Question about variable and constant bitrates

[JamieLee2k]

I have been for a while using Mpeg Encoder (mainconcept) to encode (S)VCD's to DVD and I get to the point where I have to either choose Variable Bitrate or Constant Bitrate and this is where I get confused sometimes, I have searched the forums in this and found nothing to even give me a decent answer.

If you choose Variable Bitrate then I would think it sets it at default (whatever that may be)
If you choose Constant Bitrate then it gives you an option to choose your own bitrate, but How do you really know what to choose.

I do normally choose Constant bitrate and select it anywhere in the range of 3000 to 4500, but I have never figured out why I have to choose them. (just followed a guide to get the bitrates).

If I choose a lower bitrate would that lower the quality? and whats the difference between, higher and lowering the bitrates?

I have noticed that changing the size of the bitrates does change the size of the final product and it's really hard to get it to fit on a DVD, but after using DVD2One I can shrink it down no problems.

[Mr Grumpy]

Put simply, because that's all I can do, the bitrate governs how much information about the 'picture' is stored, and generally, the higher the bitrate, the better the quality.

MPEG films don't carry all the information about every frame - they work on the principle of knowing what information has changed from the previous frame, which unless the image changes completely each frame change, reduces the amount of information stored.

Now, Constant bit rate (CBR) use the same "bandwidth" (quality) across the film, regardless of how much the frames are changing, whereas Variable bit rate (VBR) varies the bit rate, allocating higher bitrates to the action scenes and lower to the still scenes. In VBR you normally choose an average bit rate which governs how big your final MPEG will be, and you can think of this as the same as the bit rate in CBR (i.e. use the same value as you would if you were going to do CBR). You usually also define minimum and maximum bit rates (to fall within the set standards or the capabilities of your player)

Generally, I think VBR gives better results, but the trade off is it takes longer to encode as the encoder needs to perform at least 2 passes through the file.

There are manual ways to work out the bit rate so that it fits on a CD/DVD, but I always use Fit2Disc (or it's free version FitCD or one of the other free bitrate calculators) - they are worth their weight in gold for a lazy git like me!

Well, that's the first time I've ever tried to put that into words - hope it makes sense.

[FulciLives]

There is a nice bitrate calculator on this very site:

https://www.videohelp.com/calc/

Just enter the running time of the clip you are encoding along with the bitrate of the audio you will be using (MP2 and AC-3 are roughly the same size at the same bitrate so for calculation purposes it doesn't matter which you use allow AC-3 will sound better than MP2 in general).

The program will then give you the bitrate you should use.

This applies to both CBR and VBR. For VBR the bitrate value given should be what is used for the AVERAGE. For a DVD the MIN should be 2000 and the MAX should be 8000 give or take.

- John "FulciLives" Coleman

[Vejita-sama]

Check out what I wrote about CBR vs. VBR for x(S)VCD encodes:
https://www.videohelp.com/forum/viewtopic.php?t=169977

Basically CBR (constant bitrate) uses the same bitrate through out the entire encode. While VBR (variable bitrates) raises or lowers the bitrate during the encode. For a VBR encode you enter three values:

min - the minimum bitrate the encode will drop to
average - the desired average bitrate
max - the max allowed bitrate during the encode

During a VBR encode the bitrate will be decreased during low motion scences (but not below the min value) and raised during high motion scences (but not above the max values). It will be raised/lowed so that it works out to the average value. Hence you can still predict file size (ave bitrate x runtime).

Max = DVD player max 9800
ave = from bitrate calculator
min = 300~500

you want to set the min REALLY low to give the encoder more room to play with. The encode will most likely never drop that low (remember it has to average out) but it does allow the encoder to go to the max bitrate during really high motion scences should the need arise.

For encodes >6000kbit/s (some would set the number lower) just use CBR as it's much faster. If that is if your encoding at near max bitrate anyway there's no need for a VBR encode.

[FulciLives]

I know that even commercially made DVD discs dip below 2000kbps from time to time but I thought that the "pure" DVD spec called for a MIN bitrate of 2000kbps?

So to be safe this is what I use when I encode material to MPEG-2 DVD standard.

I'd like to hear some feedback on this issue though because it does make sense to make the bitrate even lower AS LONG AS it is in spec and not going to screw up on even SOME stand alone DVD players.

- John "FulciLives" Coleman

P.S.
I am off course talking about setting the MIN bitrate for VBR encoding.

[adam]

Neither the VCD, SVCD, nor DVD specs have a min bitrate required, except for the obvious fact that some bits have to be applied otherwise there is no data at all. You can go as low as you want and still be compliant.

Many commercial DVDs drop down to as low as 500kbits during scene changes. All dvd players are required to play the full range of bitrate allowed, which basically means anything up to 10.08 total and 9800kbits video, and actually all decoders can easily decode well in excess of even 15MBits per sec because they are required to for raw channel data. So even 100kbits should be absolutely no problem for any hardware dvd player. All of the stories about people having problems playing back low bitrate encodes are probably largely due to user/software error (TMPGenc!!!), and if not that then its probably dvd-/+r/w, cdr-rw incompatibility.

Every SVCD and DVD I have ever made used a min bitrate of 0, and I have yet to find a compatible player that had any problems.

[ZippyP.]

Every SVCD and DVD I have ever made used a min bitrate of 0, and I have yet to find a compatible player that had any problems.

My Pioneer has problems. I set the minimum at 300 with padding enabled to avoid it.