GB2031197A

GB2031197A - Pseudo-random noise generator

Info

Publication number: GB2031197A
Application number: GB7839564A
Authority: GB
Original assignee: Marconi Instruments Ltd
Current assignee: Marconi Instruments Ltd
Priority date: 1978-10-06
Filing date: 1978-10-06
Publication date: 1980-04-16
Also published as: DE2850240C2; FR2438382A1; DE2850240A1; FR2438382B1; GB2031197B

Abstract

Arrangement for generating a sequence of pseudo-random numbers constituting a band limited noise signal for testing a voice channel of a p.c.m. telephone system. A pseudo-random pulse sequence generator 1 comprising a feedback shift register feeds a digital filter 6 comprising a series in - parallel out shift register 9. At each clock the outputs of register 9 are each multiplied by an individual binary number and the results are summed at 11. Each binary number at 8 is then multiplied at 12 by a common number 13 to adjust the amplitude and by varying numbers at 14 according to a companding law. The generator 1 may be preset at intervals by inputs 7 so as to reduce the sequence length and thus change the peak to RMS ratio of the signal. <IMAGE>

Description

SPECIFICATION Noise sources This invention relates to noise sources which are intended for use in connection with the testing digital telephone systems. Digital telephone systems generate a certain amount of quantising distortion and it is customary to measure the level of this distortion in primary level pulse code modulation telephone stages by injecting a band-limited noise signal in analogue form into a voice channel of the system. The injected noise is chosen to lie within a particular narrow frequency band and by measuring the level of analogue noise signals present in a voice channel at the output stage of the telephone system a measure of the quantising distortion can be obtained.In a digital telephone system it is becoming increasingly necessary to measure the quantising distortion introduced by each stage, and points at which the noise can be injected and measured in analogue form are not necessarily available. The present invention seeks to provide a noise source in which the noise is presented directly in a digital form having the characteristics necessary to enable the quantising distortion to be determined.

According to a first aspect of this invention, a noise source includes a digital noise generator which produces a stream of pseudo-random digital words representative of frequency band-limited noise, digital means for varying the amplitude of all the digital words by a common adjustable factor and means for selectively altering the amplitude of the digital words so generated in accordance with a non-linear amplitude law.

Preferably the digital noise generator comprises a pseudo-random sequence generator which feeds a digital filter arranged to produce from the sequence of digital bits the said stream of digital words having the required frequency characteristic.

Preferably again the pseudo-random sequence generator comprises a multi-stage shift register in which the outputs from selected stages of the register are combined at an exclusive-OR gate, the output of which is applied to the first stage of the register to form a closed feedback loop.

A pseudo-random bit sequence has a finite length which depends on the number of stages in the shift register, and when the pseudo-random sequence has been completed the entire sequence is repeated exactly. Thus whilst the bits in the sequence occur in what appears to be a statistically random order, the sequence is nevertheless predetermined by the characteristic of the sequence generator. The length of the sequence affects the ratio of the peak value to RMS value of the noise signals which are derived from it. The length of the pseudo-random sequence can be reduced from the maximum length, (which is determined by the number of stages in the shift register) by periodically re-setting each stage of the shift register to a predetermined value.The sequence then continues to run from this predetermined value and the length of the sequence is determined by the number of bits occurring between re-setting intervals.

Preferably the digital filter comprises a multi-stage shift register in to which the pseudo-random bit sequence is fed under the action of a clock signal, the contents of each stage of the register being multiplied by a predetermined factor chosen in dependence on the required frequency bandwidth of the noise, the result of all multiplications being summed within each clock period so as to produce a particular digital work. As the pseudo-random sequence is advanced bit by bit through the shift register under the action of the clock signal a new digital work is produced at each clock period.

According to a second aspect of this invention a method of producing a sequence of digital words representative of frequency band-limited noise includes the steps of generating a stream of pseudorandom digital words representative of frequency band-limited noise, varying the amplitude of all the digital words by a common adjustable factor, and selectively altering the amplitude of the digital words so generated in accordance with a non-linear amplitude law.

Preferably the stream of pseudo-random digital words is obtained from a pseudo-random sequence of digital bits which is digitally filtered to produce said stream of digital words having the required frequency characteristic.

Preferably again the pseudo-random sequence of digital bits is generated by means of a multi-stage shift register in which the outputs from the selected stages of the register are combined at an exclusive OR gate, the output of which is applied to the first stage of the register to form a closed feedback loop.

According to a third aspect of this invention the digital words representative of the frequency bandlimited noise produced in accordance with the first or second aspects of this invention are used in combination with at least one stage of a digital telephone system so as to provide a measure of the quantising distortion introduced by that stage.

The invention is further described by way of example with reference to the accompanying drawing, which illustrates a digital noise source in accordance with the present invention. A sequence generator 1 consists of a multi-stage digital shift register 2 haying twelve stages, the outputs from stages 1,4, 6 and 12 being combined at an exclusive OR gate 3, the output of which is fed back to the input of the first stage of the register 2 to form a closed feedback loop. The register 2 is clocked under the action of an 8 kHz clock source applied to terminal 4 and as the sequence generator 1 is clocked, a pseudo-random bit sequence is obtained at the output of the exclusive-OR gate 3 and this is applied to a digital filter 6. The sequence generator 1 produces a pseudo-random sequence of 4,095 bits after which the sequence repeats exactly.This is the maximum length of a sequence which can be obtained from a sequence generator using a twelve stage register and is equal to 212-1 bits. For many applications the length and pulse repetition period of this sequence may be too long and the sequence generator is periodically re-set, the intervals between re-setting necessarily being less than the natural sequence length of the sequence generator 1. The sequence generator is re-set by entering a predetermined pattern of ones and noughts into the register 2 via inputs 7 from a code source not shown.

The pseudo-random sequence is conveniently arranged so that the stages may be re-set by entering a logic 1 into each stage, whilse still producing a sequence which exhibits required characteristics. The relevant characteristic is the value of the peak to RMS noise value and in practice the register 2 is re-set to that the portion of the pseudo-random bit sequence which is fed to the filter 6 includes the necessary number of amplitude peaks. By arranging that the shift register 2 may be re-set by entering logic is into each stage, the inputs 7 can be connected in parallel to a single switchable logic level source.

The filter 6 is a digital filter which is itself of known configuration and which operates to produce a stream of digital words on output lines 8, which are representative of noise lying within a predetermined narrow frequency band. In this example a frequency band of 100 Hz width lying between 450 Hz and 550 Hz is chosen. The filter 6 consists of a multi-stage shift register 9 containing 127 stages each stage being connected to a respective digital multiplier 10, of which for the sake of clarity only seven are shown.

Each multiplier 10 acts to multiply the logic 1 or 0 in the corresponding stage of the shift register 9 by a sixteen bit word having a value which is chosen with regard to the bandwidth of the noise which is to be produced. In general the value of each multiplication factor will be different.

The band-pass characteristic of the digital filter is determined by the number of stages in the shift register 9, the greater the number of stages the better the filtering, and the amplitude of frequency components lying outside the 100 Hz bandwidth is minimised. A shift register 9 having 127 stages provided a sufficiently good pass-band characteristic.

The pseudo-random bit sequence from the sequence generator 1 is clocked into the serial shift register 9 and for each clock pulse the contents of the shift register 9 are multiplied by the appropriate multiplication factor and the results of the multiplication process summed together in a summation circuit 11 to produce a sixteen bit word which is fed out in parallel over the sixteen lines 8.

The stream of digital words produced on lines 8 is representative of the required digital noise, but in order to use the digital words to enable a measurement of quantising distortion to be made in a telephone system it is necessary to adjust the overall amplitude of the noise to enable performance to be assessed at difference input levels. Clearly quantis ing distortion is dependent on the amplitude level of a signal applied to the telephone system and in order to obtain the required variation in amplitude the digital words are applied to an amplitude scaler 12.

In the amplitude scaler 12 each digital word is multiplied by a common adjustable factor applied over line 13.

Because most of the information in a voice signal occurs at relatively low amplitude levels, it is customary to use non-linear analogue-to-digital encoders in a digital telephone system which obey what is usually termed a "companding law". As a result of the companding law more quantising levels are available for relatively low amplitude signals than are available for higher amplitude signals which are accordingly converted to digital words with a lower degree of accuracy. In order to provide a true measure of the quantising distortion introduced by a stage of a telephone system, it is necessary to convert the digital noise in accordance with a corresponding non-linear law and this is achieved at the converter 14, which may be conventional linear to non-linear converter which operates in accordance with a known companding law.The output of the converter 14 consists of a sequence of eight bit digital words and in order to retain the required degree of accuracy twelve bit words are applied to it from the amplitude scaler 12, which in turn receives sixteen bit words over lines 8.

For a particular sequence generator 1 and a digital filter 6, the same sequence of digital words will always be obtained from the converter 14 thereby allowing exact repeatability of test measurements performed using the noise source. The sequence of digital words obtained from the converter 14 can be conveniently entered into a memory and read-out as necessary when it is desired to perform a test measurement on a telephone system. If desired the sequence of digital words can be entered into a plurality of read-only memories, it then being necessary to use only the read-only memory in conjunction with the telephone system to enable measurementsto be made.

Claims

1. A noise source including a digital noise generator which produces a stream of pseudo-random digital words representative of frequency bandlimited noise, digital means for varying the amplitude of all the digital words by a common adjustable factor and means for selectively altering the amplitude of the digital words so generated in accordance with a non-linear amplitude law.

2. A noise source as claimed in claim 1 and wherein the digital noise generator comprises a pseudo-random sequence generator which feeds a digital filter arranged to produce from the sequence of digital bits and said stream of digital words having the required frequency characteristic.

3. A noise source as claimed in claim 2 and wherein the pseudo-random sequence generator comprises a multi-stage shift register in which the outputs from selected stages of the register are combined in an exclusive OR gate, the output of which is applied to the first stage of the register to form a closed feedback loop.

4. A noise source as claimed in claim 3 and wherein each stage of the shift register is periodically re-set to a predetermined value so as to produce a pseudo-random sequence having a number of bits which is less than the maximum obtainable from the shift register.

5. A noise source as claimed in claim 2,3 or 4 and wherein the digital filter comprises a multi-stage shift register into which the pseudo-random bit sequence is fed under the action of a clock-signal, the contents of each stage of the register being multiplied by a predetermined factor chosen in dependence on the required frequency bandwidth of the noise, the result of all multiplication being summed within each clock period so as to produce a particular digital word.

6. A method of producing a sequence of digital words representative of frequency band-limited noise includes the steps of generating a stream of pseudo-random digital words representative of frequency band-limited noise, varying the amplitude factor, and selectively altering the amplitude of the digital words so generated in accordance with a non-linear amplitude law.

7. A method as claimed in claim 6 and wherein the stream of pseudo-random digital words is obtained from a pseudo-random sequence of digital bits which is digitally filtered to produce said steam of digital words having the required frequency characteristic.

8. A method as claimed in claim 7 and wherein the pseudo-random sequence of digital bits is generated by means of a multi-stage shift register in which the outputs from selected stages of the register are combined at an exlcusive OR gate, the output of which is applied to the first stage of the register to form a closed feedback loop.

9. A method of determining quantising distortion introduced by a stage of a telephone system by applying to that stage the digital words representative of the frequency band-limited noise produced in accordance with any of the preceding claims.

10. A noise source substantially as illustrated in and described with reference to the accompanying drawing. ~~~~~~~~~~~~~~~