Question 26/XII - Setting objectives for mixed analogue/digital circuits
(continuation of Question 26/XII studied in 1985-1988; reviewed wording)

Considering

(a) that there exists a draft Recommendation G.1xx;

(b) that still for a long time there will exist mixed analogue/digital
circuits on international and national networks during the period of
transition to all-ditigal networks;

(c) that mixed analogue/digital circuits may have a complex structure
and they may be composed by using PCM equipment and group codecs
(G.795), transmultiplexers (G.793, G.794), modems (G.941, V.37),
transcoders (G.761), equipment with different coding methods other than
PCM ones;

(d) that the existing draft text of Recommendation G.1xx sets objectives
for mixed analogue/digital circuits formed by using only PCM equipment;

(e) that the transmission quality of mixed analogue/digital channels
should not be impaired considerably,

the CCITT has decided to study the following Questions:

1. What parameters should be considered for mixed analogue/digital
circuits of different structures, using various analogue/digital
conversion methods, and with digital telephone sets on each and as well
as with a digital telephone set on one end and an analogue telephone set
on the other end?

2. What numerical values of the parameters should be set including
planning rules?

3. What rules of addition of impairments should be used depending on the
type of analogue/digital conversion?

4. What new Recommendations on objectives for mixed analogue/digital
circuits should be drawn up?

5. What additions and amendments should be introduced into the existing
CCITT Recommendations?

6. How should draft Recommendation G.136 (Supplement No. 29, Volume
III.1 of the Blue Book) be completed, using the study results of other
Questions?

Note 1 - Study of this Question is interest to Study Groups IV, XII and
XVIII.

Note 2 - The problem of the "total noise design" should also be studied
with regard to the addition law of the "circuit noises (analogue)" and
the "MOS equivalent to noise due to quantizing distortion".