Transcode target bitrate

[n0hc]

Hey everybody I have a video conversion question and was hoping that you could help. I work for a company that gets mpeg4 streams via a multicast IP and we transcode this to mpeg2 so that we can send it out over a 256qam to our cable tuners. What I am wondering is if there is any specific way that I can get the right target bitrate that I should be transcoding the mpeg4 stream to? The details:

Input: MPEG-4 AVC (H.264) Video 1920x1080i 29.97fps (stream type 27 (gave me this info may be particular towards device))

Output: MPEG-2 VBR
-target video bitrate 14meg (adjustable)
-target peak video bitrate 16meg (adjustable)
-audio passes through

Since I send this out on a 256qam I put two streams per QAM. a 256 qam has the capacity of about 34meg usable bandwidth. Split in half would be 17meg for a peak rate. But I just wanted to make sure that I was converting this to the proper bitrate. Here is the ffmeg information about a saved file of the mpeg4 input stream:

[mpegts @ 0x24757a0] max_analyze_duration reached
[mpegts @ 0x24757a0] PES packet size mismatch

Seems stream 0 codec frame rate differs from container frame rate: 59.94 (60000/1001) -> 59.94 (60000/1001)
Input #0, mpegts, from 'TS 2013-10-01T071314 dev4_in_in.m2t':
  Duration: 00:00:10.70, start: 22175.148567, bitrate: 6782 kb/s
  Program 3 
    Stream #0.0[0x31]: Video: h264 (High), yuv420p, 1920x1080 [PAR 1:1 DAR 16:9], 74.95 fps, 59.94 tbr, 90k tbn, 59.94 tbc
    Stream #0.1[0x34]: Audio: ac3, 48000 Hz, 5.1, s16, 384 kb/s
At least one output file must be specified

Please let me know if there is anything that I can do tweak anything to make the video stream better. Thanks!

[n0hc]

No idea on this? Is there anywhere technical that you guys would know of to ask a question like this? Thanks!

[jman98]

It's a great topic, well within what should be asked here. We have people who can help you, but I'm not one. Unfortunately this is one of those posts that for no reason at all just doesn't get attention. But sometimes when I post it the thread it helps and others start getting paying attention, so maybe someone will come along who can help.

[Selur]

Please let me know if there is anything that I can do tweak anything to make the video stream better.

use a better mpeg-2 encoder, ffmpegs/libavs mpeg-2 encoder isn't that great so unless you are stuck to linux and can't use wine I would recommend to use another encoder.

a. better post mediainfo output ('74.95 fps, 59.94 tbr, 90k tbn, 59.94 tbc' just looks wrong)

What I am wondering is if there is any specific way that I can get the right target bitrate that I should be transcoding the mpeg4 stream to?

what would qualify as 'right'?
a. I mean you are limited to 16-17Mbit max due to bandwidth restrictions.
b. you reencode to a lossy format so independent of what bit rate you choose you will lose quality
-> using the highest data rate you can use seems to be a good idea

[Cornucopia]

In that ffmpeg shot, you gave us the input bitrate (6782kbps, well within your 14MbpsAVG & 16MbpsPEAK limits), but nothing about the output. Since YOU are the one doing the conversion, you can specify what you want. Remember to always leave a little extra room for encoder error (rarely are they able to hit the target rate right on the money).

You, of course, also have to make sure that the 2nd program in your channel isn't peaking too much at the same time as the 1st program's peak, but if you are hard limiting those peaks so they never either one get to 17Mbps, then you should be OK.

The syntax for getting the right target bitrate (Peak & Average) will change depending upon which MPEG2 encoder you are using. Are you using ffmpeg for that? If so, show an example of your current syntax. Me, I would recommend other encoders for better MPEG2 (HCEnc, QuEnc, TMPGEnc, Mainconcept, etc.).

Scott