Question 18/XII - Transmission performance of digital systems
(continuation of Question 18/XII studied in
1985-1988)

Considering

(a) that many new types of digital processes are being considered for
introduction into telephone networks resulting in mixed analogue/digital
connections;

(b) that distortions (e.g. quantization, circuit noise, etc.) due to
these processes can significantly affect overall transmission
performance of telephone connections;

(c) that the MNRU (Modulated Noise Reference Unit) of Recommendation
P.70 has been adopted as the reference system in terms of which the
transmission performance of digital processes for telephony should be
expressed;

(d) that Supplement No. 14 provides guidance on the use of the MNRU in
conducting subjective tests using the system of Recommendation P.70; and

(e) that methods of Supplement No. 14 are most suitable for evaluating
the relative transmission performance of waveform digital processes
(e.g., PCM, ADPCM),

asks the following Questions:

1. What changes can be made in Recommendation P.70 and Supplement No.
14,

a) to improve their application for waveform digital processes intended
for telephony (e.g. synchronous tandeming of PCM and ADPCM systems);

b) to enable their application for wideband processes (e.g. 7 kHz
programme) and low bit rate (< 16 kbit/s) processes;

c) to enable their application for non-waveform digital processes.

2. In what terms (for example QN and QW determined using the system of
Recommendation P.70) should the subjective effect of distortion of
digital processes be expressed and what magnitude should be recommended
for different digital processes to ensure satisfactory transmission
performance?

3. What objectively measured values can be recommended for different
digital processes?

Note - Objective methods are being studied under Question 13/XII.
Attention is also drawn on Question 14/XII.