GB2192116A - Telephone apparatus - Google Patents

Abstract

In a P.C.M. telephone set in which a custom integrated circuit 1 permits P.C.M. digit values to be modified by an external processor 3, voice switching is achieved by modifying the digit values of one or other P.C.M. stream to obtain the necessary attenuation. <IMAGE>

Description

SPECIFICATION Telephone Apparatus The present invention relates to telephone apparatus.

According to one aspect of the present invention in telephone apparatus in which a succession of speech signal samples are arranged to be digitally encoded for transmission as a corresponding succession of digital code groups there are provided means selectively digitally to modify the digital code groups in respect of some at least of the speech signal samples to effect amplification or attenuation of the corresponding speech signal.

According to another aspect of the present invention in a telephone system in which speech signals are periodically amplitude-sampled and the sample values digitally encoded for transmission over the system as a succession of digital code groups, telephone apparatus comprises means to compare the digital code groups in respect of transmitted and received speech signal samples and means responsive to the resuit of this comparison selectively digitally to modify the digital code groups in respect of some at least of the sample values to effect attenuation of the corresponding speech signal.

The telephone apparatus may be a loudspeaking telephone, and the attenuation may be utilised to effect voice-switching of transmitted and received speech signals.

According to another aspect of the present invention in a P.C.M. telephone system in which at least one telephone set of the system utilises a custom integrated circuit which allows a P.C.M.

stream of digit values to be read and modified by an external processor, voice-switching at said telephone set is effected in said processor by comparing the average levels of the transmitted and received speech signals represented by the respective P.C.M. streams of digit values and selectively digitally modifying the P.C.M. digit values in one of the streams to effect the required attenuation of the corresponding speech signal.

The pulse code groups making up each P.C.M.

stream may be converted to logarithmic code to facilitate said digital modification.

Telephone apparatus in accordance with the present invention will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 shows the apparatus schematically, and Figure 2 shows schematically a model for the apparatus of Figure 1.

Referring to the drawings the telephone apparatus is arranged to act as a loudspeaking telephone by taking advantage of the use of a custom integrated circuit which now forms the major component of a digital telephone set. The integrated circuit 1 allows the pulse code and modulated (P.C.M.) steam of digit values passing to and from a code 2 to be read and modified by an external processor 3. In this way a loudspeaking telephone function can be provided cheaply without using analogue components apart from the codec, which is already provided in a digital telephone set.

The integrated circuit 1 is arranged to transmit and receive P.C.M. coded samples in burst mode, with two bytes each of eight bit values being transmitted in either direction during each of a succession of 250 ,us frame periods. Extra bits may be added to provide a signalling channel, which may be used to control peripherals connected to the integrated circuit.

The two bytes in either direction in any one frame period are held over for one frame period before being passed on, either to line or to the codec 2.

During this frame period the microprocessor 3 can read the bytes and modify them as required.

The microprocessor receives an interrupt from the integrated circuit 1 every 250 ps and then reads the P.C.M. bytes which have been received from the line and from the codec 2. Using a look-up table the Alaw-encoded bytes are then converted to a true logencoded form before processing continues.

A software single pole digital filter is then applied to both channels to compute the average level of each (filters 1 and 2 of Figure 2). Another single pole filter is used (filter 3 of Figure 2) for the purpose background noise compensation.

On a voice-switched loudspeaking telephone stability is achieved by attenuating the speech channel in one direction while the other is being used (taking advantage of the fact that both parties rarely speak simultaneously). Thus the software compares the average levels of the speech channels to decide which is the greater, and then attenuates the other by subtracting a fixed amount from every received byte. This translates directly into an attenuation by a number of dBs since the speech is now in a true log-encoded form. A look-up table is then used to convert the log-encoded speech bytes back into A-law-encoded P.C.M., before they are written back to the integrated circuit 1.

Volume control of the received speech from the line is also performed by attenuation as described above.

The loudspeaking telephone can also be placed in a state where there is a medium amount of attenuation used in each channel (idle) if the transmit channel level is the greater, and the background noise compensation software indicates that speech is not present. The background noise compensation software works by comparing the filtered transmitted speech level with the biassed output of a trough detector piece of software which follows the troughs in speech very quickly but only rises towards the peaks slowly. Thus, since speech has regular gaps in it the filter will indicate that it is speech, but a constant level of background noise will cause the trough detector output to rise to a point where it is close to the actual filtered speech level (within the bias value) and then the comparator will switch and indicate that it is merely background noise.

The microprocessor also uses a signalling channel for its control, i.e. loudspeaking telephone on/off, volume, mute on/off etc. The signalling channel can also be used to alter parameters used within the loudspeaking telephone software such as switching depth, and the offsets that are used to weight each channel's level before being used for comparison. Thus, the loudspeaking telephone can be used on exchanges with different transmission levels without needing any alterations to the masked program or any hardware adjustments.

Claims (6)

1. Telephone apparatus in which a succession of speech signal samples are arranged to be digitally encoded for transmission as a corresponding succession of digital code groups, wherein there are provided means selectively digitally to modify the digital code groups in respect of some at least of the speech signal samples to effect amplification or attenuation of the corresponding speech signal.

2. Atelephone system in which speech signals are periodically amplitude-sampled and the sample values digitally encoded for transmission over the system as a succession of digital code groups, wherein telephone apparatus comprises means to compare the digital code groups in respect of transmitted and received speech signal samples and means responsive to the result of this comparison selectively digitally to modify the digital code groups in respect of some at least of the sample values to effect attenuation of the corresponding speech signal.

3. A telephone system in accordance with Claim 2 wherein the telephone apparatus is a loudspeaking telephone, and the attenuation is utilized to effect voice-switching of transmitted and received speech signals.

4. A P.C.M. telephone system in which at least one telephone set of the system utilises a custom integrated circuit which allows a P.C.M. stream of digit values to be read and modified by an external processor, wherein voice-switching at said telephone set is effected in said processor by comparing the average levels of the transmitted and received speech signals represented by the respective P.C.M. streams of digit values and selectively digitally modifying the P.C.M. digit values in one of the streams to effect the required attenuation of the corresponding speech signal.

5. A P.C.M. telephone system in accordance with Claim 4 wherein the pulse code groups making up each P.C.M. stream are converted to iogarithmic code to faclitate said digital modification.

6. Telephone apparatus substantially as hereinbefore described with reference to the accompanying drawings.