Companding system (lincompex) using pre- and de-emphasis

COMPANDING SYSTEM (LINCOMPEX) USING PRE- AND DE-EMPHASIS

0002Field of the Invention The present invention relates to the application of pre-emphasis and de-emphasis techniques to a linked compressor-expander (Lincompex) telecommunications system.

0003Background of the Invention

0004Lincompex systems are well known in the telecommuni- cations art. Such a digital Lincompex system is taught in U.S. Patent No. 4,271,1499 (the " '499 patent") issued June 2, 1981, to Leveque, the Inventor of the present application. The '499 patent only teaches the transmitting of a voice signal over a Lincompex system. However, it has also been found that Lincompex systems can be utilized to transmit data having a complex waveform. Such a system that overcomes the inherent problem of data and envelope overlap (i.e., complex waveform) is fully disclosed in U.S. Patent No. 4,907,217 (the " '217 patent") , also to Leveque, issued on March 6, 1990, entitled "System and Method of Transmitting A Complex Waveform over a Communication Channel Utilizing Lincompex Techniques" .

0005In the voice transmission system, as disclosed in the '499 patent, a voice signal is transmitted using Lincompex techniques. This voice signal is band limited and exhibits a control tone 4 (according to the teachings of the '499 patent) which is also band limited and does not overlap the voice band 2, (see FIG. 2(a)) .

0006Figs. 1(a) and 1(b) illustrates the Lincompex System of the '499 patent. In the modulator of FIG. 1(a) an input 20 of the Lincompex system modulator receives voice information for transmission. A control tone generator or envelope circuit 24 monitors the input voice signal. A compressor 22 compresses the introduced input voice signal. Compression is performed by dividing the signal by its envelope in pseudo-real time to produce a compressed voice signal. To develop the control tone, the system of FIG. 1(a) supplies the envelope signal developed from the output of the envelope detector 26 to a logarithmic (log) amplifier 28 which then develops a signal representative of the logarithm of the envelope signal . The output of this logarithmic amplifier 28 is supplied to a control terminal of a voltage control FM oscillator 30, which generates a frequency that varies about a center frequency F_c in relation to the variation of the input voltage supplied to its control terminal from the logarithmic amplifier 28, to develop an envelope signal as an output of the control tone generator (envelope circuit) 24.

0007A summer 32 then sums the compressed voice signal developed at the output of the compressor 22 with the control tone developed at the output of the FM oscillator 30 to form a combined information signal.

0008In this speech transmission system, this combined information signal output from the summer 32 is provided to a transmitter 34 which transmits the signal over a desired transmission medium 36. In a typical embodiment, a single side band transmitter would normally transmit the modulated combined information signal across the airways in a known manner. As illustrated in FIG. 1 (b) , the prior art Lincompex demodulator includes a receiving unit 15 which receives a modulated combined information signal from the transmission medium 36, which normally includes an antenna for receiving radio waves from the atmosphere, and supplies the received modulated combined information signal to a receiver 38. The receiver 38 demodulates the transmitted signal to produce a demodulated version of the combined information signal. Typically, this receiver 38 would be a single side band receiver which mixes the received modulated combined information signal with the channel frequency to produce a base band combined information signal.

0009To recover only the voice signal from such a combined voice/envelope information signal, a low pass filter 40 removes the voice information 2 of FIG. 2(a) from the combined information signal to recover the compressed voice signal containing only the voice information 2. This compressed voice signal is transmitted according to the Lincompex techniques at a substantially complete modulation of the transmitter 34 of Fig. 1(a) . The compressed voice signal is passed through a fading regulator 58. The fading regulator 58 is a fading regulator similar to the one described in the '499 patent which removes any audio level variations not removed by the automatic gain control circuitry in the receiver 38. This compressed modified voice signal output from the fading regulator 58 is then expanded to produce the necessary dynamic range for the recovered voice signal to be supplied at the output 60.

0010Accordingly, an expander 42, similar to the one described in the '499 patent, is utilized which essentially multiplies the compressed modified voice signal developed at the output of the fading regulator 58 by an envelope signal which is recovered by a control tone conversion circuit 27.

0011The control tone conversion circuit 27 comprises a band pass filter 46 which recovers only the control tone which is represented by the control tone information signal 4 in FIG. 2(a) . The control tone originally developed by the FM oscillator 30 of FIG. 1(a) is then passed through a frequency discriminator. The frequency discriminator 48 measures the instantaneous frequency of the control tone and produces a voltage level representative of this measured frequency. In this case, the voltage level also represents the logarithm of the original envelope signal . The anti-logarithm circuit 50 is utilized to recover the original envelope signal. This logarithmic signal is then supplied to an anti-logarithm circuit 50. The anti- logarithm circuit 50 is utilized to recover the original envelope developed by the envelope detector 26 of FIG. 1(a) . This original envelope signal is used by the expander 42 to recover the original voice signal by expanding the compressed voice signal to provide the original signal to the output 60. This control tone conversion circuit 27 is similar to the control tone conversion circuit described in the '499 patent.

0012With respect to the transmission of data, the Lincompex techniques used may or may not be different from the transmission of voice signals due to the complex nature of the data waveform. This technique is fully discussed in the '217 patent. The frequency compensation technique, however, will be briefly described below.

0013As described in the '217 patent, a data signal is transmitted using Lincompex techniques. The problem associated with transmission of such data signal arises from the frequency band overlap occurring within such a complex waveform as shown in FIG. 2(b) where the data signal overlaps the control tone of the data signal. FIGS. 3(a) and 3(b) illustrate, collectively, a Lincompex System where the input signal is frequency shifted prior to the compression/expansion operation to enable transmission of a wideband complex waveform using Lincompex techniques, and the received digital signal is frequency compensated to account for frequency drift. In

0014FIGS. 3(a) and 3(b) , as in all of the figures of the present invention, like elements throughout the drawing figures are identified with like numbers.

0015Whereas the FIG. 1(a) prior art Lincompex system normally receives speech - or voice at input 20, the embodiment of FIG. 3 (a) normally receives a data input such as a 16-tone parallel-tone or multi-tone data signal as illustrated in FIG. 4(a) at its input 20. However, it should be understood that any complex data waveform including voice may be transmitted over a communications channel.

0016A mixer 100 is provided to frequency shift the input data frequency spectra shown in FIG. 4(a) , to a desired higher frequency F_osc to ensure that no overlap, as shown in Fig. 2(b) , between the input data band, when compressed, and the control tone signal frequency band occurs. Accordingly, an oscillator 102 supplies the mixing frequency F_osc to a frequency multiplier or mixer 104 where it is mixed with the data input A of FIG. 4(a) to produce the mixed data B of FIG. 4 (b) . A filter 106 is then provided to band pass filter the mixed output to remove an undesired one of the two side bands produced by the mixing process. The frequency response of the filter 106 is illustrated in FIG. 4(c) . After filtering, only a single side band is left which 35 is frequency shifted to a desired frequency at the output D of the filter 106 as illustrated in FIG. 4 (d) . This frequency shifted data input of FIG. 4 (d) is then treated by prior art Lincompex techniques by a compressor 22, envelope circuit 24, summer 32 and transmitter 34, in the manner described with respect to FIG. 1(a) to produce a combined data and envelope signal for transmission on the transmission medium 36. As explained previously, the compression operation enlarges the frequency spectrum of the compressed data as shown in FIG. 4(f) . The envelope circuit 24 generates an envelope spectrum E which would have a selected bandwidth depending on various factors including the accuracy of compression to be achieved. However, it is important that the bandwidth of the envelope spectrum not overlap that of the compressed data so that full compression and recovery of the data can be accomplished by the Lincompex system. FIG. 4(b) illustrates a prior art Lincompex demodulator. The demodulator of FIG. 4(b) , differs from that of Fig. 1(b) in that frequency shifted data is obtained from expander 42. This frequency shifted data may be provided via a line to an output where it may be detected by a detector sensitive to the frequency shifted tones contained within the frequency shifted data. Alternatively, as shown in Fig. 1(b) , a receiver mixer 110 including a multiplier 108, frequency oscillator 112 and filter 114 may be utilized to shift the frequency shifted data back to its original frequency band. Accordingly, the digital information may be readily recovered.

0017The above described Lincompex systems have been described with respect to single side band transmission and reception. However, it has also been found that Lincompex signals can be transmitted and received utilizing frequency modulation. Such a system is fully disclosed in U.S. Patent No. 5,058,202 (the " '202 patent")^', also to Leveque, issued on October 15, 1991, entitled "System and Method of Transmitting And Receiving A Lincompex Modulated Signal over A Communication Channel Utilizing Frequency Modulation Techniques" .

0018Traditionally, pre-emphasis and de-emphasis have been used in wideband FM systems. In FM systems, the noise power at the output of a FM discriminator increases as the square of the modulating frequency. To prevent the higher modulating frequencies from suffering disproportionate signal-to-noise degradation, pre-emphasis increases the amplitude of these frequencies before transmission so that all modulating frequencies could have the same signal-to- noise ratio at the discriminator output. Then, de-emphasis reduces the amplitude of the higher frequencies at the discriminator output to prevent distortion (i.e., non-flat frequency response) .

0019Pre-emphasis and de-emphasis have not been applies to narrowband FM and amplitude modulation systems because (1) all existing radio equipment would require modification, (2) increasing the higher modulating frequencies would increase adjacent channel interference, and (3) increasing the modulation factor of higher modulating frequencies can require more precise receiving detectors. Consequently, pre-emphasis/de-emphasis with Lincompex has never been considered because Lincompex, not very common in telecommunication systems, has traditionally been applied to only AM/SSB systems.

0020SUMMARY OF THE INVENTION An object of the present invention is to provide a system and method for transmitting and receiving a Lincompex modulated signal over a communications channel utilizing pre-emphasis and de-emphasis techniques which overcome the disadvantages discussed above. A further objective of the present invention is to reduce noise in a Lincompex modulated signal .

0021These and other related objectives are achieved by providing a method for transmitting and receiving an 5 information waveform over a communication channel, comprising the steps of:

0022(a) amplifying high frequency components of an information waveform;

0023(b) transmitting output of said step (a) over a communication channel according to Lincompex techniques; (c) receiving output of said step (b) from said communication channel according to Lincompex techniques; and

0024(d) attenuating said high frequency components of output from said step (c) . The objectives of the present invention are further achieved by an apparatus for transmitting and receiving an information waveform over a communication channel, comprising: means for amplifying high frequency components of an information waveform; a Lincompex modulator transmitting output of said pre- emphasis circuit over a communication channel 25 according to Lincompex techniques; a Lincompex demodulator receiving output of said Lincompex modulator from said communication channel according to Lincompex techniques; and means for attenuating said high frequency components of output from said Lincompex demodulator. These and other related objectives of the present invention will become more readily apparent from the detailed description given hereinafter. It should, however, be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of changes and illustration only, since various modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

0025BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description hereinbelow and the accompanying drawings which are given by way of illustration only, and thus do not limit the present invention. FIGS. 1(a) and 1(b) illustrate a prior art Lincompex modulator and demodulator;

0026FIGS. 2(a) and 2(b) illustrate typical relationships between the frequency band of typical human voice and the control tone of this voice, and that which may be encountered by typical data and the frequency band of this data's control tone;

0027FIGS. 3(a) and 3(b) illustrate a prior art modulator and demodulator; FIG. 4 (a) -4 (f) illustrate the frequency spectrum of the data signal transmitted using the prior art Lincompex techniques of Fig. 3 (a) ;

0028Fig. 5 illustrates the system f or transmitting and receiving a Lincompex modulated signal over a communi- cations channel utilizing pre-emphasis and de-emphasis according to the present invention;

0029Figs. 6(a) and 6(b) illustrate an example of the pre- emphasis and de-emphasis circuits; and

0030Figs. 7(a) and 7(b) illustrate examples of the 30 transfer functions for pre-emphasis and de-emphasis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0032Fig. 5 illustrates the system for transmitting and receiving a Lincompex modulated signal over a communications channel utilizing pre-emphasis and de- emphasis according to the present invention. In the system of Fig. 5, a pre-emphasis circuit 200 receives and emphasizes an information waveform. A Lincompex modulator 202 is connected to the pre-emphasis circuit 200, and receives the output therefrom. The Lincompex modulator can be any prior art Lincompex modulator, for example, such as illustrated in Figs. 1(a) and 3(a) .

0033The Lincompex modulator 202 modulates the emphasized information waveform and transmits a modulated information waveform on transmission medium 36. The transmission medium 36 utilized in the present invention may be any type of transmission medium for example, an atmospheric radio wavelength, a telephone line, coaxial cable or fiber optic cable, or any other path usable for the transmission of the selected combination information waveform such as voice or data. Also, the components of the information waveform can be sent via separate transmission mediums.

0034A Lincompex demodulator 206 receives and demodulates the modulated information waveform from transmission medium 36. The Lincompex demodulator 206 can be any prior art Lincompex demodulator, for example, such as illustrated in Figs. 1(b) and 3(b) . A de-emphasis circuit 208 is connected to the Lincompex demodulator 206 and de- emphasizes the output thereof. The pre-emphasis circuit 200 embodies any method or means for amplifying the high frequency components of the information waveform. For instance, the embodiment of the pre-emphasis circuit 200 may have a transfer function as shown in Fig. 7(a) . As shown in Fig. 7(a) the pre-emphasis circuit does not affect the low frequency components of the information waveform, the frequency components of the information waveform up to the roll-up point f_PR. The pre- emphasis circuit 200, however, increasingly amplifies high frequency components of the information waveform, the frequency components above the roll-up point f_PR.

0035Fig. 6(a) illustrates a simple circuit embodiment of pre-emphasis circuit 200. More complex pre-emphasis circuit embodiments are well known, as are software implementations. The de-emphasis circuit 208 embodies any method or means for attenuating the high frequency components of the information waveform. For instance, the embodiment of the de-emphasis circuit 208 may have a transfer function as shown in Fig. 7(b) . As shown in Fig. 7(b) the pre-emphasis circuit does not affect the low frequency components of the information waveform, the frequency components of the information waveform up to the roll-down point f_PD. The pre-emphasis circuit 200, however, increasingly attenuates high frequency components of the information waveform, the frequency components above the roll-up point f_PD . Fig. 6(b) illustrates a simple circuit embodiment of de- emphasis circuit 208. More complex de-emphasis circuit embodiments are well known, as are software implemen¬ tations. The pre-emphasis and de-emphasis circuits 200, 208 can have transfer functions such that the cascade freqency response combination of the pre-emphasis and de-emphasis circuits 200, 208 is unity. Alternatively, the transfer function of the pre-emphasis circuit 200 does not need to be the inverse of the de-emphasis circuit 208. For instance, the de-emphasis circuit 208 can have a greater roll-down or the pre-emphasis circuit 200 can have a greater roll-up.

0036By incorporating the pre-emphasis and de-emphasis circuits 200, 208 as part of the Lincompex device, existing radio equipment is not impacted. Furthermore, since the Lincompex device contains all required filtering to band limit the modulating waveform spectrum, adjacent channel interference does not become a problem. Also, in practice, the inventor has not found the need for more precise receiving detectors.

0037Furthermore, the pre-emphasis and/or de-emphasis can be tailored to the characteristics of the compressor and/or expander of the Lincompex system to find an optimal pre- emphasis/de-emphasis which takes into account any reproduction limitations or peculiarities of the Lincompex compressor and/or expander. Lincompex modulator/ demodulator operation is set by standards (e.g., standards regarding attack times, release times, compression ranges, expansion ranges,, control tone, slew rates, etc.) . Too much pre-emphasis can be applied so that the Lincompex system will not properly develop the envelope of the information waveform. This results in distortion after demodulation and de-emphasis. Thus, based on de-emphasis output, the pre-emphasis and de-emphasis can be modified to achieve maximum undistorted benefit . As a further alternative, the pre-emphasis and de- emphasis circuits can be used with a Lincompex modulator and non-Lincompex receiver to provide a more articulate receive signal than compression alone. A similar variation of the present invention applies to an autobypass operation. As described in the '499 patent, the autobypass operation is where the demodulator switches out of Lincompex demodulation due to a low signal-to-noise ratio. Accordingly, the output signal is a Lincompex compressed information waveform, the control tone having been automatically removed by the demodulator. The compressed information waveform output during a bypass operation has higher articulation due to pre-emphasis. De- emphasis may or may not be applied to this information waveform. Additionally, application of de-emphasis could be made user selective.

OPERATION OF THE PRESENT INVENTION

0039In operation, the pre-emphasis circuit 200 receives and emphasizes an information waveform (voice or digital) . The Lincompex modulator 202 modulates the emphasized infor¬ mation waveform and transmits the modulated information waveform on the transmission medium 36. The Lincompex demodulator 206 receives the modulated information waveform from the transmission medium 36 and demodulates it. The de-emphasis circuit 208 de-emphasizes and outputs the demodulated information waveform.

0040The present invention provides unexpected improvements in the area of noise reduction as compared to that achieved by a Lincompex system alone or a non-Lincompex system using pre-emphasis and de-emphasis. For example, a 7dB increase by pre-emphasis at a given frequency and a subsequent 7dB decrease from de-emphasis at the given frequency produced a 7dB reduction of interfering signals at the given frequency. In a Lincompex system, the noise across the link is f lat and is slaved to the envelope of the information waveform, i.e., the information waveform is transmitted at substantially full modulation. Using pre-emphasis and de- emphasis with the Lincompex system significantly reduces the noise power in the higher frequency components of the information waveform. For instance, , during periods of no speech, the Lincompex demodulator provides no output. Consequently, during these silent periods the pre-emphasis and de-emphasis are unused. Since pre-emphasis and de- emphasis only apply during words/syllables/tones, the advantages/effects of pre-emphasis and de-emphasis are slaved to the original information waveform.

0041Therefore, Lincompex allows selective application of pre-emphasis and de-emphasis to further reduce inter¬ ference. For example, Lincompex slaves the one voice heard to a particular speaker (envelope of the voice heard) ; thus, pre-emphasis and de-emphasis will reduce interfering speakers and noise. Only the speech of the particular speaker is returned to normal (i.e. flat frequency characteristic) by the pre-emphasis and de-emphasis attenuation operation.

0042Besides the objectively measured advantages, the combination of pre-emphasis/de-emphasis with Lincompex provides unquantifiable, unexpected subjective advantages.

0043For example, the effect of changing the noise present during a desired word/syllable helps a human listener because the noise is "tagged" differently (by rolling off its higher frequencies) than would be expected or anticipated by the listener. This helps the listener identify the desired voice and reject the undesired noise or interfering speech.

0044The 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 departure from the spirit of scope of the invention, and all such modifications would be obvious to one skilled in the art are intended to be included within the scope of the following claims: