Compression Rate = Quality... Rule of Thmb?

Question

Are there any rules of thumb for compression rates and resulting quality? For instance, I've heard 5Mbps is the bottom end for "as good as commercial movie DVDs". I guess measures that describe "quality" are subjective or varied... but terms that are significant to me are things like "Movie DVD's" "Betacam" "VHS" "no perceptable loss, even in motion scenes".
I often need to decide if a client will need one or two disks, based on the proposed amount of footage, before production. I make a rough bit-budget ahead of time. So, if the bit-budget calls for a 4.2 bit rate and that is too ugly for what I think my client needs, then I'll need to go to 2 disks. So, knowing in advance the bit rate -to- quality relationship would be very helpful.
Let's say the constants are:
Decent DV footage, let's say "average" screen action
Edited on FCP
Compressed with Compressor-2, Constant Bit Rate, to Mpeg-2
Thanks, Ralph
Desktop G4 / 1.5 G / Mac OS X (10.3.6)
Desktop G4 / 1.5 G / Mac OS X (10.3.6)

Answer 1

Not really. A lot comes down to what you see on screen when you try a few test encodes. Higher data rates don't always equate to higher quality - particularly at the upper end of the scale... can you see a difference between 8MBPS and 9.4MBPS, for example?
The fact is that it is only through experience that you get to know what tends to look good for the material you have got... therefore encode lots of things and see how it turns out!
Quite often the client will demand a single disc set, for example. Your experience will tell you that their 4 hour sport video won't look very good and so you have to have that conversation with them. No matter how good the source footage is, when you reduce the bitrate too far to get a file that fits the disc they want the quality will suffer - but you can turn to MPEG1, half D1, etc to help get them to replication, of course.
In general, the middle of the bit rate scale will be a good enough place to start. If you have really clean source footage then a software encoder like Compressor might do the job alright, but a better encoder, such as BitVice, MegaPEG.X, Canopus Procoder will do a better job with the same bit rates. A hardware encoder like the sonic SD2000 might give a different result again, whilst Cinemacraft could be set up to run a multi-pass VBR encode over a period of several days and squeeze every last drop of quality out of the footage.
There are simply too many variables involved, and I'd be sceptical of a 'one size fits all' approach, I'm afraid.
5MBPS for 'as good as commercial DVDs' is nonsense in this context. 5MBPS on really clean betacam source footage run through an SD2000 is a dream compared to 5MBPS on noisy, poorly lit mini-DV put through Compressor... it's unwise to make generalisations, IMO.