GB2093293A

GB2093293A - Signal Processing Method

Info

Publication number: GB2093293A
Application number: GB8123662A
Authority: GB
Original assignee: HINTON RICHARD OLIVER
Current assignee: HINTON RICHARD OLIVER
Priority date: 1980-11-17
Filing date: 1981-08-03
Publication date: 1982-08-25
Also published as: GB2093293B

Abstract

A method of signal processing includes detecting and encoding the intervals between the minima and/or maxima of an input signal in the presence of natural noise or interference without encoding the amplitude values of the said minima and/or maxima. <IMAGE>

Description

SPECIFICATION Signal Processing Method The present invention relates to signal processing and is particularly applicable to electronic signal amplification techniques, to sensory aid and simulation systems and to analogue to digital conversion systems.

In accordance with a first aspect of the present invention, a method of signal processing includes detecting and encoding the intervals between the minima and/or maxima of an input signal in the presence of natural noise or interference without encoding the amplitude values of the said minima and/or maxima.

The present invention is predicated upon the discovery that the human sensory system, and in particular the ear, is more sensitive to the times of occurrance of maxima and minima in a signal than it is to the amplitude of these extrema. This can enable significant simplification to be made in various types of signal processing associated with the human sensory system and in particular can permit intelligible information to be processed using a reduced bandwidth.

Signal amplification techniques make it possible to raise intensity levels of signals obtained from various sources such as transducer devices and devices that detect signals from various electrical representations such as facilitated by modulation techniques. By amplifying the amplitude, signals become compatible with other devices such as recording, transmission and reproduction systems.

Amplifiers find a large number of applications such as sound and video systems, hearing aids and sensory simulation systems. In the latter case, a lack of input dynamic range becomes noticeable since the human auditory system functions for signals having a dynamic range of 120 dB or more while electric amplifiers barely reach 100 dB. This effect causes distortion andfor insensitivity.

According to a second aspect of the present invention, there is provided a signal processing circuit which comprises means for differentiating an input signal and an infinite clipper having a fast response time with respect to the bandwidth of the input signal for clipping the differentiated signal. In the case of an audio signal, the bandwidth of the clipper should extend at least up to 50 KHz but is preferably in excess of 100 KHz.

The faster the response of the clipper the better the quality of the output signal.

The technique off differentiation followed by infinite clipping has in the past been proposed by Licklider and Pollack in a paper published in 1 948. Although this technique does solve the problem of input dynamic range it was found that the performance is strongly degraded by the raised background noise. The present invention differs from the proposals made by Licklider and Pollack in that it recognises the importance of the response time of the clipper, a factor not appreciated by Licklider and Pollack. Indeed, clippers having the required response time were not available at that time.

In the Licklider and Pollack paper, it is proposed to add a repetitive (20 KC) signal in order to suppress spurious signals in the interval between speech signals. The fast clipper proposed by the present invention obviates the need for the superimposition of a repetitive signal and relies instead on the existing noise which has a superior, gaussian, distribution. Of course, where the input signals have a particularly low noise level, for example if steps have already been taken to filter out noise then it is possible and indeed desirable to add noise having a gaussian distribution in order to achieve the desired distribution of times between transitions in the clipper output. It should also be mentioned that in Patent Specification No. 1501874 (Inventor R. C.

Weston) there is disclosed a technique of measuring the intensity levels of the signals at the so called stationary point that is to say the maximum and minima. The fact however that the Patent still takes into consideration the amplitude or intensity levels results in the proposal lacking input dynamic range. Thus, whereas in the present invention a single binary bit is required to indicate the timing of a maximum or minimum, in Patent Specification No. 1 501874 a plurality of bits are required not only to identify timing of a maximum or minimum but also the intensity level.

Consequently, by the use of the invention it is possible to reduce significantly the amount of information required per second to permit, for example, a speech signal to be transmitted intelligably over a telephone line.

In accordance with a preferred feature of the invention, the clipped signal is applied to a retiming circuit, for example a D-type flip-flop operative to sample the output signal of the clipper at predetermined instants and to transmit an output signal having a constant repetition frequency off which the binary level at any instant is dependent upon the value of the clipped signal at the previous sampling instant. When signals obtained from the output of the system are supplied to the human sensory system they are found to be well suited to the activation of the sensory neurones to which the signals are supplied. This has the advantageous effect that the system effectively reduces the information content of its input signal without significantly.

altering the sensation perceived when the signal is directly supplied to the human sensory system.

A signal which is suitably encoded by the system of the present invention may be transduced, transmitted or stored by devices of significantly simplified construction as compared with the prior art. Furthermore, instead of using ordinary transducer devices to obtain the input signal significant simplification and improvement in construction may be possible.

One way of carrying out the invention is described in detail below with reference to drawings which illustrate only one specific embodiment in which; Figure 1 is a block diagram of the fundamental system of encoding, synchronization and stimulus recovery with some connections for possible peripherals, Figure 2 shows the electronic circuitry of such a system.

The Figures show a system for electronic signal amplification, encoding and reproduction comprising, a differentiator which obtains its signal from transducers or other electric sources.

Differentiated signals may also be obtained from transducers of special design or from other electric sources. The differentiator may be approximated by various different high-pass filters. A linear filter characteristic of 6 dB per octave, principally gives obtimum results in signal recovery. Differentiation is obtained by C, and the input impedance of the circuitry.

In accordance with the invention, differentiated signals in the presence of gaussian noise or interference, are supplied to be broadband clipping circuitry. The bandwidth and the corresponding slewing rate of the circuitry need to be significantly greater than the bandwidth of the input signal. The transistorized preamplifier (class A or B) needs to give a good low noise performance. The actual clipping process takes place in either the comparator or the transistor circuit. After employing a low-pass filter (R2, C2,) the output of the schmitt-trigger may be applied to ordinary signal storage, transmission or reproduction devices. The j-k flip-flop will trigger at the positive going edge of its input signal.

Under appropriate circumstances off stimulus recovery, such as loss of information can be irrelevant in terms of perception.

After synchronization by the D-flip-flop the signal can be applied to digital storage, transmission or reproduction devices. The binary digital signals are applied to the stimulus recovery circuitry. The two monostables must give short pulses of a duration shorter than the clock period at both positive and negative going edges of the binary waveform.

The filter (R3, C3) will give pulses, which, if shaped properly will have only one effective extremum during one period of the clock.

In certain applications, a low intensity signal, obtained by halving the clock, mixed with the input signal prior to the clipper, as a sort of dither, can improve performance.

Using the circuitry, the advantages of the invention as specified, can be employed.

It will be seen that an important consequence of the signal processing technique proposed in the present invention is the reduction of the number of bits which need to be transmitter per second, that is to say the baud rate, in order to retain a given intelligability. This can apply to not only audio signals but also to video signals.

Within the field of audio signals, the reduction of the number of features necessary to represent a sound or speech signal considerably simplifies the task of speech and voice recognition.

Furthermore, the reduction of the baud rate in such fields as video transmission and recording may enable significant simplification of the systems necessary, giving the potential capability of aliowing a television picture to be reproduced using only the bandwidth available with audio tape recorders.

Claims

1. A method of signal processing which includes detecting and encoding the intervals between the minima and/or maxima of an input signal in the presence of natural noise or interference without encoding the amplitude values of the same minima and/or maxima.

2. A signal processing circuit which comprises means for differentiating an input signal and an infinite clipper having a fast response time with respect to the bandwidth of the input signal for clipping the differentiated signal.

3. A signal processing circuit as claimed in Claim 2, wherein the response of the infinite clipper extends at least up to 50 KHz.

4. A signal processing circuit as claimed in Claim 2 or 3, wherein a retiming circuit is provided operative to sample the output signal off the clipper at predetermined instants and to transmit an output signal having a constant repetition frequency of which the binary level at any instant is dependent upon the value of the clipped signal at the previous sampling instant.

5. A signal processing circuit constructed, arranged and adapted'to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.