Constant Bit Rate vs. variable Bit Rate

[venkmanincognito]

I have been told that Variable Bit rate results in higher quality videos than Constant Bit rate. If you set CBR to 2500 kb/sec and the vbr to 2400 avrg, 2500 max, and 1500 min will VBR really be better than CBR? I am just confused because in some frames there will be less than 2500 kb/sec in VBR but in CBR every sec is 2500. I am not particularly concerned with file size as much as quality so I was wondering which one is better and why. Thanks.

[Mirror_Image]

You've actually answered your own question, which brings up another question:

With all other factors being equal, how could having a bit rate that is occasionally equal, never higher, often lower, and sometimes much lower, possibly give better video quality?

[venkmanincognito]

So when making a SVCD the maximum bit rate is about 2500 kb/sec (I think). If your length of video is small enough to encode the whole thing at 2500 kb/sec (eg. 25 min or so) you will get better quality w/ CBR @ 2500kb/sec than VBR no matter what settings you use for VBR. That is what I thought but I had been told over and over again that VBR gives better quality but takes longer.

[adam]

Even if your not gaining any bitrate ex: cbr 2.5mbits vs vbr 2.5mbits, there are still some benefits to using vbr as long as its multipass vbr. The added passes aid in motion search precision and I'm sure there are some other benefits as well but all of these are very minor and not really noticable.

If your video will fit using cbr ~2.5 mbits then there is really no reason to use vbr, it will only take longer to encode.

The point of vbr is to achieve the SAME level of quality that you can get with cbr but in a much smaller filesize. For instance if you encoded at an avg of 2mbits you would probably get about the same quality as you would get with cbr at 2.5mbits but you would be able to fit considerably more content onto each disk with vbr.

[kinneera]

You can also use single-pass VBR/constant quality with a max of 2500Kbps, and then the encoder has the discretion to conserve bits when it determines they aren't necessary to maintain the perceived quality of the video. This allows you to fit more video into the same space without requiring too much more time than a CBR encode.

[Nelson37]

As I read the SVCD spec, it calls for an AVERAGE bitrate of no more than 2500. Although some players cannot handle bitrates much over 2800 or so, many can exceed this to a great degree. Comparing CBR 2500 and VBR Max 2500 ignores the point of VARIABLE bitrate. If you consider that perhaps 10% of the flic would benefit from a bitrate over 2500, but that benefit would be a major improvement, comparing CBR 2500 with VBR AVG 2500 MAX 2800-3500 will show the improved quality attainable.

You really need multiple passes to achieve this improvement (try CCE).
The actual numbers will vary with filesize and quality requirements, I often use min 500, avg 1600-2200, max 3000. The only one I vary is the average, and consistently obtain very good quality encodes. Others will argue various methods, to which I can only reply " try it and let your eyes decide".

[adam]

The svcd specs say absolutely nothing about avg bitrate. They call for a max video bitrate of no higher than 2.6Mbits and a total max bitrate (video + audio) of no higher than ~2748kbits.

Since most people use audio in their encodes you have to drop your video max bitrate a little to stay in spec, 2.5Mbits is logical number to use.

Some dvd players can handle max bitrates as high as 3.5mbits as you say, my apex definitley does, but I choose to remain within the svcd specs so that I am always guaranteed compatibility on all Svcd players.

2.5Mbits cbr vs 2.5Mbits vbr is actually a pretty good comparison, at least as far as this discussion is concerned. Both of these settings represent the best you can realistically get, as far as bitrate is concerned, out of a svcd.