transcoding; no visibile difference in quality, or is it just me?

[xizar]

In my effort to save hard drive space, I've transcoded some AIC stuff to H.264 using mpeg streamclip, and am trying difference settings.

Using two different quality settings, I'm getting a 2.2gig file and a 200meg file, and then going back to look at the differences in quicktime.

Here's the thing: I can't see one. Finder says that the data rates are 50kps vs 7kps. (These number are all approximate. Except the .264.)

Am I as inept a videophile as an audiophile? (MP3s sound as good to me as cassettes and stuff.) The text looks as legible in one as another.

Some possibly relevant things are that these are screen captures from World of Warcraft on a 25x14 screen scaled down to 12x7 to begin with, and I'm watching all of this stuff on my computer. Also, anything I do with these is destined for YouTube.

 

[smokescreen76]

h.264 compression is a very efficient codec and if done well the data that is thrown away is mostly visually invisible.

The way that the human eye works is different to the way that a computer reads data.

- So you can throw away every other red pixel and the human eye barely notices. There you go - you just saved some space.
- Throw away some blue pixels - again the human eye can't tell the difference.
- That bit of sky is repeated for at least 10 seconds - just tell the computer to play the same bit of video for ten seconds - you've just saved a ton of space.


There are two problems with doing all this. All those calculations and instructions being carried out are pretty intensive on the computer processor. You need a pretty powerful computer to play back large h.264 videos.

The other problem is that if you re-encode the video (something that happens all the time in video editing and grading) then the data falls apart very quickly. Those pixels that were thrown away are gone for good and cannot be recovered. It can look pretty nasty.

Hope this helps you understand.

 

[jtara]

AIC is designed with editing in mind. As such, there is no temporal compression. Not even lossless temporal compression. Frames are compressed, but each frame is complete in itself.

MPEG4, on the other hand, does temporal compression. That is, not only are frames compressed, but some data is compressed ACROSS MULTIPLE FRAMES. A "history" of several frames is needed in order to reconstruct any frame.

So, a lot of the bloat in AIC has more to do with making things convenient for editing than it does with perceived quality. AIC needs about 4X the data/bandwidth of a codec (such as H.264) that uses temporal compression.

Therefore it's not surprising that you're seeing less degradation that you might have expected.

Edit: I see now you are comparing different settings for your H.264 compression. smokescreen76 has provided a great explanation of what you are seeing.

 

[xizar]

Thanks a bunch for both of your explanations.

I'm not going to worry too much, then, about "needing" the larger version for this kind of stuff.

Out of curiosity, is there a "better" format that I can use for stuff I need to keep around without having to set aside a lot of drive space?