decoder sets clock-rate for application/x-rtp wrong?

[Olivier Crête]

Hi,

In this case, it is a bug in the RTSP server, for MPEG ES streams (MP3)
it should use 90000 as the RTP clock rate and ignore the actual sample
rate of the content.

Is it in particular very invalid to use payload type 14 with any clock
rate other than 90000, they should use a 96-128 payload type for MPA
with a clock-rate thyat is not 90000.

That said, we care about interop, so I'll relax the caps on rtpmpadepay
to accept any clock rate.

Olivier

On 02/12/14 11:33 AM, Peter Maersk-Moller wrote:
> Hi.
> 
> Trying to decode a AV stream from a Wowza server using RTSP, GStreamer
> 1.4.4 seems to fail finding a suitable decoder, when the stream contain
> a mpeg audio stream (mpeg version 1 - here MP3). Looking further into
> the matter, it seems like it is the rtpmpadepay that fail. Now if i use
> AAC it works.
> 
> Looking at the capabilities for rtpmpadepay I notice that rtpmpadepay
> ONLY accepts application/x-rtp input with a clock rate of 90000 which
> indeed is the clock-rate for payload type 14 according to RFC2250,3551.
> However, it appear gstreamer uridecodebin (and probably rtspsrc or
> others) are setting the clock-rate for application/x-rtp from the audio
> rate description in sdp rather than from payload type. If that is the
> case, then that is a bug. Most payload types have a specific clock-rate
> such as payload type 14, which is 90000, but the sample rate can for
> MPEG1 audio be 32000, 44100 or 48000 and for MPEG2 audio 16000, 22050
> and 24000. So they should set the parameters
> 'application/x-rtp,clock-rate=90000,rate=44100' if the sample rate is that.
> 
> Part of the sdp file may look like this
> 
> m=audio 0 RTP/AVP 14
> a=rtpmap:14 MPA/44100/2
> a=control:trackID=1
> 
> So rtspsrc (and perhaps others) need to be changed to set clock-rate
> primarily from the payload types.
> 
> The reason it works with AAC is that the rtpmp4adepay module accepts a
> wide range of clock-rates so a clock-rate of 44100 is accepted even
> though the rtp clock-rate is 90000. Perhaps however, as the clock-rate
> is set to 44100, this may be the source for the annoying messages about
> the the computer being slow while plenty of CPU is available. This could
> also be the reason why playing a live RTSP stream  often can lead to
> very very slow video, but here I am just guessing.
> 
> The mail about problems for GStreamer playing live streams may be found
> here
> http://lists.freedesktop.org/archives/gstreamer-devel/2014-November/050612.html
> 
> Would be nice if we can find the source of why GStreamer have problems
> playing live streams.
> 
> best regards
> Peter
> 
> . And indeed for AAC, rtsp
> 
> 
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> 

-- 
Olivier Crête
olivier.crete at collabora.com