CCE and Quantization

[MarkGrigsby]

I'm converting some high quality source material from high-res to DVD-standard, and am getting some strange results from CCE...

I am setting a 'quantization factor' of 60 for 1-pass VBR, and when I convert 'true' 720p material (by feeding 30 progressive frames to CCE), the average bitrate is about 6Mbps, which is what I'd expect. However, when I convert IVTC'ed 720p material (by feeding 24 progressive frames to CCE), the bitrate is really low - about 3 or less.

Am I missing something, or does the 'Quantization factor' need to be set carefully depending on whether the input material is e.g. film-based or studio based?

Would appreciate advice in this complicated area!!

[johns0]

It all depends on the film itself,29.97 fps with no pulldown at 720x480 full screen with lots of action and color will usually require a lot of bitrate while dark slow moving widescreen will take less.

You will just have to do some quess work to get a close bitrate.Lower the q till you get a higher bitrate,

[MarkGrigsby]

I get that, but why for the same 'Q' value does my high quality 30fps 42 min clip produce an output of 1.8Gb, compared with the 24fps 42 min clip size of 700mb?!

[johns0]

Basically i just told you,files encoded this way are unpredictable due to the way they encode.

[MarkGrigsby]

Ahh, I think I understand now (sorry, this is all very new to me and soooo confusing!).

Is this right.... The 'Q' factor in CCE gives it an indication of how important quality is to you (lower the better). Then, when it encodes, it uses the amount of bitrate that IT thinks needs to be used to achieve that quality. Because of this, certain films (e.g. with lots of dark sections) can almost 'fool' CCE into using lower bitrate because the it thinks the 'quality' is good enough. But a resulting encode at 3 Mbps is never really going to match DVD quality, so the Q factor needs to be adjusted in such cases to force CCE to up the bitrate.

Sorry for my dumb questions, but your help is MUCH appreciated

[adam]

Quantization is essentially the amount of information that will be thrown away. A highly compressible movie will use less bitrate at the same Q value then one that is less compressible. You should not be trying to aim for a certain bitrate, rather you want to aim for a certain Q value since this is more or less representative of the quality of the output. If you are satisfied with a Q value of 60 (I'd personally try to get it lower) and the filesize is smaller than expected then all the better.

Personally I prefer to use multipass encoding instead so I can maximize my bitrate ensuring that I get the lowest possible Q value with that source. But 1-pass encoding like you are doing can produce excellent results in about half the time as well.

Like Johns0 says, if you are doing quantization based encoding then bitrate will vary, often drastically, according to the compressibility of your source.

[MarkGrigsby]

Thanks Adam! You mention you'd aim for a Q value lower than 60 - what would you usually use?