Bitrate problem - PVR-150

[m0dts]

Hi All

Quite a specific request here but i'l give it a go..

I'm using a PVR-150 Hardware MPEG-2 capture card to capture in CBR 1400kbps video and 192kbps audio.

I know CBR is not totally constant if looked at in a small timeframe but the average should be correct over a longer period, the output file sizes using this format when saved to disk make the bitrate about 200kbps higher than what i specified - ie around 1800kbps if i capture for one minute or even for a few minutes and calculate bits per second etc..


The project i'm working on is a DVB-S transmitter for Ham Radio and it needs a fixed bitrate program stream stream which i can do with some fiddling of the numbers above but i would like to know why the numbers do not add up!

Thanks for looking..

Rob

Project Info http://www.m0dts.co.uk/datv.htm

[jagabo]

I don't know what you might be able to do with the PVR-150 but some general information:

Many programs that report bitrates are not very accurate with short videos. Try a longer cap, like 30 minutes.

Some programs only look at the MPEG headers to report bitrates. Use one that actually looks at the data through the entire video. GSpot does this.

Use Bitrate Viewer to examine your video's bitrate. It shows a graph of bitrate over time.

Many encoders (both software and hardware) have a hard time meeting the requested bitrate when stressed -- ie, trying to encode a large frame size with a low bitrate, or vice versa. 1400 kbps will be ok with 352x288 video but is very low for 720x576.

[m0dts]

Thanks for the tips...

Interestingly when i have looked at the video with GSpot and Bitrateviewer it shows average bitrate to be 1399kbps and 1400kbps respectively so that looks good, the 192kbps audio is CBR and very steady so no problem.

I recorded a 1min file 12781kB for 61.6 seconds
12781/61.6=207.483kB/s
207.483*8=1659.87kb/s

Should be 1592kb/s, the difference is not 200kb/s that i'm seeing later on but it's a start..
I will try some longer files and see if it's still different.


I wonder if i've missed something obvious there..?


i used: cat /dev/video0 > test.ps

#PVR-150 card settings..
v4l2-ctl -c audio_layer_ii_bitrate=9 #192Kbps
v4l2-ctl -c audio_stereo_mode=3 #Mono
v4l2-ctl -c stream_type=0 #MPEG-2 Program Stream=0
v4l2-ctl -c video_bitrate=1400000 #Set Video Bitrate
v4l2-ctl -c video_bitrate_mode=1 #CBR
v4l2-ctl -c video_aspect=1 #4:3
v4l2-ctl --set-fmt-video=width=704,height=576 #Set resolution
v4l2-ctl --set-input=2 #Composite input

Hope someone has some ideas!

Thanks

Rob

[jagabo]

There's always some overhead for the container. Also be sure you don't have K=1000 vs K=1024 related errors. For example, when Windows says a file is 2.00 KB it means 2048 bytes, not 2000 bytes.

[m0dts]

Ok that's a good point... i was using K=1000

looking at real size in bytes is different again... it's 13402112 bytes
that equals 1740.528kb/s

more like what i had before

I wonder if there is some specification for the container for program streams and also transport streams?
I am going to assume already that it is encoder dependant as usual!


Maybe this is not going to be as easy as i thought... ha