Maximum encoding quality...

[dylanpank@msn.com]

Just a general thought, one which may have been answered elsewhere, but I thought I'd ask anyway (haven't found it on a brief search)

When asking for maximum quality for an MPEG-2 file I'm returned a value of 3742kb/s for PAL files (576, 25fps) yet commercial DVDs generally have much higher data rates (usually around the 6-7Mbs second unless constrained by space).

What I wonder is, is the 3.7Mb/s datarate the best quality that MPEG-2 requires, or merely the best FFmpegX (via the various engines), or, more convoluted still, is the quality of FFmpegX at 3.7 equal to those commercially authored DVDs at 6-7Mb/s (which, my guess, probably rely on realtime hardware MPEG-2 encoders).

That said - I've seen some pretty appalling commercial DVDs - checkout the PAL Northfork DVD for some pretty appalling artefacts!

[Tommyknocker]

Was this for two hours? Remember, DVDs are on 9 GB disks.

[dylanpank@msn.com]

This is a general point, not related to a particular project, but 2 hours is a reasonable benchmark to use.

Anyway a 2 hour mpeg2 file (plus a 384kb/s sound track) will fit on a single layer/4.4GB DVD with a datarate of about 4500kb/s but FFmpegX flags this up as an excessively high "Blue" datarate. Setting the FFmpeg maximum recommended "green" rate will bring in a final movie size of about 3.5GBs with soundtrack, sometime less depending on how much the second pass can save.

However most two hour commercial DVDs come in at about 6-7GBs for the movie (give or take a few 5.1 AC3 sound tracks that is). I just wonder what the discrepancy between what FFmpegX thinks is the best quality and what "Hollywood" thinks is the best quality is.

[Tommyknocker]

I think he is saying the max bit rate he can get for a two hour MPEG is around 3.7, while commercial two hour DVDs are are around 6-7. Since the commercial DVDs are working with twice as much disk space a single-layered disks, this sounds about right.

[dylanpank@msn.com]

That's not it, because you can set the disc size in FFmpegX to 8000Mb or simply chose 2 x 4GB discs as your destination.

Anyway as I said, the maximum recommended quality is significantly LESS than can fit on a single sided 4.4GB DVD anyway.

[DeusExMachina]

I could be wrong, but I think the discrepancy revolves around constant bit rate vs. variable bit rate.
Many commercial DVDs use constant bitrates, so the value you see is the value they spit out. If you use variable bit rate, the bitrate from the bit rate calculator is a benchmark, but not set in stone. The actual bit rate value can go as high or higher than 9000 kbs, and as low as or lower than 2000. If you encode a file as VBR, and have the bit rate set at, say 2500, then after the encode open the resulting file in ffmpegX, you may find that the perceived bit rate is significantly higher than 2500. Mine generally tend to register at 9000 kbs, even when I encode them at 2500. This also depends on which encoder you use. Ffmpeg seems more apt to ignore your specified bit rate.

Hard won knowledge from a previous issue. But then again, maybe I never understood that one, either, so major, feel free to interject with appropriate corrective zeal.

[major]

mpeg-2 has no standardized quality/bitrate levels, and the standard does not define a reference encoder. So each particular implementation of an mpeg-2 encoder will have its own efficiency, that is, a given quality for a given bitrate.

ffmpeg is very efficient, and the "green" values (which should be taken as average bitrate values, there can still be higher VBR peaks) are based on quantization and PSNR observed behavior. You can of course increase the overall (video+audio) bitrate up to 9.8Mbps which is the upper limit for DVD, but this will not bring much noticeable increase in quality by using ffmpeg in respect to "green" values. I believe that if you exceed by more of about 20% over the green limit, there will be "no" increase in quality. mpeg2enc is however less efficient, and you may gain some more quality with it if you increase bitrate AND decrease Qmin.

Please note that ffmpeg (as well as mpeg2enc) does not ignore the bitrate value. It will try to conform to it as an average bitrate (as far as allowed by the quantization range).

[dylanpank@msn.com]

Thanks Major, that's useful info.