DV Compression Quality Differences?

December 21, 2005 12:53 PM

DV video compression question - Do I have to recapture all these tapes?!

In anticipation of the demise of my digital-8 DV camcorder, I'm capturing all of the tapes I shot with it, (about 40) into 20-minute (4.4GB) QT movies in DV/DVC Pro format. I'll be exporting those files to DVD for archival storage.

Here's my problem: I'm about 20 tapes in and I've discovered that each time I relaunch my capping program, myVCR, it returns the compression settings to the default, (DV compression; 29fps; 720x480; medium quality; 16:48 sound). The problem is that I want high quality and I fear that I've capped over half my tapes at the medium quality settings.

As I understand it, DV compression has a constant data rate of 3.6MBytes/sec, reagrdless of any settings. Is it possible that the quality slider in the video compression dialogue doesn't really do anything? What would ever be the point of compressing a fixed 3.6MB/sec stream at any quality other than the highest?

The bottom line is this: is there a difference in playback quality between a DV stream captured at Medium Quality and a stream captured at High Quality, if both output files have the same data rate and the same file size? Are other factors changed by the Quality setting? Frame rate?

Thanks for reading all this, and extra thinks to those who actually understand what I mean.

posted by squirrel to computers & internet (9 comments total)

Well I don't have any familiarity with the actual program that you are using, but I have done my fair share of capturing.

It seems simple but you might want to just look at two video's side by side, on the PC and outputed to a tv to see if there's any discernable difference.

The medium quality setting might be a grouping for a set of settings, that by some glitch in the program comes up again each time you relaunch, but leaves your previous settings intact? or perhaps that setting is for one of the other compression types (sorenson, divx) and has no bearing on the DV compression.

The screen size, frame rate, and sound settings all seem high quality, except for that mysterious "medium quality" setting.

I hope you don't have to recapture... I know what a pain that is.
posted by Mark5four0 at 2:29 PM on December 21, 2005

The data is spooling from your tape directly to the disk and is not being recompressed, since it starts in DV format and ends up in DV format. I'd bet it's getting reset to "medium quality" because your camcorder is telling your computer "hey, this is medium-quality video." That is, medium-quality likely refers to the bitrate that the camcorder is using.

Try recapturing one that was captured at medium quality at high quality and see if its size is any different.
posted by kindall at 3:47 PM on December 21, 2005

kindall: that's the funny thing about DV. It has a constant data rate of ~3.6mBps regardless of the settings. All my hour-long captures are 13gb, which I chop and resave (note, not export, not recompress) them into 4.2gb chunks for export to DVD-R.

So my first caps, which I know to have been at High Qual and my later caps, which I know to be Med Qual both have the same data rates and the same file footprint size.
posted by squirrel at 3:52 PM on December 21, 2005

Maybe this will help clarify. Or maybe not.
posted by squirrel at 3:58 PM on December 21, 2005

If the size is the same, then you're all set regardless of the quality slider, unless you are noticing actual quality differences.
posted by kindall at 4:45 PM on December 21, 2005

What kindall said. I'd compare a pair of HQ/MQ caps side-by-side, and if there's a difference in file size then you'd have to recapture. But I suspect that you've already captured in the best format possible.
posted by goodnewsfortheinsane at 5:12 PM on December 21, 2005

What kindall said. If you're capturing from a DV camera over FireWire, the date on your disk is the same as the data on the tape, regardless of settings*. It's just data.

(* Unless you're using a crummy program)
posted by cillit bang at 6:19 PM on December 21, 2005

There is zero difference between compression qualities with a fixed data rate codec. It's an I-Frame CBR codec. Don't sweat it. The slider does nothing. Really. It exists as a default for standard QT codecs (but can be turned off by the developers of a codec.)

Last, DV is DV is DV.
DVCPRO 25 is the same as miniDV is the same as DVCAM is the same as Digital Hi-8. (the data is the same really.)

If you capture the footage...you don't need to recompress it...just store the QT files and you should be able to edit with them sometime in the future.
The only difference is a tape storage of locked vs. unlocked audio.
16 bit, 48khz.
posted by filmgeek at 9:53 PM on December 21, 2005

Thanks, everyone. Filmgeek, I was hoping you would show up!
posted by squirrel at 9:41 AM on December 22, 2005