Changing buffer size of a encoded video

[Martí]

Hi,all!

I recently bought some home made VCD's and they play a bit choppy. I saw (with bitrate viewer) that they have a buffer size of 20. How can I change this buffer size to the standard 40? (I believe this could resolve the trouble, as the other parameters of the video are good).

Thanks in advance,

Marti

[Truman]

VBV buffer size 20 (40KB) is enough for a VCD standard (352x240, 1150+224 kbpsCBR, GOP 15). You can change it by reencoding only.

[kinneera]

I'm pretty sure you can change the buffer size by re-multiplexing with bbMPEG.

[Truman]

I hoped it would be that easy, but no, it isn't.

[Martí]

Thanks a lot!

Martí

[kinneera]

Well perhaps in the hopes of learning something then, I was wondering if you could explain to me what the "Buffer Sizes" fields under "Program Stream Settings" do in bbMpeg? The defaults for VCD (46) and SVCD (230) correspond to the reported correct values for VBV for their respective stream types.

[Kdiddy]

I believe ithe "buffer size" is the buffer size of the overall program stream...not just the video in respect to BBmpeg. Example, I was trying to make an mpeg2 XVCD for my neighbor's player. With CCE, you cant change the buffer size, its default to 112 for mpeg2, I thought however, if I muxed with BBmpeg with a VBV of 40, it would change this. Well I got massive PTS/DTS underflows, but I allowed it to continue anyway. When burned & played, it played very choppy. This same method done with TMPG (which allowed me to set my mpeg2 VBV buffer to 40), worked just fine. That is my only "knock" on CCE, I wish it would allow for VBV buffer adjustments in its video streams.

[Truman]

I can not explain it yet. I just noticed that I got buffer underflow errors (picture freezes when playback or played very, very choppy), when I forgot to change the VBV buffer size for 480x480 XVCD (~2000kbps).
Here the default 20 (40KB) is definitely not enough, I changed it to 112 (224 KB). But there were no way to change it by multiplexing.
Remember TMPGEnc.Beta12? Under MPEG tools there is a VBV buffer size optimization tab. Since TMPG do not have this option anymore, I do not believe, that it works properly. But I have to "repair" a couple of XVCD, so I will try it sooner or later.

[kinneera]

I too discovered that bbMPEG is not the tool to use for multiplexing non-standard VCDs, but I was never able to isolate the exact problem.

The reason the buffer size issue still doesn't make sense to me is because a buffer isn't actually encoded. After all, a buffer is just a temporary space created in memory during decoding to mediate between an unpredictable I/O stream and a process that needs a predictable stream, is it not? So logically, one would think that the buffer is just a variable specified in the header somewhere, which could be changed if necessary. But perhaps it's not so simple...

[VidGuy]

If you can find TMPGEnc v12 (not 12a, but 12), in it's MPeg Tools it had a menu thing called "VBV Buffer Optimization".

I don't know exactly what it did, but whenever I burned a VCD that had Buffer overflows on muxing from BBMPeg, I use this feature and the overflows go away.

[Truman]

Well, the VBV buffer size is written to the MPEG sequence header. When you mux with bbMPEG and change the buffer size there, the information in the sequence header wont be changed. Yes, as mentioned you can try to run it through the TMPG tool "VBV Buffer Optimization". It writes two new headers, "system headers". This may help or not. I am going to test it today.

[Truman]

OK, the TMPG tool "VBV buffer optimization" worked for me. It changed the VBV buffersize of 2 XVCD from 20 (40KB) to 46 and 48. Now I can play it without stops or freezes.

It isn't that easy with SVCD, that's might be the reason why this option was kicked. According to the standard (MP@ML) the maximum VBV buffer size is 112 (224KB). After "optimization" you may end with a larger VBV buffer size easily.
Also it workes with one video and one audio stream only. If you muxed 1 video and 2 audio streams, one audio stream will be deleted. Also the tool will write 2 new system headers, one for each stream. Normaly it is one system header for both streams.

So I went a different way. I run the elementary video stream
through "VBV opt", so the new .mpv is actually a (video only-) system
stream, and muxed one video and two audio streams afterwards with TMPG. Everything seem to work fine, the new VBV buffer size is written into the sequence header and there is only one system header for all 3 streams. Since the .mpv isn't a real elementary stream anymore, I can not use the bbMPEG muxer. I am not too sure whether the TMPG muxer work properly at all.

However, I think you can try it to "repair" a MPEG if you have no other chance, but generally it is better to take care to do a proper encoder configuration.
Hope it helps.