CBR is not constant after all, and my DVD player freezes

Hi all,

I recently posted in this forum about my JVC DVD player freezing when it got to scenes of Titles produced by MS Movie Maker 2 using TMPGEnc for the MPEG encoding of the DV-AVI file ( https://www.videohelp.com/forum/viewtopic.php?t=172413 ). However it did not freeze on those scenes when my VAIO software “Click To DVD” was used for the encoding. CBR at 8kbps were used in both cases (PCM audio). Also, other DVD players tested and PC players such as WinDVD played either encoding just fine. I’ve since determined the problem, so I thought I would share what the real culprit is.

Using the tool Bitrate Viewer (great tool for discovering my problem), I was able to see that CBR really doesn’t produce a constant bit rate after all (by either TMPGEnc or C2DVD). That was news to me. I noticed that a scene (transition from miniDV footage to a blank, black screen) that caused my JVC player to freeze went from 7602 to 6730 kbps within a second when encoded by TMPGEnc. This same scene went from 7317 to 6945 kbps in the MPEG from C2DVD. Now, I’m a newbie at this (though I’ve been doing a heck of a lot of research for a week), so I came up with a theory that DVD players may have only a limited tolerance for changes in the datarate after being told that it is constant (I assume CBR or VBR is something that is established up front to the decoder by the MPEG format?). And some DVD players have higher tolerances than others, and some encoders will have less variance from a constant rate than others. So, in my case, TMPGEnc gave more variance in a CBR than my JVC was able to deal with.

I tested this theory by encoding with TMPGEnc using VBR instead of CBR, thinking if the player is expecting VBR in the first place, then bitrate changes will be fine. Unless otherwise specified, my other settings are based on the advice given at http://dvd-hq.info/Compression.html . Here is what I found using my JVC XV-SA75GD:

1) 2 Pass VBR 2min – 8max kbps 6kbps average, 224 VBV (no padding):
----- Played fine.

2) 2 Pass VBR 2min – 8max kbps 6kbps average, 0(auto) VBV (no padding):
----- Played fine.

3) MVBR set at 8min-8max VBR (padded) (to simulate CBR at 8kbps):
----- Player froze again at the same blank, black screen

4) AutoVBR 2-8kbps padded:
----- Played OK, but the text titles on otherwise blank screens flickered.

5) 2 Pass VBR 2-8kbs 6 average, padded:
----- Played OK, but the text titles on otherwise blank screens flickered.

6) 8kbps CBR 224 VBV
----- Player froze again at the same blank, black screen

7) Using MPEG created by Click To DVD at around 8kbps CBR
----- Played fine.

Conclusions:
+ VBR at 2-8kbps, 6 average (NOT padded) has the best playback on my JVC.
+ TPMGEnc CBR from and VBR with a constant bitrate range do not work on my JVC
+ Padding[/list]the bitrate will cause ficker in simple title screens.

Has anyone out there seen similar problems with CBR not being constant, or have any more informed light to shed on the hypothesis that DVD players have different tolerances for CBR variance? Even if not, I hope this posting might someday help someone like me who was baffled about their DVD player freezing on simple blank or title screens using CBR.

Thanks,
Jay

Even though CBR stands for constant bitrate, that's really not what it is. All CBR means is that the buffer remains constant, basically there are still fluctuations in bitrate just not as much.

Now changing from ~7mbits to ~6mbits is nothing. DVD players should easily be able to handle changes from 9.8mbits down to 500kbits. That's what its internal buffer is for, and I guarantee you nearly every commercial DVD on the market has such a drastic bitrate change on it at some point. Also the constance of the bitrate should not matter either. All DVD players are required by spec to be able to play a constant bitrate of 9.8mbits, of course this doesn't necessarily apply to burnt media which is harder to read.

I have to admit that your results seem strange. I don't see how making your stream more or less like CBR should affect playback, but it seems this is what is happening. I hate to be pessimistic, but I honestly just think the problem lies with TMPGEnc in general. I have run countless test encodes and I have rarely had any luck getting a DVD compliant mpg out of it. The problem could be with the setting of the VBV, TMPGEnc has had numerous bugs with this in the past, or it could be with TMPGEnc's GOP settings which seem to randomly break the rules.

TMPGenc is a very high quality encoder, I just find it far too inconsistent for DVD encoding so I guess you will just have to keep experimenting with your various settings until you find one you like.

Hmmm... And I thought I had read nothing but good things about TMPGEnc. My delima is that I am producing this DVD for about 30 people, and I would like to minimize any compatibility problems with that many DVD players. So far, the VBR results from TMPGEnc have worked on the three DVD players that I've had access to. Your preferred encoder for DVD is MainConcept, eh?

I read that commercial DVDs almost always use VBR, and my player has thus far had no problems with these. And the VBR encoded MPEGs by TMPGEnc have huge bitrate changes - from 7Mbps to 300kbps in about two seconds (that's another strange thing - TMPGEnc seems to ignore my request to have a 2Mbps lower limit - assuming I can believe Bitrate Viewer). Still, even with this huge change in bitrate, my player has had no problems with VBR. It only seems to have problems when the encoding is supposedly CBR but has too much variance from a constant value.

Oh, and it didn't have to be a fade to cause it to freeze. It was simply an instant switch from normal miniDV footage to a black, blank screen.

TMPGEnc seems to set VBV to 112 if left at 0(auto) for my movie clip. I believe the other guide said to set it to 224 if done manually. Not sure if that is anything to worry about, but I'm open to any suggestions you may have about VBV settings or any other options that might make my DVD compatible with as many DVD players as possible.

Thanks for your reply,
Jay

112 is 224 , just a different way of saying the same thing .


i go with what adam and racer-x says ..

"from 7Mbps to 300kbps in about two seconds" <-- thats a good thing , means your encoder is working proper ..

bit rate is per GOP avg, at a certain frame it can be higher or lower ..

Yes -- ive seen broken GOPS also in TMPGEnc , but some versions have little/less of this -- the last version is not one of them ...