Render as... producing AVI high quality question

mfhau wrote on 8/3/2003, 1:42 AM

I just need to get my mind around this - I render a 10min veg to a PAL avi which then produces a 2.4GB avi file which I then show using a data projector and it is fanstastic.
After this I create SVCDs for sharing and playing on DVD players - slight quality drop here but totally acceptable - mpg is aroung 200MB (using TMPG).
OK - no problem here - what I can't put my finger on is what makes up the avi as opposed to the mpg. Is the avi "overbloated" and how does it comapre to actualy dvd quality and the subsequent variations. I have read a lot of stuff - I am just trying to understand the difference between the avi and the svcd file size.

Comments

farss wrote on 8/3/2003, 2:57 AM

Good question.
DV only compresses at the frame level, at the time the system was designed that was as much smarts as could be engineered into things like cameras given that the compression system had to work in real time.

MPEG1 and 2 use both spatial and temporal compression. With mpeg say you have 10 seconds of video where nothing changes, only one frame of data is required and some little bit of data that says, display for 10 seconds. Obviously its a lot more complex than that but thats it in a nutshell!

MPEG1 can only use Constant Bit Rate (CBR) wheras MPEG2 can use Variable Bit Rate so in the example I gave above after the first frame of the 10 second freeze the bit rate can drop to zero. For many reasons it probably doesn't.

Technically both MPEG1 and 2 are capable of higher resolution than DV, DV resolution is fixed in the spec. MPEG1,2 and 4 permit very high resolution and colour depth. Thats not to say they are always used that way.

If you want to know more there's plenty of info about these topics, mostly though its hard work wading through it, I hope my very simplistic example gives you enough of a clue to make some sense of it.

Grazie wrote on 8/3/2003, 3:07 AM

Farss - Excellent! I've read loads on this stuff, mostly went over my head! - But your "10 Second" example drove it home for me. Chances are, if it had this effect on me, it will be of similar effect on others. "K.I.S.S." - I werote the manual on this . . .

Regards - Grazie

mfhau wrote on 8/3/2003, 4:12 AM

great - excellent explanation
so when the slight quality drop off happens going from avi to svcd mpg this is as a result of the rerendering??
and this can be improved by tweaking TMPG or whatever encoder you are using - correct??
this is not an an add for tmpg but how does it rate against VV/main concept?

thanks

Former user wrote on 8/3/2003, 8:34 AM

As Farss mentioned, DV is compressed on a frame level. The compression is about 5 to1. In MPEG, the compression rate is much higher. Most of the frames are made up of the information around them. As Farss also said, MPEG is capable of higher bitrates, but honestly I don't think you can ever make it look better than the original, be it DV or VHS. Compression will always introduce some quality loss and artifacts. In some cases, these do not affect your viewing, but in other cases they are very noticeable.

DV is no more bloated than say a CD track compared to an MP3. Compression makes some assumptions about what can be thrown out and what can be kept in order to keep a small file size, with a certain quality level.

DaveT2