Feasibility of using digital filters to interconnect p.c.m. and f.d.m. transmission systems

A time-division multiplex (t.d.m.) and a frequency-division multiple (f.d.m.) system may be interconnected by means of gates and lowpass filters. The use of digital filters (d.f.s) for this purpose appears attractive when the t.d.m. system uses pulse-code modulation (p.c.m.), because digital-analogue and analogue-digital conversion are unnecessary at the interface between the d.f. and the p.c.m. system. The problems involved in applying d.f.s in this application are discussed. It is concluded that the interconnection of p.c.m. systems with f.d.m. systems using quite low carrier frequencies requires the development of digital circuits operating at several hundred megahertz.     

Upper bound on signal/quantising-noise ratio for television d.p.c.m. systems

Consideration is given to an upper bound on signal/quantising-noise ratio for television d.p.c.m. systems. The 7.2 dB gain for entropy coding assumes ? entropy of the bit stream of the quantiser output. The calculations are based on Laplacian signals, because television signals at d.p.c.m. quantiser inputs are approximately Laplacian.     

Reducing quantising noise in p.c.m. coders

An analogue-feedback method has been developed to reduce quantising noise in p.c.m. systems. The improvement in signal/noise ratio, however, depends on the loop delay, saturation limit of the coder and the number of digits used. The overall characteristics of the feedback p.c.m. systems have been found to be better than those of the conventional p.c.m. for bit rates up to 50 × 103 bit/s.     

Hybrid p.c.m.?an experimental video system

Hybrid pulse-code modulation (h.p.c.m.) is a variant o. multilevel p.c.m., in which the least significant digit(s) is made to describe the quantising noise exactly. A theoretical and practical feasibility study has been made of the application of h.p.c.m. to the transmission of 625-line Pal colour signals over low-quality narrowband circuits.     

Detection and partial correction of isolated errors from the received data in a 1st-order d.p.c.m. codec

A detection and partial-correction (d.p.c.) system located at the receiver in a 1st-order d.p.c.m. system enables isolated transmission errors that would cause gross distortion in the recovered signal of a conventional d.p.c.m. decoder to be corrected from the received data, providing that the input signal does not exhibit a rapid change in its slope during a sampling period. The d.p.c. system is disabled when this occurs.     

P.C.M. A-law decoder using a circulating method

This letter describes an algorithm for p.c.m. A-law decoding and a simple realisation based on the use of a circulating method. Results of initial measurements fall within the required p.c.m. specifications for speech.     

Simple delay-line p.c.m. coder

A simple p.c.m. coder, using a delay line as the coding network and having signal/noise ratio and other characteristics similar to conventional p.c.m. systems, has been developed. The coder can easily be extended to ternary and multilevel working, and the reconstructed message, available at the transmitter, may be used for purposes of monitoring and further reducing quantising noise.     

Digital Bandlimiting Filters for P.C.M. Transmission Systems

Bandlimiting filtering is required at the input to p.c.m. transmission and switching systems to prevent aliasing. The required filtering can be performed by a combination of analog and digital filters or by analog filters alone. This paper discusses a combination of a simple analog filter and digital filter suitable for Large Scale Integration. Both recursive and transversal digital filters are considered each in a variety of realizations. The realizations include two filters which exploit the companded nature of the p.c.m. code by using logarithmic signal processing to simplify multiplication. Comparisons are made between the realizations and the most suitable types for Large Scale Integration are found to be a recursive look-up table filter and a transversal logarithmic filter.     

Pitch synchronous 1st-order linear d.p.c.m. system

A system called p.s.f.o.l.d. is described which exploits the correlation between successive pitch periods of a speech signal. This system is a differential one and can employ various types of encoders. We describe a p.s.f.o.l.d. system using a 1st-order d.p.c.m. encoder and show that for a speech utterance this system has a peak signal/noise ratio which is 6 dB larger, and has an increase in dynamic range of 13 dB, compared with a 1st-order d.p.c.m. codec.     

Spectra of biternary digital f.m.

Spectra of digital f.m. have been published recently for general n-level transmission. However, biternary signals, as used in p.c.m. systems, are not covered by the published formulas and require a special treatment. Spectrum density and spectrum distribution are derived for biternary transmission, numerical curves are presented for certain values of f.m. deviation, and the results are compared with ordinary binary transmission.     

Optimised hybrid unidigit p.c.m. system

The coding efficiency of unidigit p.c.m. systems has been improved by using a secondary feedback loop, and the gain/frequency characteristic of the coder has been equalised. The overall signal/noise characteristic of this hybrid system is found to be better than those of all other unidigit systems; it is inferior to the conventional p.c.m. system only for bit rates higher than 60 × 103 bit/s.     

Noninteger sampling to subcarrier frequency ratios in d.p.c.m. of PAL signals

A simple predictor having three coefficients is described for d.p.c.m. encoding of composite PAL colour signals. Noninteger ratios of sampling frequency fs to subcarrier frequency fsc can be selected. For a colour-bar signal, it is shown that the s.n.r. of the d.p.c.m. system using fs/fsc = 2.5 is approximately the same as when third-previous sample prediction is used, i.e. when fs/fsc = 3.0.     

Complementary dynamic m.o.s. logic circuits

Allowing both p and n channel groups of transistors to be blocked between transitions of c.m.o.s. gates leads to complementary dynamic m.o.s. circuits which, in many cases, are significantly less complex than their static counterparts. The value of the concept and a method of synthesis are demonstrated with a practical example. Systematic application to frequency dividers yields very simple new structures.     

Lyapunov estimates of oscillator transients in p.c.m. systems

Lyapunov techniques are applied to the problem of estimating phase overshoots in a p.c.m. system with controlled oscillators. Two possible approaches are examined for use in highly and sparsely connected networks, respectively. The case of full interconnection is also dealt with.     

Optimum p.c.m. regenerator performance in presence of crosstalk

Minimum values of p.c.m. regenerator NEXT noise figure and FEXT noise figure are derived. The performance of regenerators with optimised cosine rolloff is within 0.2 dB of these minima, and that of commercial regenerators can be within 2 dB of the minima.     

High-performance c.c.d. channel filter for a p.c.m. codec

A monolithic c.c.d. filter for p.c.m. codec was fabricated employing both a minimum-phase design with a substantially reduced number of taps and almost perfect elimination of excess capacitances. Results, such as negligible degradation in frequency responses, considerable reduction of common-mode signal, low filter noise (?78 dBm) and wide dynamic range (84 dB) were successfully obtained.     

P.c.m. nonlinear counting encoder

Companded encoders of high resolution, such as may be required for p.c.m. transmission of high-quality sound-programme signals, can be realised using the `counting? principle. In the reported implementation, a reference waveform of ramps of different slopes are generated by integrating the output from a d.a. convenor.     

Correction of errors in p.r.b.n. identification

When fitting a continuous model to p.r.b.n. results, there are several sources of error. The letter deals with the incorporation of interclock sampling and folding-error corrections for the normal m sequence, the maximally orthogonal sequence and the modified m sequence. It further shows how to correct for d.c. offset errors and for bias and ripple in the autocorrelation function where they occur. A comparison is made between the three sequences using an analogue model and the advantages of each sequence are pointed out.     

Speech-transmission performance of p.c.m. systems

A method is described for calculating a quantity representative of the transmission performance of a p.c.m. system. The value obtained depends upon (a) the number of quantised output voltage states, (b) the companding law and (c) the volume of the input speech signal relative to the overload point of the system.     

Various approximating functions for cascade or ladder simulating RC active networks

The use of m.r.p.e.r. (multiple real pole equal ripple) and m.c.p.e.r. (multiple critical pole equal ripple) approximating functions for the design of RC active filters is shown. A study of various LC ladder structures using Cauer,3 m.r.p.e.r. or m.c.p.e.r. solutions has been performed. The results indicate that no significant difference can be expected if either of these solutions is used. It is shown that more components are required for m.r.p.e.r. or m.c.p.e.r. solutions, and that no improvement over coupled sections of lower order can be observed.