Method and apparatus for reducing noise in a linked compressor-expander telecommunications system.

0001The present invention relates to telecommunications systems in general, and more particularly relates to noise reduction in the transmission of any band limited complex waveforms including those representing human speech such as that generated by linked compressor-expander systems (Lincompex) and that generated by parallel-tone or multi-tone data in a system transmitting envelope data separately from compressed signal data.

0002Lincompex systems have generally been known since the early 1960s. These systems were originally implemented with analog technology but now may be implemented digitally such as taught in U.S. Patent 4,271,499 to Leveque, the inventor of the present application. U.S. Patent 4,271,499 is incorporated herein by reference.

0003The basic principles of a Lincompex system as disclosed in the '499 patent will now be described with reference to Figures 1(a) and 1(b). Incoming signals are applied to an input terminal 20 and are split into speech signal and control paths 21a and 21b respectively. In the control path 21b, the envelope of the input signal level is detected by an envelope detector 26 and a signal is generated by detector 26 proportional to this detected level. A compressor circuit 22 in the speech path uses the signal from the envelope detector to adjust its gain according to the detected signal level so as to output signals of substantially constant and compressed amplitude. In general implementations, the signal is a voice signal so that the gain of the compressor is adjusted at a syllabic rate. The control path signal is then converted to a logarithmic signal in a logarithmic amplification circuit 28 to compensate for the voltage to decibel conversion which occurs when compressing a voltage signal representing a logarithmic voice signal. The logarithmic signal is applied to a voltage controlled oscillator 30 which produces an output frequency related to the amount of speech signal compression conducted upon the corresponding syllable in the compressor circuit 22. The compressed speech signal and the control frequency signal are then combined in a summer 32, amplified, and input to a transmitter 34 for transmission over a transmission medium 36. The envelope detector 26, logarithmic amplifier 28, and the voltage controlled oscillator 30 form a control tone generator 24.

0004At the receiving end, a modulated signal is input to a receiver 38 after being transmitted over the transmission medium 36a, and is split back into speech and control signal paths 39a and 39b respectively. The control signal frequency is isolated from the demodulated signal by a band pass filter 46 and the frequency thereof is detected by a frequency demodulator 48. This frequency signal is then passed through a logarithmic to linear network or anti-log amplifier 50 to reattain the speech envelope level signal. The compressed data signal from receiver 38 is passed through a low-pass filter 40 to remove high frequency components. The envelope level signal is then applied to an expander circuit 42 in the speech path which amplifies the compressed signal, back to the original amplitude input signal.

0005One unresolved problem relating to Lincompex use in a communications network arises when the receiving station demodulator circuitry is in an operative state but no signals are being transmitted by the transmitter. In such a circumstance, the demodulator has only a noise input. Referring to Figure 1(b), during normal speech transmission, the compressed speech frequency band and the control tone frequency band are as indicated in Figure 3(a) and are received at receiver 38 of Figure 1(b) as indicated at (a). The frequency band of the envelope of the original speech signal is shown in Figure 3(b), while the restored voice signal frequency band is shown in Figure 3(c). As is apparent, the control tone frequency is used to expand the compressed speech signal to the restored voice parameters as shown in Figure 3(c) which are output from the expander circuit 42. Reference (b) indicates where the envelope signal, represented by the control tone signal, appears in the control tone conversion circuit 44. With only noise present at the input, as illustrated in Figure 4(a), random noise responses with a frequency characteristic shown in Figure 4(b) will be expanded as shown in Figure 4(c) according to the fluctuations in the noise input as shown in Figure 4(a). The designations (a), (b) and (c) of Figure 1(b) apply also to Figures 4(a)-4(c). Such random fluctuations of noise at the demodulator are very disconcerting to an operator and are thus undesirable.

0006It is thus an object of the invention to reduce random noise level fluctuation in a telecommunications system employing a control tone in the demodulation and recovery of a transmitted data signal.

0007It is another object of the present invention to reduce the noise fluctuation in a Lincompex communication system used in the transmission of speech or data signals.

0008These and other objects of the present invention are fulfilled by providing a method of reducing random noise level generation at a demodulator of a Lincompex system comprising the steps of determining a threshold level below which proper Lincompex demodulation does not occur, developing an artificial control tone signal having a level substantially at said threshold level and having a frequency effective to bias random noise level generation at the demodulator to a substantially constant minimum level, and injecting the artificial control tone signal into the demodulator to produce the constant minimum level at an output thereof in the absence of communication signal transmission from a transmitter.

0009Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

0010The present invention will become more fully understood from the detailed description given hereinbelow with reference to the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: <ul id="ul0001" list-style="none"><li>Figures 1(a) and 1(b) are block diagrams illustrating a prior art Lincompex compressor and expander circuit respectively;</li><li>Figure 2 is a block diagram of a demodulator circuit as one embodiment of the present invention in which random noise level fluctuation in the absence of communication signal transmission is reduced;</li><li>Figures 3(a) to 3(c) are charts illustrating the frequency band content of various signals at different points within the Lincompex system;</li><li>Figures 4(a) to 4(c) are charts illustrating the frequency content of random noise input to a Lincompex demodulator;</li><li>Figure 5 is a block diagram of a demodulator control tone detector according to a second embodiment of the present invention; and</li><li>Figure 6 is a block diagram of a third preferred embodiment of the present invention.</li></ul>

0011The present invention reduces the effect of random noise fluctuation in the demodulator of the receiving station of a Lincompex system during nontransmission of communication signals by injecting a small amount of artificial tone into the demodulation circuitry to produce quieting of the system. The frequency of this artificial tone should be such that it will produce biasing of noise toward a constant quiet level when no meaningful communication signals are being transmitted. The effective level of this artificial tone signal should be sufficiently small so that at Lincompex transmission threshold and above, (signal levels at which Lincompex demodulation is acceptable) there is no significant detrimental effect on the recovered signal, while the level should be large enough so that below the Lincompex transmission threshold, the biasing effect is produced so that the demodulator unit "self quiets".

0012As shown in Figure 2, in a first embodiment of the present invention, the artificial tone is transmitted locally at a low power level from an artificial control tone generator 60, which is added to the signal from the receiver 38 in a summer 62 to be transmitted to the remainder of the control tone conversion circuitry 44. Under circumstances when no speech or communication signal is being received by the receiver 38 or such signal is inadequate, the artificial tone injected into the control tone conversion circuit 44 has a signal level related to the output of the receiver, forcing the control tone conversion circuit to output an expansion factor signal which causes the expander 42 to attenuate random noise to a quiet condition.

0013Figure 5 illustrates a second preferred embodiment of the present invention wherein a signal-to-noise ratio computation network 64 is used to obtain an estimate of the signal-to-noise ratio of the demodulated control tone plus the artificial tone at an output of the control tone conversion circuit 44. The network 64 develops a signal indicative of the computed signal-to-noise ratio to adjust the amplitude of the injected artificial tone from the artificial tone generator 60 in order to maintain a predetermined minimal signal-to-noise ratio. This will achieve a similar biasing toward self quieting as the embodiment of Figure 2.

0014The network 64 can be implemented, for example, to measure the average power of the signal at the output of the conversion circuit 44. This can be performed in a number of ways, for example, by measuring the envelope of the control tone and dividing it into the average value of the control tone.

0015A third preferred embodiment of the invention is illustrated in Figure 6. In this embodiment, the control tone channel signal-to-noise ratio is detected by a detector circuit 66. The detector develops a signal representative of the signal-to-noise ratio of the control tone channel which is applied to the artificial tone generator 60 to vary the amplitude of the artificial tone in inverse proportion to the detected signal-to-noise ratio to accommodate various noise levels so as to dynamically control the reduction in noise fluctuation according to the actual random noise level input. The detector circuit 66 can be implemented to measure the average power of the signal at the output of bandpass filter 45 similar to the network 64 to obtain the mean noise power and consequently the signal to noise ratio of the control tone channel.

0016It is to be noted that although an artificial tone frequency is injected into the demodulator of the Lincompex system to achieve the effect of noise reduction, the same effect may also be obtained by applying another type of artificial variable during the subsequent demodulation processing. For example, since the control tone frequency in effect is a variable representative of the transmitted speech envelope, the apparent envelope may be artificially modified during subsequent processing to achieve an effect similar to the use of the artificial tone injection. For instance, the apparent envelope could be decreased nonlinearly at low signal-to-noise ratios.

0017Finally, since all Lincompex-type systems, whether they be analog, digital, or microprocessor based, use a "control tone" of some type, the features of the present invention will also be applicable to such systems.

0018The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, but all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.