GB1350850A - Measuring noise in pcm systems - Google Patents

Abstract

1350850 Measuring noise in PCM systems STANDARD TELEPHONES & CABLES Ltd 14 Sept 1972 42625/72 Heading G1U Quantizing noise in a PCM system having a non-linear coding characteristic is measured by injecting a test tone into a spare time slot at the transmitter, the test tone being amplitude modulated by the inverse function of the coding characteristic, filtering out the test tone fundamental frequency at the receiver (after decoding) and measuring the average energy remaining. Amplitude modulating the test tone in the manner mentioned produces equal amounts of test tone energy in each quantizing step, even though such steps will be of different widths (in amplitude) in accordance with the coding law. As described, for a speech PCM system using logarithmic compression, a 1 kc/s test tone amplitude modulated by a decaying exponential waveform is produced 5, Fig. 2, repeatedly sampled by a gate 24, coded 1 into a spare time slot, and transmitted 26. At a remote station the transmitted samples of test tone are decoded 27, and routed by a 3. 4 kc/s low pass filter 28 into two parallel channels, the upper one of which rectifies and integrates 210 the whole band up to 3. 4 kc/s, which includes the test tone and the quantizing noise, while the lower channel eliminates the test tone by a 1 kc/ s high pass filter and merely rectifies and integrates 213 the band 1 kc/s to 3. 4 kc/s, which includes only the quantizing noise, and the outputs of these two channels are compared. The arrangement can be operated in either of two ways. Either the amount of quantizing noise can be measured by adjusting a calibrated amplifier 212 in one channel until the comparator indicates equality, or by setting the calibrated amplifier at an appropriate level the comparator can actuate an alarm when the amount of quantizing noise becomes too high.