Output Streaming versus video bitrate descrepancy

[karinann]

I've seen an anomaly that I can't quite get my head around. When using VLC in both Windows XP and Linux versions, I am transcoding a real time input stream originating from a PCIe based ASI input card. The input stream is an HD MPEG Transport Stream with an average bit rate for the entire stream around 18 Mbits/sec. If I simply view the stream in VLC and look at the statistics tab the input and output streaming rates track pretty well.
However if I set the transcoding functions, using 768 kbits/sec for the video using MP4V, the output streaming bitrate is about 40 to 50 percent higher than what I would expect. Interestingly if I use an SD source who's stream bit rate averages around 4 Mbits/sec and use the same video bitrate in the transcoding engine, I get an output stream rate consistent with the video bitrate of 768 kbits/sec plus a little bit for transport stream overhead. The computer I am using is a server with two Quad core Intel processors at 2.8 GHz for each processor, 8 CPUs total. The CPU is using only about 10 percent of total capability, so I don't believe I'm running out of processing horsepower. One thing I have not experimented with is some file buffering on the input stream. I'm actually generating a UDP input stream from my capture hardware and inputting this UDP stream to VLC. As far as VLC error messaging goes I am not seeing any dropped frames or other messages for either the input or output streams. Just to reiterate I see this anomaly on both Windows XP and Linux based systems. Thanks for the bandwidth. Karin

[Rémi Denis-Courmont]

Codecs don't always obey the bitrate, or only within certain limits. Patch welcome.