On XVCD does increasing only the bitrate improve quality?

[mattscholey]

Hi,
I'm making XVCD's using TMPGEnc. I can change the bitrate to 2500 and this will play on my DVD player but if I change the resolution it won't play on my DVD player (green squares, frames freeze, sound speeds up). What I'm wondering is does increasing the bitrate alone improve video quality, or is increasing bitrate pointless without increasing the resolution? (If increasing bitrate alone improves quality I'd be fascinated to know why.) And can you suggest a way to get over this problem of playing clips with increased resolution?
Any help would be great.
Thanks
Matt

[vcdfreak]

Yes, bitrate is the most important factor to reduce compression artifact(blocks) in the video during encoding.

Increasing the resolution will demand higher bitrate bandwidth due to most information/data is needed to stream through the video system every second. Increasing the resolution without increasing the bitrate will hurt the video quality.

[vcdfreak]

Correction: 'more information/data' instead of 'most information/data'.

[EddyH]

Raw video even at standard VCD resolution uses a huge amount of data (about 8mb/second! (64000kbit!)) that even the fastest CD drive would be pushed to supply... VCD video is heavily compressed with lossy MPG compression so a large amount of the data is chucked out to make it undemanding enough for a primitive little 2-speed system to cope with... (and so more than two minutes can fit on each disc) - hence if you've got 64000kbit/s going down to 2500kbit/s rather than 1150, far less of the video data will have to be compressed away, and it'll look a whole lot better.

For a demonstration of this without having to sit through a lot of encodes, take a high rez scan of a photo and save it to various qualities of JPEG, from max to minimum, see how the apparent 'quality' or (urgh-that-looks-compressed-to-hell-ness) of the result changes in respect to final file size

Higher resolutions mean higher original data rates that need to be dealt with, too - for example the less common of the 'high' DVD resolutions, 704x480, is four times the size of VCD - so about 32mbyte/s or 256000kbit/s!! (and here's me making vcds with average rates of 512kbit ).. so for the same 'apparent' quality as 1150kbit, you'd have to increase the rate to 4600k. Of course it doesn't work quite like that with the oddness of mpg compression and the advances made in MPG2 (it's more like 3600k..) but you get the idea. Compressing it at the same final size for four times the picture would look a lot worse.

For all that though, MPG is very efficient at what it does, and the 50-ish times reduction in filesize can be very hard to tell with a decently encoded video... there's a lot of redundancy in what the human eye takes in!

[Dodzzz]

Yes, with 2500 kbps at 352 the video gets more stable, no more wavy blocks, mosquitos, etc..