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I get it's a variable bit rate, and the actual size is a guess.
What doesn't make sense is the random change in quality that's not reflected in the settings. And if I choose 'high' then the dual layer size jumps to 4Gb, so it's clearly not maximising the bit rate at 2.3Gb.
where does it show the actual bit rate?
If I'm happy with the quality at 1.7Gb, where can you set the same quality for the dual layer setting?
Again, where's the logic in changing the bit rate behind the scenes between single layer and dual layer?
As you already knew the actual bitrate is
variable. your
logic may make perfect sense for contant bitrate (CBR) profile (which is not offered in power2go), but it might not be applicable for variable bitrate encoding.
The profile you selected that still labelled with a bitrate in number is just a compression level reference and the profile won't give you an absolute bitrate because the actual bitrate is not available before the video really got encoded and compressed that are based on vidoe scene complexity and other graphic processing factors scientifically.
Technical wise, video compression (encoding) is very complicated in mechanism and it is a big topic, and variable bitrate encoding does not act on 100% precise or contant number basis.
You may consider the profiles a general quality level reference.
For example, if your video is just full of purely black or white static scene for an entire hour, you can try selecting a profile that labelled with extremely high bitrate, the produced video won't be encoded with the high bitrate you selected.
In my own test, the dual layer DVD that produced from a short source video (less than 10 minutes) via power2go, the maximum bitrate of the produced VOB videos is a bit higher than videos in single layer DVD folder, but the average bitrate is the same between these two disc folders.
To save your time and effort to dig in the bitrate differences, produce your project into disc folder first, then burn the folder into discs. Or, use smartfit to produce your DVD discs.
But, if you really need a disc production tool that produced video disc in precise and contant bitrate, you may need to look for other software. The file size will be precisely measured before the disc produced, and the produced bitrate will be identical to what you selected.
For what I know so far, Cyberlink's power2go or powerdirector all act on variable bitrate basis to encode or produce videos.