SINGLE CHANNEL AUDIO SIGNAL TRANSMISSION SYSTEM

• US 3,684,838 A
• Filed: 03/15/1971
• Issued: 08/15/1972
• Est. Priority Date: 06/26/1968
• Status: Expired due to Term

• Current Assignee: Leonard R. Kahn
• Original Assignee: Leonard R. Kahn
• Inventors: Kahn, Leonard R.
• Primary Examiner(s): Richardson, Robert L.
• Application Number:
  US05/124,406
• Time in Patent Office: 519 Days
• Field of Search: 325/59,61,32,49 343/200 332/41 179/15.55R
• US Class Current: 704/227
• CPC Class Codes: H04B 1/66 for reducing bandwidth of signals; for improving efficiency of transmission H04B1/68 takes precedence

Abstract

A system and method for transmitting relatively high fidelity audio signals (e.g., speech and music signals having frequencies from about 100 cps to about 3,000 cps or higher) across an audio signal transmission channel characterized by a passband which effectively transmits only a portion of the frequency components making up the audio signals (e.g., a telephone line with a passband of about 300-3,000 cps or higher, or a radio frequency carrier wave restricted to a comparable low fidelity signal modulation, for example). In the preferred embodiment of the invention, a relatively narrow-sub-band of frequencies are blocked from the frequency spectrum of the signal to be transmitted in a range (e.g., about 2.1 - 2.275 kcs) where the effect of such blocking cannot be perceived by a listener at the receiving station. The signal components in a lower frequency range (e.g., about 100 - 275 cycles) which have a significant effect on the naturalness of the audio signal ultimately heard at the receiving station are translated in frequency to derive a frequency-displaced signal falling within the frequency range of the blocked sub-band, and combined with the remainder of the audio signal. The resultant signal is then transmitted across the relatively low fidelity transmission channel. At the receiving station the frequency-displaced portion of the signal is isolated from the remainder of the transmitted signal, restored to its original frequency range and re-combined with the remainder of the signal to substantially re-establish the original signal, less only the insignificant blocked frequency sub-band. In a modified embodiment of the invention, substantially the entire audio signal is translated upwardly in frequency by an increment of 200 cps, for example, to derive a frequencydisplaced signal prior to transmission of the signal. All but the highest frequency components of the translated signal thus fall within the passband of the relatively low fidelity transmission channel. Such frequency-displaced signal is then transmitted across the low fidelity channel and is restored to its original frequencies at the receiving station.

Claims

1. A communication system providing a relatively high fidelity audio signal and an audio signal transmission channel characterized by a pass band which is narrower than and respectively transmits only a portion of the frequency components making up such relatively high fidelity audio signal, signal processing means for processing the relatively high fidelity audio signal components and improving the fidelity thereof as utilized in receiver means receiving an audio signal input only from such transmission channel, said signal processing means comprising:

means for eliminating a narrow sub-band of frequencies from the audio signal;

means translating low frequencies of such audio signals which do not fall within said pass band to obtain a frequency-displaced signal having frequency components which fall within a relatively narrow sub-band of less than 500 cps;

means applying such frequency-displaced signal to the relatively low fidelity transmission channel at the portion of the eliminated sub-band;

means receiving such frequency-displaced signal;

means for restoring such frequency-displaced signal to its original frequency range; and

a summation circuit for adding the stored frequency-displaced signal to the receiver means.

2. A communications system according to claim 1, wherein the range of frequencies blocked from the relatively low fidelity transmission channel is somewhat greater than the range of low frequencies translated in frequency to fall within the blocked sub-band frequencies.

3. A communications system according to claim 2, wherein the blocked sub-band of frequencies is in the upper midrange portion of the relatively low frequency audio signal.

4. A communication system providing a relatively high fidelity audio signal and an audio signal transmisstion channel characterized by a pass band which is narrower than and respectively transmits only a portion of the frequency components making up such relatively high fidelity audio signal components and improving the fidelity thereof as utilized in receiver means receiving and audio signal input only from such transmission channel, said signal processing means comprising:

a band rejection filter connected in series with the transmisstion channel for eliminating a narrow sub-band of frequencies therefrom;

means translating low frequencies of such high fidelity audio signals which do not fall within said pass band to obtain a frequency-displaced signal having frequency components which fall within a relatively narrow sub-band of less than 500 cps;

means applying such frequency-displaced signal to the relatively low fidelity transmission channel at the portion of the narrow sub-band;

means receiving such frequency-displaced signal;

means including a demodulator coupled to an oscillator for restoring such frequency-displaced signal to its original frequency range; and

a summation circuit for adding the restored frequency-displaced signal to the receiver means.

5. A communications system according to claim 4, wherein the frequencies blocked by the rejection filter are greater than 1.5 kilocycles per second.

6. A method of transmitting a high fidelity audio signal over a low fidelity transmission line which is narrower than the high fidelity audio signal which includes the steps of:

a. attenuating a first band of frequencies from the audio signal, said first band lying withiN the limits of the frequencies passed by the low fidelity line;

b. separating a second band of low frequencies from the signal, said second band lying below the lowest frequency passed by the low fidelity line;

c. transferring the second band of frequencies to said first band by modulation means; and

d. applying the signal minus the first and second bands of frequencies plus the transferred frequencies to the low fidelity transmitting line.

7. A method as claimed in claim 6 wherein the transmitted signal is transformed into a high fidelity signal at the other end of the line by the steps which include:

a. applying the received signal minus the first band of frequencies to a summation circuit;

b. separating the transferred frequencies from the received signal;

c. demodulating the transferred frequencies to produce the original second band of frequencies; and

d. applying the second band of frequencies to the summation circuit.