question about vbr v/s cbr and 2 pass vbr

[perfection]

1) Assuming i have a 20 minute xvid movie with a low bitrate of merely 1500 kbps. this means using DVD specifications of maximum audio and video bitrate it could easily be re-encoded to fit on a dvd5. In SUCH cases does 2 pass VBR give really give better quality than CBR.

2) Is it correct to understand that vbr iencoding s a better quality option when the data is higher than the dvd disk capacity and therefore needs to be squeezed. This happens because the scenes that requires lower btrate for storage use leser disk space thereby allowing better quality of high birate scenes and therefore an overall a better quality and higher length permissable ontothe destination media - is a right understanding of the concept??

3) How is 2 passing better than a single passing vbr encoding

[guns1inger]

You are making a lot of assumptions without fully understanding all that you are dealing with. A few extra facts to throw into the mix to help you along

1. At the maximum permitted bitrate for audio and video, you can fit approx. 65 minutes on a single layer, single sided DVD. There fore your 20 minute video will easily fit three times over.

2. Xvid/Divx compresses more efficiently than mpeg-2. In order to maintain the same quality as the file you have now, you will need to use around 4 times the bitrate. So a 1500 kbps Xvid will require around 6000 kbps on average to maintain the around same quality.

3. Every time you encode a video with a lossy codec you do damage. How much and how visible varies and depends on a lot of factors.

So now to your questions.

1. Given you could be using a CBR encoding at around 8700 kbps for this video, CBR is probably as good, if not better than VBR in this instance. Once you start looking at bitrate in the low - mid 7000's or less, VBR starts to really show it's value.

2. Mostly right, but again it depends on what bitrates you are looking at, and what the content of the video is. If the footage is just some guy sitting at a desk talking for three hours, and nothing else happens, you might be able to CBR at a low bitrate and still get a very good image.

3. 2-pass or multi-pass (CCE allows for more than two passes) has several advantages over single pass VBR. In 2 pass encoding the first pass analyses the video content to work out how best to allocate the data available. The second pass does the actual encoding. This has two advantages. The first is that you get a very predictable and accurate filesize. Very important when you are encoding for DVD. The second is that the data is allocated where it is best needed. Single pass encoding can often starve the video of data at inappropriate times either because it is saving data early in case it needs it later, or because it blew it's load early and now doesn't have enough left.

[perfection]

Thanks

where can i get the dvd specs on max video and audio bitrates for DVD format?

[redwudz]

To the upper left on this page in 'WHAT IS' DVD.

[guns1inger]

It is worth noting that some older or cheaper players can have problems reading the data fast enough from burned discs if you use the full allowable bitrate. Many recommend you limit the video bitrate to the mid-8000 range.