Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J1/00—Frequency-division multiplex systems
  • H04J1/02—Details
  • H04J1/14—Arrangements providing for calling or supervisory signals
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J1/00—Frequency-division multiplex systems
  • H04J1/02—Details
  • H04J1/12—Arrangements for reducing cross-talk between channels

Definitions

• the present invention relates generally to communication systems and, more specifically, to SCPC satellite communication systems for transmitting an auxiliary signal such as a telegraph signal along with a voice signal on a common SCPC system channel.
• companding involves amplifying low signal levels to a greater degree than high signal levels thereby compressing the dynamic range of the output signal. The purpose of this is to place the transmitted signal within a linear range of the channel thereby avoiding or at least significantly minimizing non -linear distortion by the channel.
• An example of this would be 2/1 Db compression wherein if the input signal has a dynamic range of 2 to 10 Db's , the compressed signal will only have a range of 1 to 5 Db's.
• Expansion in the receiver then restores the original signal (e. g. in the above example back to 2 to 10 Db's) .
• Such arrangements are commonly used in voice transmission and typically give a significant gain to the voice signal when compared with systems wherein no companding takes place.
• the inventors have found through their experiments that significant degradation of a composite FDM signal having both voice signals and telegraph signals results if companding is applied to it.
• the inventors' experiments show that when strong discrete tone signals such as found in FSK-TTY are companded along with a void signal the strong TTY tone signals capture the operating point of the voice signal.
• the normal operating point for the companded voice signal is usually set for a 1000 Hz crossover point.
• the presence of strong individual discrete tones such as those found in FSK-TTY transmission, will have a very marked effect on the compander operating point for the voice signal.
• TTY test tone to noise ratio
• Another object of the present invention is to provide an improved SCPC system wherein a telegraph signal such as TTY is added to a voice signal by frequency division multiplexing the signals onto a common channel.
• Yet another object of the present invention is to reduce interference between telegraph and voice signals in an SCPC system wherein such signals are frequency division multiplexed together onto common individual channels .
• the present invention contemplates new methods of and apparatus for providing multiple service signals over a common channel.
• a processor is provided for receiving a first signal, such as a voice signal, and performing at least one processing operation on the first signal. Only after the first signal has been processed is an auxiliary signal added to it.
• the auxiliary signal has a frequency different than that of the first signal so that a composite FDM signal is formed containing the processed first signal and the auxiliary signal.
• the composite signal is then modulated with a common carrier and transmitted to a receiver.
• the received signal is split into the first signal and the auxiliary signal.
• the first signal is then processed in the receiver to restore the original first signal which was provided to the transmitter.
• FIG. 1 is a block diagram of the transmitter and receiver in accordance with the present invention.
• Figures 2A through 2D are waveform diagrams illustrating the waveform at various points in Figure 1;
• FIG. 3 shows a modification of the transmitter arrangement in accordance with the present invention.
• a communication system 10 including a transmitter 12 and a receiver 14 for transmitting a composite FDM signal having voice and TTY components over an SCPC communication link.
• the combination of voice and TTY signals are shown inasmuch as they serve as an excellent illustration of the particular advantages brought about by the present invention. However, other signals could be used, as will be discussed hereinafter.
• the original baseband voice signal A (see Figure 2A) is applied to a voice processor 16 for processing into a form which is well suited for transmission over the SCPC communication link.
• the frequency range for the voice signal is shown as being between 300 Hz and 3400 Hz, although obviously different ranges could be employed.
• a voice processor includes a compressor, as discussed previously. It is also common to use pre-emphasis circuits and echo suppression circuits in such a voice processor to further improve the overall system processing gain.
• a typical processor might include compression circuits for compressing at 2/1 Db's and pre-emphasis circuits to provide pre-emphasis of approximately 6 Db /octave.
• the compression circuit will give approximately an 11 D /b gain while the emphasis circuit will give approximately a 6 D/b gain for an overall 17 D /b system processing gain improvement.
• Voice processors for these purposes are well known in the art. For example, the Harris voice processor No. 2034 is well suited for such operations.
• the processed baseband output of the voice processor may have a form as shown , solely by way of example, as signal B in Figure 2B .
• This output is applied to one input of a summing circuit 18.
• the particular shape of the output wave B will depend on the particular type of processing which the original voice signal A is subjected to in the voice processor 16.
• the other input to the summing circuit 18 is a baseband 4-channel FDM-TTY signal C , such as shown in Figure 2C.
• This composite TTY signal is provided from 4 TTY keyers 20.
• Each TTY signal comprises a pair of FSK frequencies with one frequency representing a mark and one frequency representing a space, as is conventional.
• the frequency range in which these 4 FDM-TTY signals are shown is between 3400 Hz and 4600 Hz. Any particular individual frequencies may be used for the discrete FSK frequencies , provided that proper spacing is maintained between each individual frequency to ensure proper distinction in the receiver between a mark and a space signal, and non-interference between one pair of TTY signals and another.
• f 01 3540 Hz
• f 02 3660 Hz
• f 03 3780 Hz
• f 04 3900 Hz
• the voice signal and the TTY signals lie in different frequency ranges .
• some filtering must be applied at some stage to the voice signal prior to the actual addition to eliminate any high frequency components which might interfere with the TTY signals.
• the particular frequency ranges shown for the voice and TTY signals are exemplary only, and other suitable ranges could be readily used. Also, either a greater or lesser number of TTY signals could be used.
• Figure 2D reflects the fact that while the voice signal has been processed in the voice processor to change its waveform from that of the original voice signal A, the TTY signals have not been processed. In particular, the TTY signals have not been subjected to compression along with the voice signals . The significance of this is that it avoids the problem previously mentioned of the capturing of the operating point of the voice signals by the TTY signals if the voice signals were to be compressed with the TTY signals. Applicants found that avoiding such capture by combining the TTY signals with the voice after the voice sig ⁇ nal has been processed significantly reduces the interference between the TTY signals and the voice signals . At the same time, the advantages of improved overall processing gain due to the voice processing are still retained.
• the composite signal is passed through a modulator 22 where it is modulated in a conventional manner with a common RF carrier for transmission over a common channel of an SCPC satellite relay.
• This transmission over an SCPC satellite relay link is particularly attractive in light of the recent increased use of such relays for telephone use.
• the present invention could readily be used with other communication link arrangements not utilizing satellite relays (e . g. cable transmission) , if desired, and is not in any way limited to satellite usage .
• FIG 1 also shows receiver circuitry 14 for use in accordance with the present invention.
• An SCPC modulated composite signal having voice and TTY components is received by the circuit 14 and demodulated, in a conventional manner, in a demodulator 24 to again provide a composite baseband signal having a waveform approximately shown in Figure 2D.
• the signal D in the receiver will, in actual practice, differ somewhat from the signal D as transmitted by virtue of attenuation, distortion, noise and interference. But, notwithstanding these factors , the waveform D in the receiver should have the same general shape as that in the transmitter following demodulation, especially if proper voice processing has been used for the particular channel link in question.
• the baseband signal D is passed through a signal splitter 26 to separate the baseband signal D into a separate baseband voice signal B and a teletype signal C.
• the voice signal output of the signal splitter 26 still contains the effects of processing which was done in the voice processor 16 of the transmitter.
• the TTY output of the signal splitter 26 is fed to a highpass filter 28 while the voice output signal is fed to a lowpass filter 30.
• these filters one can attain excellent rejection on the order of 50 Db or better between the voice and TTY signals .
• the TTY signals C are ready for application to a conventional TTY printer (not shown) .
• the voice signal B still is in its processed form (i. e. the form to which it was modified in the voice processor in the transmitter) . Therefore, before this signal can be utilized, it must again be converted into the form of original baseband signal A. This conversion is accomplished in a voice processor 34 coupled to the output of the lowpass filter 30. This voice processor 34 performs a reverse operation relative to the transmitting voice processor 16.
• the receiver voice processor 34 typically includes an expander to expand the compressed waveform and a de-emphasis circuit to remove the pre-emphasis applied in the transmitter.
• the output signal A of the voice processor 34 is then in a form suitable for utilization with a telephone or other desired utilization circuit.
• TTNR test tone to noise ratio
• Figure 1 shows the use of a separate summing circuit 18 for combining the voice signal and TTY signals after voice processing
• the present invention could combine these signals in an output stage of the voice processor instead, as shown in Figure 3.
• the usual final amplifier in a conventional voice processor could be modified to receive the TTY input in addition to the processed voice signal.
• the final amplifier would act as a summing amplifier 36 to produce the signal D in the same manner as in the Figure 1 arrangement. Note that with this modification, the voice signal still would be processed but the TTY signals will not be even though they are fed to a stage of the voice processor.
• the present invention has been discussed primarily in terms of combining voice signals and TTY signals , it is to be understood that other signal combinations could be utilized which would obtain the basic beneficial results discussed above.
• the present invention will yield improved results in any composite FDM signal arrangement wherein a first signal must be subjected to processing which is unnecessary for the auxiliary signal, and where processing the composite signal will increase the interference between the first signal and the auxiliary signal.
• voice signals have been generally described above, it should be understood that other audio frequency signals could obviously be used.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Reduction Or Emphasis Of Bandwidth Of Signals (AREA)

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• 1981
  • 1981-06-24 JP JP50251981A patent/JPS57500957A/ja active Pending
  • 1981-06-24 EP EP19810902092 patent/EP0055293A1/de not_active Withdrawn
  • 1981-06-24 WO PCT/US1981/000868 patent/WO1982000074A1/en unknown

Non-Patent Citations (1)

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