channel 3 is utilized to route the conversation from input channel 3 to the appropriate party. 
     Input signals from the input channels 1 . . . 31 are converted into digital (P.C.M.) form by means of the analog-to-digital converters 28, 30, 32, 34 and others. In the preferred embodiment, the sampling rate of each of these analog-to-digital converters is 8000 samples per second. Thus, each sample of an incoming signal represents the amplitude of the signal on each input channel over a 0.000125 second interval. These samples are directed to a gain control circuit 90 which will be more fully described below. From the gain control circuit 90, the most recent of these samples are directed to fixed buffer 13 and stored on a per channel basis in identifiable segments 36, 38, 40, 42, and others of that buffer. In the preferred embodiment, the 256 most recent samples from each input channel are stored in the segment associated with that channel. These digital amplitude samples, when retrieved from fixed delay buffer 13, may be stored for variable times in variable delay buffers 44, and then transmitted on a facility when a facility becomes available. This is described in the aforementioned Clingenpeel patent. Prior to transmission, the digital samples are reconstituted to analog by the digital-to-analog converters 46, 48, 50, and others before the signals are applied to the transmission facilities 1A . . . 16A. 
     In accordance with the present invention, samples stored in the fixed delay buffer 13 in addition to being directed to the switching network 10 are also directed to the centralized speech detector circuit 11. For application to the speech detector 11, samples are retrieved on a channel by channel basis. The channel being retrieved at any given time is determined by the count of channel address counter 52 which is driven by a clock means 54. In the preferred embodiment, the most recent 128 of the 256 samples stored from each channel are delivered to speech detector circuit 11 by the sample address counter 58 which is also driven by clock means 54 before samples from the next channel are retrieved. 
     In accordance with the present invention, the samples retrieved from fixed buffer 13 are retrieved by the clock means 54 and counters 52 and 58 in a time faster than real time. In the preferred embodiment, samples are retrieved at a rate of 1024 KHz., or 128 times faster than real time. In this manner, digital samples representing input signal amplitudes over a relatively long period of real time may be retrieved in an interval many times faster than real time. In accordance with the preferred embodiment, every 0.000125 second, 128 samples are retrieved which represent the input signal history for a particular channel which had been accumulated over a 0.016 sec. interval. In this manner, the amplitude history for all channels may be continually monitored with a single centralized speech detector. 
     FIG. 2A depicts an input signal on an input channel, such as channel 1, over a 16 millisecond period of time. This signal is continuously sampled at 125 microsecond intervals and the samples are stored in fixed buffer 13. The samples are read out and reconstituted by digital-to-analog converter 60 into the time compressed signal shown in FIG. 2B. FIG. 2B depicts the signal power in the input signal for our channel which occurs over the 0.016 second interval shown in FIG. 2A but compressed in time, e.g., by a factor of 128:1. 
     Referring now to FIG. 3, the centralized speech detector of the present invention will be described in detail. As shown in FIG. 3, the time compressed signal from a particular input channel is directed from fixed buffer 13 to a digital-to-analog converter 60, the output of which is exemplified in FIG. 2B described above. 
     The output of the digital-to-analog converter 60 is directed to a speech envelope detector shown generally at 61. The speech envelope detector comprises a spectral weighting filter 62, a full wave rectifier 64 and an RC circuit comprising a resistor 66 and a capacitor 67. The spectral weighting filter smooths and shapes the signal and weights it heavily in the 800-900 Hz. range in which speech normally occurs. The output of the spectral weighting filter 62 is rectified and applied to the RC circuit 66, 67, the output of the speech envelope detector 61 being the voltage across capacitor 67. 
     The output of the speech envelope detector 61 is a voltage proportional to the average power of the input signal for a particular input channel taken over a 0.016 second interval. This voltage is amplified by an amplifier 68 and applied to a first comparison means 70 for comparing the time compressed input signal for each channel with the variable threshold V th1  for that channel. The purpose of the amplifier 68 is to ensure that the comparison means 70 operates in the proper range. The old speaker level for each of the input channels is stored in a speech history storage means 72 which comprises a digital memory containing an 8 bit byte representing the old speaker level for each input channel. These 8 bit bytes representing the old speaker level for each channel are retrieved at the control of channel address counter 52 and converted into analog signals by digital-to-analog converter 74, the output of which is directed, via Pad 76, to the first comparison means 70. Whenever the average power of the time compressed input signal exceeds the variable threshold V th1 , the comparator 70 produces a speech present (SP) signal which is directed to processor 80. V th1  is the old threshold for each channel and is merely a value specified by Pad 76 which is K db below the old speaker level. In the preferred embodiment K is 9 db. The speech present signal SP is directed from processor 80 to the controller 15 so that switching may be accomplished. 
     The speech detector circuit of the present invention also includes a means for setting a new threshold for each channel whenever the average power of the time compressed input signal differs from the old speaker level stored in the speech history memory 72. The means for setting a new threshold comprises a servo loop including a second comparison means 78, processor 80 and the aforementioned speech history storage means 72. The comparator 78 compares the time compressed input signal with the old speaker level as stored in the speech history memory 72 after that speaker level has been reconstituted to analog form via digital-to-analog converter 74. 
     If the incoming signal power on a particular channel exceeds the old speaker level for that channel, the processor 80 increments the old speaker level stored in the speech history memory 72 to a new and higher level for the next time that an input signal is compared for that channel. If the incoming signal power is below the old speaker level stored in speech history memory 72, the processor 80 reduces the old speaker level to a new and lower threshold for the next time an input signal on that channel is monitored. 
     Since it is not advantageous to update the old speaker level as stored in speech history memory 72 instantaneously, the processor 80 ensures that the old speaker level for each channel is updated only after a minimum time such as 0.016 second has elapsed. Moreover, in accordance with the preferred embodiment of the present invention, processor 80 permits the updating of the old speaker level stored in memory 72 only when speech is present as determined by the output of the first comparison means 70. In this manner, the variable threshold is only adjusted by a speaker&#39;s volume and not by noise. 
     Referring now to FIG. 4, the gain control circuit 90 will be described in detail. The gain control circuit 90 includes a first programmable read only memory (PROM) 91 and a second programmable read only memory (PROM) 93 responsive thereto. The gain control circuit 90 further includes a random access memory (RAM) 92 to which gain index codes from PROM 91 are directed and from which the same gain index codes are directed to PROM 93. Digital samples of input signals from analog-to-digital converters 28-34 and others on lines 1 to 31 are also directed to PROM 93. The amplitude of these samples is adjusted by PROM 93 in accordance with the function shown in FIG. 5A. As seen in that figure, the gain control circuit 90 applies a varying amount of gain to input signal samples having an amplitude less than a predetermined minimum such as, for example, -24 dBm. The gain adjustment to these samples is determined in accordance with a gain index code applied to PROM 93 from RAM 92. The gain index code for a particular channel is determined by the binary value of the old speaker level for that channel. The binary old speaker level for a given channel is directed to PROM 91 from the speech history storage means 72 of the speech detector 11. PROM 91 then generates gain index codes in accordance with the binary old speaker level. From FIG. 5B, it can be seen that when the old speaker level is greater than -24 dBm, the binary old speaker level is greater than 80. In this case the gain index code as determined by PROM 91 is zero, and accordingly the gain is zero. However, when the old speaker level is less than -24 dBm, a gain index code according to the function shown in FIG. 5C is generated by the PROM 91. 
     The gain index code generated by PROM 91 for each channel is stored in RAM 92 and is retrieved under the command of the channel index counter 52 at such time as the gain for that particular channel is to be adjusted. At such time the gain index code is directed to PROM 93. 
     As shown in FIG. 5C, the gain index code from RAM 92 is utilized to adjust the gain at PROM 93 in steps ranging from zero to 7.5 dB. Incoming signal sample amplitudes are thus adjusted by varying amounts within this range. The gain adjusted digital samples from PROM 93 are then directed to the fixed buffer 13 from which they are subsequently retrieved and directed to the speech detector 11 and also to switching network 10 if speech is detected. Accordingly, it is the gain adjusted digital samples which are utilized in the speech history storage means 72 of the speech detector 11 as a representation of the old speaker level. 
     From the foregoing, it can be seen that the amplitude of transmitted speech is improved for weak talkers since gain to such speakers is applied prior to transmission. Moreover, speech detection is enhanced since low level incoming signals are effectively amplified prior to speech detection. 
     While a particular embodiment has been shown and described, it will, of course, be understood that various modifications may be made without departing from the principles of the present invention. The appended claims, are therefore, intended to cover any such modification within the true spirit and scope of the invention.