(sent to both QT-api and streaming-server-developers (even though it's
slightly off-topic for ssd))
I'm trying to diagnose some issues where some streaming that I'm doing
seems to be consuming a lot more bandwidth than expected, and I'm
gettins a little confused by the output of the QuickTime player's "bit
rate" graph / fields.
Since there are many layers involved in the transmission process, and a
lot of overhead, it isn't clear what's being reported.
For example, using QCELP half-rate from QT Broadcaster (UDP) puts out 5
packets per second, with each packet being (and the effective bit rate):
on the wire: 225 bytes: 9000 bps
IP level: 211 bytes: 8440 bps
UDP level: 191 bytes: 7640 bps
RTP level: 183 bytes: 7320 bps
payload: 171 bytes: 6840 bps
The quicktime player seems to fluctuate in it's reports from 8-10 kbps,
but I don't know what it is measuring.
This isn't so bad. One issue is that when running QT player against a
particular encoder that puts each audio frame inside a separate RTP
frame. In this case, the frames are 33 bytes each (GSM), and the
overhead is much greater than the actual payload.
In this case, relative bandwidths by layer are (this is interleaved over
RTSP):
TCP on wire: 115*3000*8/60 = 46000 bps.
UDP on wire: 89*3000*8/60 = 34800 bps.
RTP data: 45*3000*8/60 = 18000 bps.
GSM data: 33*3000*8/60 = 13200 bps.
In this case, the QT player seems to report about 20-20kbps, but again I
don't know what it's measuring..
I know what I need to do to try and optimize things, and the tradeoffs
to be made (especially with GSM, which is uninterleaved), but I'd really
like to get a better idea of how the QT player is calculating things.
-SteveK
--
Steve Kann - Chief Engineer - 520 8th Ave #2300 NY 10018 - (212) 533-1775
HorizonLive.com - collaborate . interact . learn
"The box said 'Requires Windows 95, NT, or better,' so I installed Linux."
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