[Live-devel] RTP packet size

[Ross Finlayson]

> I need to do some integration testing with a proprietary piece of hardware as part of the continuing proof of concept for the project I am working on. The hardware I need to stream audio (from an uncompressed WAV file for the test) to requires a sample rate of 48KHz and we have sources prepared for that. It also requires that the audio frames being streamed are not fragmented and so I need to send a 4mS frame of audio data rather than the 8mS that the server appears to send right now. Could someone point me to the setting or parameter for the library that would enable me to set the frame size to achieve this please? I have searched through the code but so far I have not been able to identify where this is controlled.

There's a bit of confusion here, I think.  First, audio from a WAV file are 'samples', not 'frames'.  Each sample is usually only 16 bits (i.e., 2 bytes), I think.  So WAV (really PCM) audio samples are nowhere near large enough to get fragmented over outgoing RTP packets.

OTOH, the LIVE555 code works with 'frames' - delivering one frame at a time.  For the code to run efficiently, frames need to be much larger than 2 bytes, so, for streaming PCM audio, we group samples into much larger 'frames'.  We also want these 'frames' to be small enough to fit within an outgoing RTP packet.

The code for computing this 'preferred frame size' is at 201-204 of "WAVAudioFileSource.cpp".  In your case - 48 kHz audio, 2 channels, 16 bits-per-sample (I think) - this will give you a preferred frame size of 1400 bytes: i.e., 350 samples.  For a 48 kHz sample rate, this means that each outgoing RTP packet will contain about 7 ms of audio.

Now, you seem to be saying that you want want a smaller RTP packet, one that contains only 4 ms of audio - i.e., 192 samples.  But why?  Having a smaller RTP packet (almost 1/2 as small) will lead to increased overhead (because of the need for almost twice as many Ethernet packets, each with their own RTP header).  So it's probably not something that you really want.

(Note also that RTP packets do not get seen by receivers - only by the lower-level LIVE555 reception code.  An audio receiver still sees only a sequence of audio samples, regardless of the underlying RTP packet size that was used to transmit them.)

So, I don't think that you have any real need to change anything.


Ross Finlayson
Live Networks, Inc.
http://www.live555.com/

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