I don't use the MainConcept encoder that comes with Vegas, so someone else can jump in here with specifics. But in general...

CBR means that your video bitrate remains constant, even if some scenes are very easy to encode (no motion, or black) and other scenes need all the bits they could get (lots of detailed motion, lots of video noise). CBR is obviously simple to understand and execute, and you know exactly how large your MPEG2 file is going to be if you know how long your video runs and the bitrate.

VBR means the encoder looks over your video frame-by-frame. It then decides to give only a few bits to those simple frames described above, and gives a lot of bits for the difficult frames. In this way you maximize quality for a given average bitrate.

Some encoders just kind of guess at the bitrate as they go along, so you never know precisely just how big the finished MPEG2 file will be at the outset. Other encoders use 2 or more passes to get a precise handle on just where the bits are needed the most. With this approach, if you say you want 7Mb/sec average, you'll get EXACTLY 7Mb/sec averaged, since after the first pass the encoder knows what overall size you are expecting.

Your DVD also contains an audio program. You have a choice of audio formats. The simplest solution is to just use the uncompressed wav (PCM) audio, which eats up about 1.6Mb/sec, which is a lot if you only have about 6Mb/sec total bitrate (video+audio) to work with. So people use Dolby AC-3 to compress the audio. Then it eats up only 0.2Mb/sec. Much better.

The 600 thing:

The average bitrate you need to shoot for is just 600 divided by the total number of minutes of your video. If you have a 90-minute masterpiece, then your average total bitrate has to be no greater than 600/90=6.67Mb/sec. But that total includes the audio program (0.2Mb/sec), so the video portion has to be no greater than about 6.5Mb/sec. If you're doing CBR encoding, that's what you set the bitrate to; for VBR encoding, I'd set the minimum=0, average=6.5, maximum=9.

That's all there is to it.

Note that 600/60=10, so this means that for any DVD with less than 60 minute's worth of material you could, in theory, run at a 10Mb/sec bitrate. The problem is that this is at the upper limit of the DVD spec, and some players with marginal specs choke on such a rich bitrate. Some people say the average bitrate should never be higher than about 8.5-9Mb/sec. With these bitrates it makes little difference whether you choose CBR or VBR.

To clarify a point here, the speced bitrate is for the total mpeg stream including all audio, subtitles etc. I think the spec requires a player to handle 10.8 MBits / sec so allowing a bit of room for the audio is how the 9 MBit figure is arrived at. I'm told you can push it a lot higher and SOME players will cope.

There is a freeware program around somewhere that'll give you a graph of the bitrate of an mpeg file and many of the Windoz DVD players will show you the bitrate during playback, good way to see how VBR does it's thing.

For sure VBR is the way to go, why encode a lot of data for say a 3 second title, if nothing changes from frame to frame then no data is needed in the video stream.

Also I have a freewhere bitrate calculator which seems very accurate at telling me which bitrate will JUST fit the files onto the DVD.

What exactly happens to the bit rate if you encode a black screen for a few seconds yet have your minumun set to 5mbs? In other words if your clip only gives out 3mbs but you tell it to do 5 what will fill in the other two? I imagine that even the most simple clip will be greater than 10mbs so there will always be some compression.
thanks,
Carson

THANK YOU ALL SO MUCH for your input. You have all helped to clarify a LOT!

I just have TWO follow up questions.

When talking about setting the bitrate to 6.5, 9, etc. I get confused because in Vegas's mainconcept MPEG2 custom settings box, the settings for CBR/VBR are in the millions. So are you meaning 9,000,000 (but just talk in single digits cause it's quicker and easier - kind of like KB and MB and GB)? Perhaps it's a dumb question, but I had to ask.

This leads into my second question, in DVDA is THAT bitrate (also a single digit decimal number) ALSO abbreviated for millions? You know what I'm trying to ask here? I hope so.

Thanks again!

Generally speaking, VCD/DVD video bitrates are always going to be between 1,000,000 and 10,000,000. So, if you see 6.5 or 6500 you can assume 6,500,000 is what is meant. 6.5Mbps, 6500Kbps, and 6,500,000bps are essentially (more or less) equivalent. Yeah, there's the 1024 vs. 1000 thing going on, but it doesn't make enough difference to notice.

Cheroxy, in your 3mbps -> 5mbps question, the encoder is going to decode the 3mbps stream to an uncompressed stream, then recompress it to 5mpbs. So it's not really filling in the extra space; it's decompressing the stream.

Unless the encoder is capable of 2-pass VBR (which vegas isn't) you're better off just going with a CBR. Normally when you are doing a VBR 1-pass you don't really gain that much becasue the encoder doesn't know exactly where to reduce or increase the bitrate. It just kind of does it on the fly. With 2-pass it can go through and analyse the video first, then it knows precisely where you need a higher bitrate.. I have seen MANY cases where a CBR will look much better than a VBR even on some high end encoder cards. If you have the space on the disc you're better off going with a CBR over a VBR.

Jsnkc: You might be right, I don't know much about the MainConcept encoder. I would have assumed, however, that ANY MPEG2 encoder would show improvement in quality in VBR --if-- there was more than 60 minute's worth of material i.e. the average bitrate had to be less than the maximum allowed under the DVD rules.

You're right about the "1-pass" situation: the only way the encoder can do a good job of allocating more bits here and fewer there while meeting an overall size target would be if it would be allowed to first look over the entire avi on a first pass. The actual allocations would be done on a second pass.

But some encoders aren't allowed to make two passes. So instead, they have a mode where they let you set a "quality level," and then make their bit allocation based on what traditionally has been necessary in order to meet that quality level. The gotcha is that the encoder has no idea how big the final file will be, since it has no way of knowing if your original avi is comprised of simple scenes or complex ones. So in that case you need to estimate what quality level you desire, then look at the final file to see if it will still fit on your DVD blank. If it's too big, you need to go back and reduce the quality level. If there's lots of room to spare, you go back and increase the level.

The CinemaCraft encoder that I use allows one to do up to 10 passes, though there is little to be gained after 3. I have read that the newest MainConcept encoder is pretty good, and maybe it will be incorporated into the next version of Vegas/DVD-A.

I've only used CBR because I thought this might be the case. That, coupled with a number of issues reported by some users of other applications (DVD workshop comes to mind) that seemed to go away if done in CBR. I haven't produced anything longer than an hour, so space isn't as important to me as it is to some.

I'm sure there are definite benefits to PQ using multi-pass VBR when circumstances demand. The quality of the original material seems to be a much more significant factor than compression scheme employed however.

VBR for me is essential if I'm putting 90 or 120 minutes on a DVD-5 blank. There is a HUGE difference between a 120 minute program encoded at 4.8Mb/sec CBR and the same program encoded at min=0, avg=4.8, and max=9 in CinemaCraft's VBR mode. The former is horrible, and the latter is almost completely artifact-free.

The issue will be alleviated to a certain extent when we get dual-layer blanks by this time next year.

Since you are talking about a small video just test for yourself. Render at the default VBR setting, then try a few CBR runs like 7,8,9. 9 may be getting a bit risky unless you know for sure that the target machines will run it. Anyway, I essentially did the same thing for my videos and decided I couldn't tell a difference so I always go with the default VBR setting. Now i'm using footage shot with home digital camcorder. I think it would take higher quality source than I have to really differentiate the encoder used. I tried TMPg encoder which many people seem to like, but it was much slower and I didn't notice any difference.

I always got too much judder with the CBR settings on the Main Concept encoder in Vegas so I went back to VBR and it was good. I use 8, 6, and 192kc with 31 quality.

JJK