Whats the difference between constant & variable.

[roy wood]

Hi all, what difference does it make in visual terms between a Bitrate set at 8000kbps Variable and 8000kbps Constant.

Also what are the Visual differences between the Quality slider being set at 70% or 100% in Share/ Create Vid File/ Compression. Does it make the Video Sharper,Steadier,Smoother or what? Please.

[maddrummer3301]

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[2Dogs]

Hi all, what difference does it make in visual terms between a Bitrate set at 8000kbps Variable and 8000kbps Constant.

I trust you are not an aging glam-rocker!

It's almost the wrong question. Variable bitrate will use higher compression and therefore lower bitrate on parts of the video that don't change much - it's really noticeable if you make up a slideshow, where using variable bitrate will make the finished file size much smaller. So you might have asked "what difference does it make in visual terms between a Bitrate set at 9000kbps Variable and 8000kbps Constant?"

You can then use that two ways - you can fit more footage on a DVD, or you can use a higher maximum bitrate, to improve the image quality a bit. Since the DVD standard limits the maximum bitrate, however, for most videos around one hour long (for a single layer DVD) you won't see much difference in image quality between a DVD made with constant bitrate and one with variable.

The big downside with variable bitrate is that it's best to use two pass encoding, which takes almost twice as long.

[roy wood]

Thanks for your replies guys. So if I've got a lot of movement or stills use fixed bitrate otherwise not a lot of difference. Regards Roy.

[2Dogs]

So if I've got a lot of movement or stills use fixed bitrate otherwise not a lot of difference. Regards Roy.

Actually the reverse! If you have lots of movement and/or stills, use the highest variable bitrate you can - but you'll only see any significant benefit if your project is long enough to limit the maximum constant bitrate, and of course the render time is roughly doubled for two-pass variable bitrate encoding.

For example, assume you have a 90 minute project, with lots of stills. Using constant bitrate (and a single layer disk) you might be limited to a maximum of 6000 kbps to be able to fit it onto the disk.

Choosing variable bitrate might allow you to use 8500 kbps. You should be able to detect a discernible difference in image quality between 6000 kbps CBR and 8500 kbps VBR.

Most of the time it's easier and quicker to just go for CBR.

[roy wood]

Thanks for coming back on that 2Dogs. I think I've just about got my head around it still 'You don't have to be an electrician to turn on the lights just now where the switch is' Thanks again Roy.