LINE TRANSMISSION SYSTEMS

Abstract: In a local area PCM telephone network in which pulse coded speech signals are Walsh-code modulated and trans­ mitted in burst mode between a subscriber's apparatus and the exchange, the data rate may be doubled for a given line symbol rate by transmitting each Walsh-code symbol at one of two amplitude levels in dependence upon the values of the added data bits.

Description: The present invention relates to line transmission systems. In particular the invention is concerned with line transmission systems in which information to be signalled takes the form of one or more streams of binary digit values. Such systems include, for example, local area subscriber's pulse code modulation telephone links. According to the present invention in a line transmission system in which a first stream of binary digit values to be signalled over a path during respective ones of a succession of time intervals are represented in transmission by a square-wave signal having a predetermined period,. a binary one value being represented in transmission by said square-wave signal having a predetermined phase and a binary zero value being represented in transmission by said square-wave signal having the opposite phase to said predetermined phase, a second stream of binary digit values is arranged to be signalled over said path by means of changes in amplitude of said square-wave signal. Preferably each binary digit value is represented by substantially one cycle of a square-wave signal having a period substantially equal to one of said time intervals. A binary one value in said second stream of digit values may be represented by a relatively high square-wave signal amplitude and a binary. zero value by a relatively low square-wave signal amplitude Binary values in said second stream of binary values may be signalled during respective time intervals that are substantially coincident with respective ones of said succession of time intervals. Binary digit values in said first and second streams may be derived from a common source or from independent sources. A line transmission system in accordance with the present invention will now be described by way of. example with reference to the accompanying drawings, of which: Figures 1 to 3 show electric signal waveforms and coding tables illustrating the operation of the system, and Siguse 4 and Figures 5 and 6 show respectLve apparatus fer a tranznitter and a receiver of the system. In line transmission systems such as digital telephone systems analogue speech signals are presently amplitude-sampled once every 125 microseconds, that is, at a rate of 8 E Hz., and the amplitude samples are encoded for transmission in accordance with, say, the OOlTT A-law in which each sample is represented by an eight-bit code group. Teach analogue speech signal is therefore represented in transmission by a stream of binary digit values, or bits, at an average rate of 64 X bits/sec, and if two extra bits are added to each code group for signalling or control purposes the overall signal to be transmitted is a stream of bits at 80 X bits/sec. One method of transmitting this digital information over wire cables is to represent each binary digit value by a Walsh-code symbol W(+) of the appropriate polarity (or phase). As shown in Figure 1 of the drawing there are two Walsh-code symbols W(+) commonly used, designated Wal 1 and Wal 2, each comprising effectively one cycle of a square-wave carrier". The Wal 1 code symbol commences with a positive half cycle, whereas for the Wal 2 code symbol the square-wave is delayed by a quarter cycle so that the positive half cycle occurs during the middle portion of the respective binary digit period. In each case the binary value "1" is represented by the code symbol as shown while the binary digit value "O" is represented by the inverse of that symbol, in which the square-wave "carrier" has the opposite phase. The waveforms shown in Figure 2 of the drawing illustrate respectively a binary digit value sequence "1001" and the transmitted waveform utilising first Wal 1 and then Wal 2 line code symbols. It will be appreciated that these line codes, sometimes referred to as diphase signals, have no mean d.c. component and provide timing transitions independent of the bit value sequence, Wal 2 providing two such transitions per bit at a cost of increased high frequency energy content. Where better quality speech coding is required an analogue speech signal may be sampled at a rate of 16 K Hz, which with eight-bit coding and two signalling bits requires a signalling capacity of 144 X bits/sec. This would normally be provided by a corresponding increase in the line signalling rate, but since line attenuation increases with frequency this option is limited by the realisable performance of the detector circuitry. In the present transmission system the higherrate stream of digit values is split into two streams in which one stream, designated A in Figure 3 contains, say alternate speech sample code groups together with the signalling and control bits as in known systems, while the other stream,designated B, contains the interpolated extra code groups. The digit values in the A stream are then represented by the respective Walsh line code symbols, while the digit values of the B stream are arranged to determine the amplitude of the square-wave carrier, as shown by the line code waveforms and the line code table of Figure 3, where the binary digit value sequence "1001" from the A stream is shown combined with the sequence "1100" from the B stream. Preferably the B stream digit values are made up with extra bits to a rate of 80 X bits/sec, so that the amplitude modulation of the square wave carrier signal can be carried out in step with the Walsh symbol intervals. In this way the capacity of an existing line link can be doubled without significantly affecting the signal frequency spectrum. At the same time the multilevel code transmission will be compatible with the known systems, since the Walsh code symbols detected by a known receiver will represent the standard 64 K bits/sec plus signalling and control bits embodied in the A stream. A multilevel detector, however, will also detect the amplitude modulation to add the B stream bits to those of the A stream. If desired of course the A and B streams may represent signals from two independent sources, rather thaxi distributed signals from a common source. Referring now to Figure 4 in a trsnsmitter of the system the A stream of digit values and its inverse, each stream represented by a respective stream of Walsh code symbols, are applied respectively to the inputs of two pairs of tri-state inverting drivers or amplifiers 1 and 2, 3 and 4. In dependence upon the digit values of the B stream and its inverse, applied to OR gates 5 and 6, either the pair of drivers 1 and 2 are enabled, to provide an output current waveform of a given peak value through the resistors R and the primary winding of a line transformer 7, or the pair of drivers 3 and 4 are enabled to provide an output current waveform of one third of the above-mentioned given peak value through the resistors 3R and the primary winding of the transformer 7. It will be appreciated that the transmission of line symbols having peak values in the ratio 1:3 give rise to substantially equal received "eye openings" for the benefit of the receiver level detectors. As shown in Figure 5 the received line signals are applied by way of a differential amplifier 8 to a variable equaliser 9, this variable equaliser comprising for example, as shown in Figure 6, two common emitter transistor amplifier stages in cascade each having a variable capacitance diode in its respective emitter circuit. From the equaliser 9 the received signals are applied by way of a gain-controlled amplifier 10 and a further amplifier 11 to the inputs of three voltage level detectors 12, 13 and 14, and a peak rectifier circuit 15. The circuit 15 provides a voltage signal at its output which represents the peak level of the received signals, and this voltage signal is compared with a reference voltage V to derive, by way of an amplifier 16 and a network 17, a control voltage for the equaliser 9 and a control current for the gain-controlled amplifier 10. The effect of the gain control current on the gaincontrolled amplifier 10 is to tend to maintain the peak values of the received signals at the output of the amplifier 11 constant and substantially of the same magnitude as the reference voltage V. In dependence upon the transmitted line symbols therefore, at the mid-point of any one line symbol time interval the received signal waveform may have a value of substantially plus or minus V, or plus or minus 1/3 V. The voltage level detectors 12 and 14 compare the output signal level from the amplifier 11 respectively with plus 2/3V and minus 2/3V, signal levels of greater than these voltages being detected as a 11111 in the B stream. The detector 13 compares the output signal level from the amplifier 11 with 0 Volts, a positive signal level being detected as a "1" in the A stream. In local area service the eight-bit pulse code groups representing pulse coded speech samples, together with the two extra bits for signalling or control, herein referred to as extended code groups, are commonly transmitted between subscriber's apparatus and the local exchange in burst mode. In this mode one or more extended code groups are transmitted over a subscriber's line pair as a sequence of line code symbols in one direction followed, after a predetermined delay to allow for propagation time, by a similar number of extended code groups in the opposite direction. The exchange of bursts of pulses takes place at intervals of a corresponding multiple of the basic coding interval of 125 microseconds for 8 X Hz sampling or 62*5 microseconds for 16 X Hz sampling, For example at a 16 E Hz sampling rate a group of sixteen extended code groups each of ten bits may be assembled and trsnsmitted as a burst of eighty four-level line code symbols in each direction once every millisecond, the symbols being transmitted at a rate of, say, 192E Rz. In each one millisecond frame period therefore there is allowed a total of thirty-two line symbol periods for propagation or other delays, some of which may be utilised to transmit groups of line symbols as "headers" for the bursts for timing purposes. It will be appreciated that such long burst reauire correspondingly large buffer stores to hold the sixteen code groups as they are generated until the time comes in each frame for the burst to be transmitted, and correspondingly to hold the sixteen code groups received in reply while awaiting decoding or onward transmission. Shorter bursts, such as two-group or four-group bursts may therefore be preferred.

Claims: Claims 1. A line transmission system in which a first stream of binary digit values to be signalled over a path during respective ones of a succession of time intervals are represented in transmission by a square-wave signal having a predetermined period, a binary one value being represented in transmission by said square-wave signal having a predetermined phase and a binary zero value being represented in transmission by said square-wave signal having the opposite phase to said predetermined phase, wherein a second stream of binary digit values is arranged to be signalled over said path by means of changes in amplitude of said square-wave signal. 2. A line transmission system in accordance with Claim 1 wherein each binary digit value of said first stream is represented by substantially one cycle of a square-wave signal having a period substantially equal to one of said time intervals. 3. A line transmission system in accordance with Claim 1 or Claim 2 wherein a binary one valve in said second stream of digit values is represented by a relatively high square-wave signal amplitude and a binary zero value by a relatively low square-wave signal amplitude. 4. A line transmission system in accordance with Claim 1, Claim 2 or Claim 3 wherein binary values in said second stream of binary values are arranged to be signalled during respective time intervals that are substantially coincident with respective ones of said succession of time intervals. 5. A line transmission system in accordance with any one of Claims 1 to 4 wherein the binary digit values in said first and second streams derive from a common source.

Inventors: BYLANSKI, PETER
Applicants: THE GENERAL ELECTRIC COMPANY, P.L.C.
Application year: 1983
Main classification: H04L25
IPC codes: H04L25, H04L5, H04Q11