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Re: [Linphone-developers] RTCP packet for every RTP packet?

From: Nicholas J Humfrey
Subject: Re: [Linphone-developers] RTCP packet for every RTP packet?
Date: Wed, 21 Jun 2006 20:49:20 +0100

MPEG Audio has a fixed static payload type 14, listed here as MPA:

The sample rate of the audio isn't really relevant to MPEG audio is it? The sample rate doesn't affect the data rate, and the timestamps are based on a fixed 90Khz clock.


From RFC3551:

4.5.13 MPA

   MPA denotes MPEG-1 or MPEG-2 audio encapsulated as elementary
   streams.  The encoding is defined in ISO standards ISO/IEC 11172-3
   and 13818-3.  The encapsulation is specified in RFC 2250 [14].

   The encoding may be at any of three levels of complexity, called
Layer I, II and III. The selected layer as well as the sampling rate
   and channel count are indicated in the payload.  The RTP timestamp
   clock rate is always 90,000, independent of the sampling rate.
MPEG-1 audio supports sampling rates of 32, 44.1, and 48 kHz (ISO/ IEC
   11172-3, section 1.1; "Scope").  MPEG-2 supports sampling rates of
   16, 22.05 and 24 kHz.  The number of samples per frame is fixed, but
   the frame size will vary with the sampling rate and bit rate.

On 16 Jun 2006, at 15:59, Simon Morlat wrote:

Is there are a reason that this static payload type wasn't already
built-into oRTP?

oRTP defines the the static payload defined in RFC3551, not sure mpeg-audio
was part of it.
The clockrate you set must be the sampling rate of audio stream.


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