Imported HDV2 MPEG2 files appear to have wrong bitrate

[Mark Fry]

I'm not sure if this is the right place to ask this. It is quite a technical question about MPEG2 files.

A friend of mine has sent me some files on disc from his HDV camera (Canon XH-A1). He imported them into a recent version of Studio and exported them as MPEG2 .MPG files, posted me the disc and went off to the States for a month.

Playing them in VLC Media Player, it tells me that the video is MPEG2, 1080x1440, 50 frames (that's actually interlaced fields, not true frames, I guess), 4:2:0 and the data rate fluctuates around 25000 kbps, which is exactly what one would expect for HDV2 footage.

When I import these files into my NLE, Avid Liquid 7.2, the clip properties agree with what VLC says except that the bitrate is shown as 35 Mbits/sec. The reason I care about this is that when I fuse the finished sequence (to author a Blu-ray Disc in TMPgenc Authoring Works 4), Liquid insists on re-rendering these clips which always results in some loss of quality and occasionally makes a pig's ear of it!

I don't get the same problems when I import files from my Sony MRC1 CF-card recorder. They are M2T files rather than MPG.

I've used MediaInfo to look at the MPEG parameters associated with the video stream in various files. The significant difference between files that work OK (including AL fuse outputs) and the ones that give trouble appears to be the BitRate_Maximum/string setting. In the good files, it's "25.0 Mbps" whereas in the files that give trouble, it's "35.0 Mbps". This is in contrast to the BitRate/string parameter, which ranges between 24.4 and 25.3 Mpbs in the files that work OK, and is 24.9 Mbps in one that gives trouble, and which appears to match the numbers displayed by VLC when I play the files.

It appears that what I need to do is reset BitRate_Maximum to "25.0 Mpbs", ideally without re-encoding either the video or the audio. I've tried re-multiplexing the file, using VLC and AW4, but this doesn't have the desired effect. Can anyone suggest a suitable program to do this, or a better way around the problem?

If all else fails, I'll just have to wait until my pal gets back from holiday and see if we can figure out a better export method from Studio.

(Yes, I know that Liquid is an obsolete product. I've stuck with it because it works rather well and fits into a very easy HDV-to-BluRay work-flow, and makes pretty good DVDs into the bargin. One day I'll have to change my camera and then I'll need a new computer and NLE, too.)

[Mark Fry]

The good news is that I have found a program, called ReStream, that edits MPEG2 video-stream parameters. You have to de-mux programme- or transport-stream files first, but it successfully changed the BitRate_Maximum parameter, so that AL7.2 and AW4 reported the files as "25 Mbps". Both programs imported and played the "fixed" file with no issues.

The bad news is that this doesn't make any difference to Liquid when I fuse a sequence containing the "fixed" footage. It still re-encodes it, and it still makes a mess of the same bits. So back to the drawing board, and almost certainly back to the original Studio import/export. I've been looking very closely at all the footage over the last few days, and I've realised that Liquid is just exaggerating faults that were already present.