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.     

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.     

Pitch synchronous differential predictive encoding system

A pitch synchronous differential predictive encoding system (p.s.d.p.e.) is described, which reduces the dynamic range of voiced speech to a value similar to that of unvoiced speech. As a consequence, the signal encoded has a smaller dynamic range than the speech signal and results in an improvement in the signal/noise ratio for a given transmitted number of bits per sample. This improvement is approximately 8 dB compared with an a.d.p.c.m. codec, when the p.s.d.p.e. system uses an adaptive p.c.m. encoder and the transmission rate is 3 bit/sample.     

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.     

Adaptive difference detection and correction system for partial correction of transmission errors in linear p.c.m.

An adaptive version of the difference detection and correction system for the partial removal of transmission errors in linear p.c.m. speech without the use of channel coding is presented. The improvement in s.n.r. compared to the nonadaptive system is approximately 3 dB for transmission bit error-rates between 0.5 and 5%.     

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.     

Breaking public key cryptosystem based on systems of nonlinear equations over finite field

Tsujii et al. (see ibid., vol.23, no.11, p.558-60, 1987) proposed a public key cryptosystem which requires only O(m/sup 2/) bit operations for encryption/decryption and which also realised digital signatures; m here is the block length. The speed advantage over better known systems such as the RSA system (which takes O(m/sup 3/) operations) could have been immensely significant. Unfortunately it is possible to deduce the private key of the Tsujii system from the public key in polynomial time except possibly in a tiny minority of randomly chosen cases.<>     

Combined radix<2 and 1.5 bit/stage pipelined analogue-to-digital converter

A new pipeline architecture that combines the radix<2 and traditional 1.5 bit gain-stages is presented. The 10 bit, 60 MHz, 3 V pipelined analogue-to-digital converter has been designed in a 0.25 /spl mu/m 1p4M CMOS technology using digital self-calibration. The converter achieves more than 57 dB SNDR from a 3 V supply (10% lower than nominal 3.3 V) within -40 to +120/spl deg/C temperature range.     

2Gbps GaAs单片4bit数模转换器的设计与实现

详述了单片超高速2G bps G aA s 4b it数模转换器(DAC)的设计、制造及测试。在南京电子器件研究所标准76 mm G aA s工艺线采用0.5μm全离子注入M ESFET工艺完成流片。芯入输入输出阻抗实现在片50Ω匹配。4 b it DAC的微分非线性(DN L)为±0.22最低有效位(LSB),积分非线性(IN L)为±0.45LSB,达到5.2 b it的转换精度。该单片电路提供差分互补输出,长周期输出特性无漂移。其最高转换速率可达2 G bps,建立时间小于250 ps,电路核心部分功耗为110 mW。     

Application of solitons to communication system

In this letter, an experimental investigation of a p.c.m. communication system using soiltons as signal carriers is reported. The soliton is a pulse that propagates along a nonlinear dispersive line with constant velocity and without distortion. It is shown that by designing the system appropriately, it is possible to achieve a better performance as far as bit error rate is concerned than the corresponding system with linear dispersive channel.     

Adaptive statistical encoding of d.p.c.m.-coded videotelephone signals

Adaptive statistical encoding of d.p.c.m.-coded video-telephone signals is considered. The need for an adaptive statistical encoder to operate on short-term batch statistics is explained and the effect on the bit rate of limiting the number of codes employed by the statistical encoder is reported.     

Pseudorandom 250 Mb/s modulation of GaAs l.e.d.s

Effective direct modulation of a GaAs l.e.d. was demonstrated with a 250 Mb/s optical-repeater experiment. A GaAs l.e.d. transmitted a high-quality p.c.m. pseudorandom bit stream, which permitted the received signal to be regenerated with predicted error rates.     

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.     

Low-noise optical detection of a 1.1 Gb/s optical data stream

A low-noise, 1.1 Gb/s optical receiver has been built using a silicon a.p.d. and a GaAs f.e.t. The receiver sensitivity was evaluated using error-rate measurements, and for a bit error rate of 10?9, with no fibre, the measured optical sensitivity was ?37.0 dBm. These results are used as a basis for the calculation of maximum repeater spacings for 1.1 Gb/s systems operating at 0.85 ?m and 1.25 ?m wavelengths.     

Equalization in coherent lightwave systems using microwavewaveguides

The maximum bit rate/ distance product in recent single-frequency laser direct-detection lightwave system experiments has been limited by dispersion. An equalization technique, appropriate for coherent lightwave systems, that uses a microwave waveguide for overcoming the delay dispersion problem is considered. Results show that small low-loss waveguides can be used to greatly reduce dispersion. For example an 8 GHz bandwidth signal transmitted over 68 km of fiber can be equalized by a waveguide with a cross section of 6 mm×3 mm and a length of only 17 cm. With the waveguide equalizer, the dispersion-limited maximum bit rate/distance product for a standard fiber system can be increased to that of a dispersion-shifted fiber system at 1.55 μm, e.g., a 16-fold increase in maximum bit rate for 100 km transmission     

Synchronized chaotic optical communications at high bit rates

Basic issues regarding synchronized chaotic optical communications at high bit rates using semiconductor lasers are considered. Recent experimental results on broadband, high-frequency, phase-locked chaos synchronization, and message encoding-decoding at 2.5 Gb/s are presented. System performance at a bit rate of 10 Gb/s is numerically studied for the application of three encryption schemes, namely chaos shift keying, chaos masking, and additive chaos modulation, to three chaotic semiconductor laser systems, namely the optical injection system, the optical feedback system, and the optoelectronic feedback system. By causing synchronization error in the forms of synchronization deviation and desynchronization bursts, the channel noise and the laser noise both have significant effects on the system performance at high bit rates. Among the three laser systems, the optoelectronic feedback system has the best performance while the optical feedback system has the worst. Among the three encryption schemes, only the performance of additive chaos modulation with low-noise lasers is acceptable at high bit rates.     

Some results for an r.f.- and p.l.-modulated incoherent carrier

Possibilities of using incoherent carriers, amplitude-modulated by the message-modulated p.f.m., r.f.m. and p.l.m. have been investigated. It has been shown that, for noise bandwidths much smaller than the transmission bandwidth, efficient communication is possible, and s.n.r.s of 50?60dB are obtained for practical p.f.m.-a.m. and r.f.m.-a.m. systems. Some comparisons are also given.     

Performance degradation of two-link binary c.p.s.k. system incorporating direct phase regenerator

The letter presents an analysis for the bit error probability (b.e.p.) of a two-link binary coherent phase-shift-keying (c.p.s.k.) system corrupted by additive Gaussian noise and consisting of a direct phase regenerator (d.p.r.) followed by a coherent receiver. Equivalent signal/noise ratio (s.n.r.) degradations due to imperfections in a practical d.p.r. are evaluated at the 10?6 b.e.p. point.     

Reframing statistics of p.c.m. multiplex transmission

The statistical properties of the reframing process in p.c.m. systems are analysed theoretically. Both expectation and standard deviation of the reframing time are expressed as functions of the framing word length, number of digits per frame, and reframing strategy parameters. Results are given both for 6.272 and 8.32 Mbit/s secondary p.c.m. telephone multiplexes.