Quality versus datarate

[sussertown]

When I need to make a video smaller I often wonder if I should reduce the quality or lower the datarate. What's the diff?

[Ken Berry]

None. In effect, lowering the bitrate will lower the quality of the final product.

However, if you are talking about the 'quality' slider control, which is set by default to around 70 or 80%, then that too may affect the final quality, though it is essentially a control which balances the speed of processing of your processing with the care the program will take in doing so. So if you set the slider to 100%, this means the processing will take longer but the program will be more careful in what it does, and so the final product should be better. In that sense, it is a little like the two pass encoding setting, as opposed to single pass. With two pass, the program will, in the first pass, analyse the project and work out the best way of encoding it, and it does the encoding in the second pass. The final result is thus theoretically better than with single pass encoding, though in truth, more often than not, you might not be able to tell the difference with the nake eye. The big difference between the two, of course, is that two pass encoding naturally takes more time.

The default setting is meant to represent the best balance between final quality and an acceptable processing speed. But if you speed things up by lowering the slider below the default setting, then the program will correspondingly do things faster and errors -- and correspondingly lower quality -- may begin to creep in.

I personally never move the slider from its default position, but often will vary the bitrate depending on what quality I can expect from the initial input (e.g. there is little point in trying to push analogue video any higher than 6000 kbps since you simply can't get any better quality out of it above that rate. 4000 kbps in fact is probably more than acceptable for analogue source material). It also depends on how much I might want to fit on to a disc. Lowering the bitrate will also increase the amount you can fit on the disc, you see. Precisely by lowering the final quality, it also lowers the size of the final file.

[Ken Veal]

the Speed/Quality slider in the compression tab of the video save options box controls the file compression.If you drag the slider to the left you lose some quality but keep the "fluid motion" of the video.
The data rate is how much info your PC deals with
per sec and you may not notice much difference in quality if you redudced it from 8000 to 6000 to get more video per hour on a DVD
regards Ken

[DVDDoug]

The data rate is how much info your PC deals with per sec

I'd word that a little differently... Its the amount of data for each second of video.

Since MPEG is a "lossy" compression scheme, some data is thrown-away during compression. A 4000kbps file does not contain as much information as an 8000kbps file.... More has data has been thrown-away with the smaller, more-compressed, file, so it will have lower quality.

The same is true of MP3 files... MP3 is also lossy compression. There are non-lossy compression techniques, but you can't get as much compression that way ... you end-up with a file way to big to fit on a DVD. (The ZIP format is non-lossy compression.) The real trick with MPEG and MP3 is that they throw-away the least-important data...

Note - Of course, you cannot increase the quality of a low-bitrate MPEG , by re-coding it at a higher bitrate.... You can't get back the thrown-away data! In fact, that will probably make it worse. It will get fully decoded (de-compressed) and then the encoder will start throwing-away data again!

[sjj1805]

See also the 7th post down
http://phpbb.ulead.com.tw/EN/viewtopic.php?t=11923