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To: Joe NYC who wrote (34907)	4/5/2001 10:05:30 AM
From: pgerassi	Respond to of 275872

Dear Jozef: Bit rate is the speed of a stream usually measured in bits per second. Mb/s is in millions of bits per second. VCD by definition have the same bit rate as a CD-ROM 1.2Mb/s or 150,000 bytes/sec. Some streams including MPEG-4 DVDs have variable bit rates and usually are specified by their maximum bit rate. Typically they are no more than 1.35MB/s or 10.8Mb/s. Flask is another form of the DIVX encoder. One of the arguments sets the maximum allowed bit rate. So you should specify 1.2Mb/s for use on a CD. That allows you to have a movie at least as long as the minutes specified on the CD (80 minutes of movie on a 80 minute CD or 700MB). Take your movie for example. 720x576x25 translates to 248.832 Mb/s bit rate raw RGB data. To fit that onto a CD at 1.2Mb/s means a compression ratio of 208:1. If you add sound, that means a compression ratio of about 256:1. That is a very high compression ratio and requires substantial processing to do. MPEG-I usually achieves 150:1. That is why, MPEG-I usually specifies 320x240x30 for NTSC quarter screen (VHS quality) and 360x288x25 for PAL quarter screen as this reduces the amount of compression you would need by 4 times or 64:1. The increase of resolution is typically lost anyway unless the source is landscapes and other very slowly changing shots. MPEG-4 does not look any better at these high compression ratios either and typically the slow DVD encoder is preferred over the fast DIVX encoder. An increase to DVD speeds at 10.8Mb/s yields far more detail and allows the use of full resolution 720x576x25 screens as it only requires a compression ratio of 24:1 or so. The variable bit rate can yield further ratio reductions by allowing bursts of less than a few seconds to allow for high motion or action scenes while more highly reducing the less demanding landscapes and tiny talking actors against a fixed backdrop scenes. I hope this helps your project. Pete