[AVT] Comments on draft-ietf-avt-rtcp-report-extns-02.txt

["Alan Clark" <alan@telchemy.com>]

Timur

The 02 draft specifies signal and noise levels in terms of dBm0 with respect
to overload conditions - I'd like to change this to dBm if this is not a
problem for anyone, but retain the definitions for talkspurts vs silence
periods.

The revised definition would be

signal level: 8 bits
   The voice signal relative level is defined as the ratio of the signal
   level to a reference digital milliwatt, expressed in decibels as a signed
   integer in two's complement form.  This is measured only for packets
   containing speech energy.  The intent of this metric is not to
   provide a precise measurement of the signal level but to provide a
   real time indication that the signal level may be excessively high
   or low.

         Signal level = 10 log 10 ( rms talkspurt power (mW) )

   A value of 127 indicates that this parameter is unavailable.  Typical
   values should be generally in the -15 to -20 dBm range.

noise level: 8 bits
   The noise level is defined as the ratio of the silent period back
   ground noise level to a reference digital milliwatt, expressed in
   decibels as a signed integer in two's complement form.

         Noise level = 10 log10 ( rms silence power (mW) )

   A value of 127 indicates that this parameter is unavailable.


The goal of these metrics is to highlight excessively high or low signal
levels as these can cause many other problems (e.g. with echo cancellers and
voice activity detectors) and to identify if high levels of background noise
are present.  This is easier if we define the power levels separately for
both talkspurts and silence periods - as it avoids the reported signal level
value being affected by speech activity factor.

I've spoken to a number of DSP software vendors and it seems that these
values are readily available or can be derived from available data.

Alan Clark









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