Check out our sponsor!
Commonwealth Network

Reviews menu bar

Navigation Bar

 

DV Compression Experiments
08/13/98

Running an image through DV compression codecs multiple times

by Kent Borg <easiersaid@borg.org>


Wondering about how much my DV video will be damaged with repeated compressions, I tried a series of experiments. I started with a small TIFF graphic which has been used in similar tests by various people on the DV mailing list. <
www.DVCentral.org/thelist.html> Certainly hard edged graphics are not what DV was intended for--DV likes natural scenes far better--but graphics do occur in television. I used Adobe After Effects to make 15 generations of recompression using each of three DV codecs:

  • Radius Soft DV NTSC (which came with Edit DV 1.1),
  • Apple's DV NTSC (part of Quicktime 3.0)
  • Adaptec's DV Soft (from Adaptec's Hot Connect driver for their AHA-3945 SCSI/1394 combo card, downloaded from their web site)

The first thing I discovered is that this is not yet a mature technology. First, the Radius codec doesn't like Quicktime 3.0. Not only does it crush the contrast with each generation, it also highjacks Apple's DV codec preventing use of it. I have heard that there is a fixed version of Radius' software, but I can find no mention of it on their web site. The Radius codec does work with Quicktime 2.5, so these tests were done under Quicktime 2.5. Second, Apple's codec gives terrible quality images--at least if you are so simple minded as to install Quicktime 3.0 and double-click on a DV compressed movie. The obscure solution was to find the "Goodies" plug-in from my Quicktime 2.5 folder and copy it to my Movie Player 3.0 folder, then do a "Get Info" in Movie Player, adjust the popups to read "Video Track" and "High Quality" and check the "Enable High Quality Box". And do that for every DV movie I want Movie Player (or After Effects) to interpret in other than muddy-mode. (Jeeze! I remember when the Mac was easy to use -- Is My Mom expected to figure out this!?!) Note, this "High Quality" option does not appear in Movie Player 2.5, so this is really obscure.

Third, the Adaptec codec, which works under Quicktime 3.0, seems to ignore the "High Quality" bit and simply produces the same quality either way. It seems unfortunate to ignore the setting, but heeding it would be worse.

The results showed up two important facts. First, there is image degradation with repeated trips through the DV codec. This seems obvious for a lossy codec, but some others have thought otherwise. Second, all DV codecs do not produce the same results. Though the DV standard is precise about how DV data is to be decompressed, there is latitude in how DV video is compressed.

Who wins in this test? Radius' DV codec clearly looks best if you don't mind the blue halo off the right edge.. Second place depends on a judgement call. The Adaptec DV codec has color smearing to the left, more with each generation, but though Apple DV codec has no color smear, it looks more out of focus.

What does it all mean?

First, what you can see on a computer monitor is different from what an NTSC TV monitor can show. And by the time you consider what a VHS tape can show on your average TV set these codecs will all pretty much look the same. But don't give too much ground to this argument, for things you might want to do in post production, such as matting, will look ugly--even at the VHS stage--if you start with VHS-quality source material. It is always good to stay as high quality as you can through out production.

Second, there is no sensible reason to ever run 15-generations of DV compression in a real production. Unless you want to remind people of the early days of desktop publishing and the font crimes committed on early Laserwriters, you mostly want to make simple cuts from one shot to another, and cuts-only work introduces no extra DV compression steps; for most final frames your camera's original DV compression might well make it out the master DV print with no additional compression-related losses. For those who can't resist making material that looks like a nervous music video, think through your compression path. Work uncompressed for intermediate renders, or avoid intermediate renders by stacking all your effects together at once.

Reduced (and jpeg compressed) results are reproduced below. Click on each strip to see the three examples at actual size (also jpeg compressed). A Stuffit archive of single frame DV movies, and the original TIFF graphic, are also available: <gentest.sit 308K> I encourage you to look at these images on your own NTSC monitor.


Adaptec 15, 8 and 1st generations (above top to bottom)
Click to see actual size images


Apple 15, 8 and 1st generations (above top to bottom)
Click to see actual size images


Radius 15, 8 and 1st generations (above top to bottom)
Click to see actual size images

 

Copyright 1998, Kent Borg
Reprinted with permission

 

| Multimedia Workshop | Industry News | Online Reference | Contact Us | Find |

Copyright 1998 by Multimedia Workshop. All rights reserved