GB2179813A - Delta modulation system - Google Patents

Abstract

In a transceiver for transmitting and receiving a delta modulated signal, an analogue signal, 5, to be transmitted is spectrally analysed, 13, and the spectral analysis is used to control, in a first mode, the transmission of the analogue signal after delta modulation, 6, or, in a second mode, in which the spectral analysis indicates a high proportion of high frequencies, the transmission of a frequency inverted version of the analogue signal after delta modulation, 6. The transceiver also includes a delta demodulator, 14, and means 22, responsive to the selected mode for selectively re- inverting the demodulated signal to regenerate the original analogue signal. <IMAGE>

Description

SPECIFICATION Delta modulation system This invention relates to delta modulation systems.

Delta modulation is a form of differential pulse code modulation in which a varying signal, for example, an analogue speech signal, is sampled at a relatively high rate, for example, at 16 kilobits/sec for analogue speech signals, and coded with a "one" when the sample is at a higher frequency than the preceding sample and with a "zero" when the sample is at a lower frequency than the preceding sample.

A full description of delta modulation is to be found in a book entitled "Delta Modulation Systems" by R. Steele published by Pentel Press, London in 1974.

One of the problems associated with delta modulation systems is that with known systems a high frequency sampling rate must be used for the undistorted modulation of signals which include rapid transients. System constraints restrict the maximum sampling rate and thus with known systems distortion or aliasing of signals which include large proportions of relatively high frequencies is sometimes inevitable.

It is an object of the present invention to provide a delta modulation system in which the aforementioned shortcoming is substantially minimised.

According to the present invention a delta modulation system comprises a local transceiver suitable for communication with a similar transceiver wherein the local transceiver comprises a transmitter including a delta modulator fed via first switch means, in a first mode, with an analogue modulating signal, or, in a second mode, with a frequency inverted version of the analogue modulating signal, the first switch means being arranged to select the mode in dependence upon the spectral contents of the modulating signal whereby that signal having the smallest proportion of signals above a predetermined frequency or within a predetermined higher frequency band, is applied to the delta modulator; and a receiver including a delta demodulator connected to a second switch means, adapted to operate synchronously with the first switch means of the transmitter of the similar transceiver, arranged feed the demodulated signals directly or via a frequency re-inversion means in accordance with the selected mode to an output terminal.

The transmitter may include an analogue speech signal generator arranged to produce an audio signal for transmission in the first mode, and a frequency changer operative to produce a frequency inverted version of the audio signal for transmission in the second mode, the switch means being arranged to be responsive to the spectral contents of the audio signal as determined by a spectrum analyser, for mode selection purposes.

In order to facilitate synchronisation of the first and second switch means and/or identification of the transmission mode, the transmitter may include means for including, during the transmission of one mode, a marker signal, and the receiver may correspondingly be arranged to include means for recognising the marker signal in a signal received from the transmitter of the similar transceiver and to control operation of the second switch means in dependence upon such recognition for synchronisation purposes.

The frequency changer means may comprise a mixer fed with a local oscillator signal and with the audio signal to produce the inverted version of the audio signal for second mode operation, a filter operative to select the side band output from the mixer corresponding to the inverted version of the audio frequency signal, and a marker signal generator operative to impress on the audio signal or on the inverted version of the audio signal a marker signal. The receiver is arranged correspondingly and includes a marker signal detector responsive to an output signal from the delta modulator for controlling operation of the second switch means whereby the demodulated received signal is fed to an output line either directly, or alternatively via a frequency re-inverter in accordance with the mode of the received signal as indicated by the marker signal.

One embodiment of the invention will now be described by way of example with reference to the accompanying drawing which is a somewhat schematic block diagram of a transceiver.

Referring now to the drawing, a transceiver arranged for communication with a similar remotely located transceiver (not shown) comprises a transmitter 1 arranged to radiate signals from an aerial 2 to the remotely located transceiver (not shown) and a receiver 3 fed with signals from the remotely located transceiver via an aerial 4. Audio frequency sig nals to be transmitted are produced by an audio signal generator 5 output signals from which are fed via a delta modulator 6 to the transmitter 1.

During conditions when the output signal from the audio signal generator 5 includes a high proportion of frequencies in the upper part of a pass band (which, for speech, extends from 300 Hz to 3.3 kHz) distortion or aliasing is liable to introduced if the sampling rate of the delta modulator is not fast enough.

Thus it will be appreciated that distortion is likely to occur if the audio frequency generator produces higher frequencies in the region of 3kHz for example. It will be apparent to those skilled in the art that if the frequencies pro duced by the audio generator 5 are changed from a first mode to a second mode by fre quency inversion in a frequency changer, then a 3 kHz signal in the first mode will be translated to 300 Hz in the second mode which can quite easily be handled by the delta modulator 6 operating on a relatively low sampling rate.

Thus it is arranged that signals from the audio generator 5 are fed, in the first mode, via a first switch 7 directly to the delta modulator when the spectral content of the modulating signal, provided by the audio generator 5, does not include a high proportion of high frequencies or, in the second mode, when the spectral contents of the signal does include a high proportion of high frequencies. In order to produce the second mode (frequency inverted) signal, the modulating signal from the audio generator 5 is fed via a frequency changer comprising a mixer 8 and a local oscillator 9 operating at 3.6 kHz, to the switch 7, whereby the second mode signal comprises a frequency inverted version of the signal produced by the audio generator 5.Thus the first mode signal is fed to the first switch 7 via a line 10 and the second mode signal is fed to the first switch 7 via a low pass filter 11 which operates to remove the upper side band output from the mixer 8. The first switch 7 is operated by means of a control signal applied thereto on a line 12 from a spectrum analyser 13 which is responsive to the spectral content of the signal on the line 10.

Signals from the first switch 7 are fed to the delta modulator 6 and the transmitter 1 for radiation from the aerial 2.

Corresponding signals from a remotely located similar transceiver (not shown) are received by the aerial 4 and fed to a delta demodulator 14 via the receiver 3. Output signals from the delta demodulator 14 are fed to a summing device 17 via a second control switch 15 either directly via a line 16, or indirectly via a frequency re-inverter comprising a mixer 18 and a local oscillator 19 operating at 3.6 kHz and via a low pass filter 20 to the summing device 17. The second switch 15 is synchronised with the first switch in the re motely located transceiver (not shown, but corresponding to the first switch 7) and, in order to facilitate this synchronisation, a tone marker signal is impressed on the transmitted signal during one mode of operation.Thus it will be appreciated that the tone may be ap plied to the signal fed to either pole of the first switch 7 but in the present example a marker tone generator 21 is provided which feeds a tone marker signal to the first switch 7 via the low pass filter 11. The tone might conveniently take the form of an "out-of band" frequency such as a 20 cycle/sec fre quency marker. A marker tone detector 22 is provided in the receiver responsive to detection of the tone for controlling operation of the switch 15. Although only the local transceiver has been described, it will be appreciated that the construction of the remotely located transceiver is substantially similar to the construction of the local transceiver.

It will be appreciated that the construction of the receiver, the transmitter and the delta modulator and demodulator may take any conventional form and these and other parts of the circuit are well known and will not be further described herein especially since they are not directly concerned with the present invention.

Claims (5)

1. A delta modulation system comprising a local transceiver, for communicating with a similar transceiver, having a transmitter including a delta modulator fed via first switch means, in a first mode, with an analogue modulating signal, or, in a second mode, with a frequency inverted version of the analogue modulating signal, the first switch means being arranged to select the mode in dependence upon the spectral output of the modulating signal whereby that signal having the smallest proportion of signals above a predetermined frequency or within a predetermined higher frequency band, is applied to the delta modulator, and a receiver including a delta demodulator connected to a second switch means, adapted to operate synchronously with the first switch means of the transmitter of the similar transceiver arranged to feed the demodulated signals directly or via a frequency re-inversion means in accordance with the selected mode to an output terminal.

2. A system as claimed in claim 1 wherein the transmitter includes an analogue speech signal generator arranged to produce an audio signal for transmission in the first mode, and a frequency changer operable to produce a frequency inverted version of the audio signal for transmission in the second mode, the switch means being responsive to the spectral content of the audio signal as determined by a spectrum analyser, for mode selection purposes.

3. A system, as claimed in claim 1 or 2 wherein the transmitter includes means for generating a marker signal, for transmission in one of the modes, and the receiver includes marker detection means for operating the sec ond switch means in dependence upon a detected marker signal.

4. A system as claimed in claim 2 or claims 2 and 3 wherein the frequency changer comprises a local oscillator, a mixer for mixing the output of the oscillator with the audio sig nal and a filter for producing an inverted ver sion of audio signal during the second mode.

5. A delta modulation system substantially as hereinbefore described with reference to and as illustrated in the accompanying draw ing.