Minimum MPEG2 bitrate for DVD-R

[cosmin]

Hi,

I noticed that some MPEG2 encoders have a "minimum bitrate" in VBR mode (not just a maximum that has to be at most 9.8Mbps). Is it significant for DVD compliance, or is it just something that affects quality? (Speaking of quality, I cannot imagine why the user should constrain the bitrate to a minimum of, say, 2Mbps, when the encoder can figure out that something lower than that would be enough).

What's even more confusing, is that the default presets of those encoders set the minimum bitrate to 2Mbps. Is it a mistake to override this and set it to 0, (and let the encoder use as much as it needs, even if that's below 2Mbps)?

Best regards,

[macleod]

from my understanding, the minimum bitrate for dvdr is 300kps. the reason is not because of dvdr compliance, but rather some dvd players have problems below that amount.

[SaSi]

As macleod said, you could select a min bitrate as low as 0bps and challenge the encoder to find spots that need as low a bitrate. I think only CCE can go very low in VBR encoding, although, even theoretically, 0bps is impossible due to MPEG packet overheads.

I have seen, and follow suggestions that a min bitrate of 200bps is advisable, to ensure bitrate exists at all times.

As you said, setting min bitrate at 2Mbps is stupid and harms quality, especially if the average bitrate is close by (say at 3.5 Mbps). It defeats the chance the encoder has to use bitrate cautiously.

I use a min. setting of 200bps, a max. setting of 8Mbps and an average depending on target size (but never below 3Mbps). And if I need to go close to AVG:3Mbps, I use CCE at 3 pass VBR.

[cosmin]

Thanks.

Now it's clear to me that I need not worry about DVD standard compliance. As a matter of fact, if the VBV buffer size is set up properly (224kbytes for DVD), I don't even need to worry about a minimum bitrate. I will just set it to 0, meaning "it can be as small as possible". I know it will never be actually 0, because the VBV buffer will have to have something in it all the time -- but it's the encoder's job to take care of it.

[Skynet107]

I can't speak to the compatibility of having a min. bitrate setting of zero, but I see no reason why this would be a problem.

There IS a good reason to use a min. bitrate setting of zero. On a low bitrate mpeg2 file (for example a 3.5 hour movie made to fit one one DVD) using a setting of zero conserves the bits as much as possible. Black scenes, fade to black scene transistions, end credits etc. these are all instances where a very low bitrate is acceptable and conserves bits. If you watch the bitrate of a DVD encoded with a variable rate of 0-3.5mbits/sec it is easy to find sections where the bitrate hovers around 1.5-2.0 and sometimes even dips to near zero. And I agree CCE is very very good at low bitrate encoding. TMPGEnc by default sets the DVD compliant file to be 0-8000 mbits/sec.

[DJRumpy]

I agree. I also use a 0 min all the time. I've encoded at least 30 DVD-R's with no compatability problems to date using various friends players, as well as my own (one which is from the DVD beta era, and one that is spaking new). I use CCE for encoding though.

Just remember that your audio counts towards your bitrate total, so there will always be bitrate from that stream. In addition, even excluding overhead for the headers, even a black frame takes bits to encode. Every I-Frame in the MPEG stream (typcially at least 2 per GOP give or take), is a full frame encode, meaning it also takes bitrate to encode each I-frame, even with no motion. These factors alone would always give you more than 0 for a bitrate/per second.

Stick with 0