Speex dev - Oct 2008 - Decoding multiple encoded frames

Hi,

I've been looking at using speex over RTP. Section 3.3 of
draft-ietf-avt-rtp-speex-05 states that:

"For the purposes of packetizing the bit stream in RTP, it is only
necessary to consider the sequence of bits as output by the Speex
encoder [speex_manual], and present the same sequence to the decoder."

So, I assume that I just pass the whole contents of the RTP packet (less
the RTP header) to the decoder, regardless of whether it contains
multiple frames of encoded data. I can then see two alternatives:

	1. the decoder decodes all of the data in one go
	2. the decoder must it be called repetitively for each frame

If 1, then I can't see how to calculate how much space to allocate for
the output buffer, since I don't know the number of frames contained
within the encoded data.

If 2, then I can't see how it can be determined how many bytes the
decoder has consumed, and hence how many remain in the input buffer.

I'm probably just overlooking something in the documentation. Any
suggestions of where to look are welcome.

Cheers

Darren

Darren Longhorn a ?crit :
> Hi,
> 
> I've been looking at using speex over RTP. Section 3.3 of
> draft-ietf-avt-rtp-speex-05 states that:
> 
> "For the purposes of packetizing the bit stream in RTP, it is only
> necessary to consider the sequence of bits as output by the Speex
> encoder [speex_manual], and present the same sequence to the decoder."
> 
> So, I assume that I just pass the whole contents of the RTP packet (less
> the RTP header) to the decoder, regardless of whether it contains
> multiple frames of encoded data. I can then see two alternatives:
> 
> 	1. the decoder decodes all of the data in one go
> 	2. the decoder must it be called repetitively for each frame
2 is the correct answer.
> If 1, then I can't see how to calculate how much space to allocate for
> the output buffer, since I don't know the number of frames contained
> within the encoded data.
> 
> If 2, then I can't see how it can be determined how many bytes the
> decoder has consumed, and hence how many remain in the input buffer.
The decoder will return -1 if it can't decode anything else, so you just
do something like:

while(1) {
   err = speex_decode_int(state, bits, pcm);
   if (err==-1)
      break;
   output_data(pcm);
}


Cheers,

	Jean-Marc

Jean-Marc Valin wrote:
> 
> 
> The decoder will return -1 if it can't decode anything else, so you
just
> do something like:
> 
> while(1) {
>    err = speex_decode_int(state, bits, pcm);
>    if (err==-1)
>       break;
>    output_data(pcm);
> }
I think I still didn't fully understand. As a test I encoded some frames
(wideband, with quality set to 8), each were 70 bytes. I then did three
things:

 - passed half of it to the decoder
 - passed exactly one frame to the decoder
 - I passed two concatenated frames to the decoder.

In all cases speex_decode_int returned 0.

Unless I misunderstood what you say above, I think it should have
returned 0 only for the third test, -1 for the second test and perhaps
-2 for the first test?

Cheers

Darren