Question 30/XV - Performance characteristics of PCM and ADPCM channels
at voice frequencies (Continuation of part of Question 32/XV, studies in
1985-1988)

Considering

(1) that some specification items in the Recommendations G.712, G.713,
G.714 and G.715 need to be completed;

(2) that within a widespread digital network, it is envisaged that a
telephony connection will ultimately require only a single
encoding/decoding process for each direction of transmission;

(3) that although Recommendation G.713 gives the requirements for the
performance of PCM channels between 2-wire interfaces when used in
junction and trunk parts of the network, and another Study Group is
defining the performance of digital exchanges between 2-wire analogue
interfaces, there is a need to consider the performance of transmission
systems used in telephone terminal equipments;

(4) that Recommendations in the G- and Q-Series, dealing with the
transmission performance requirements for PCM transmission systems and
digital exchanges respectively, should, as far as possible, be
consistent with each other;

(5) that an ADPCM encoding law using 32 kbit/s is recommended in
Recommendation G.721,

1. What modifications to existing Recommendations G.712, G.713, G.714
and G.715 should be made?

2. What voice frequency characteristics of equipment using low bit rate
encoding, e.g. according to Recommendation G.721, should be recommended?

3. What voice frequency characteristics of equipment for wideband speech
coding, according to Recommendation G.722, should be recommended?
4. What are the voice-frequency performance requirements for PCM
transmission systems when used in telephone terminal equipment? In
particular, what are the transmission performance requirements for PCM
encoding/decoding and associated filtering (both in-band and out-of-
band) when interfacing directly to or incorporated within a telephone
instrument?

Points for study

1.Items left under study in the G.710-Series of Recommendations and
resolution of discrepancies with respect to the G.550-Series of
Recommendations.

2.Recommendations resulting from consideration of parts 2, 3 and 4 of
the Question.

Notes

1.Study of Point 1 should be coordinated with Study Group XI.

2. Point 2 is also relevant in the context of ISDN terminals. Close
collaboration will be required with Study Group XII who are studying
certain characteristics of subscribers' apparatus used for telephony.