Hello. I am working on implementing RFC 5574 (RTP Payload Format for the Speex Codec) in the ffmpeg and have a question concerning it. It would be nice if somebody could answered it. * Chapter 4.1.1 Registration of Media Type Audio/Speex, subpart "Optional parameters" states these SDP optional parameters: vbr: variable bit-rate - either 'on', 'off', or 'vad' (defaults to 'off'). If 'on', variable bit-rate is enabled. If 'off', disabled. If set to 'vad', then constant bit-rate is used, but silence will be encoded with special short frames to indicate a lack of voice for that period. This parameter is a preference to the encoder. cng: comfort noise generation - either 'on' or 'off' (defaults to 'off'). If 'off', then silence frames will be silent; if 'on', then those frames will be filled with comfort noise. This parameter is a preference to the encoder. And Speex documentation here (http://www.speex.org/docs/manual/speex-manual/node4.html#SECTION00417000000000000000) states this: When enabled, voice activity detection detects whether the audio being encoded is speech or silence/background noise. VAD is always implicitly activated when encoding in VBR, so the option is only useful in non-VBR operation. In this case, Speex detects non-speech periods and encode them with just enough bits to reproduce the background noise. This is called ``comfort noise generation'' (CNG). So, I am a little lost: speex doc says that VAD for CBR environment operates with CNG during silence periods. But RFC separates these two. My question is: what functionality is expected for cases: * vbr=vad + cng=off * vbr=off + cng=on P.S. I'm aware of DTX and I think it would make sense if "cng" parameter were actually controlling dtx func (description in RFC looks quite like it). Thanks.