Beth Cataldo, March 22, 2007

Uncompressed digital video has a high data-rate, which DVD drives can’t read. MPEG compression solves that problem though. And that’s where encoding comes in.

DVDSP Formats:
MPEG video dramatically reduces the digital video size while maintaining the full motion, resolution, and visual quality of the source video file. DVDSP accept two of MPEG versions, MPEG-1 and MPEG-2. MPEG-1 video has a relatively low bit rate (1.4 Mbps with a maximum of 1.8 Mbps). MPEG-1 is also restricted to a resolution of 352X240 pixels, less than half the resolution of broadcast-quality video.

MPEG-2 video is the preferred format for DVD-Video titles. It has a resolution of 720X480 at 29.97 fps with a maximum bit rate of 9.8 Mbps. The Mbps reflects the amount of data going through the DVD player. Therefore, the higher the data rate, the better the quality of picture.

Choosing bit rates:
Using the Quicktime Pro Encoder with Compressor, you can compress at bit rates ranging from 1 to 9.8 Mbps. The higher the bit rate, the better the quality of the MPEG stream and the larger the file size. Scenes with a lot of motion need a higher bit rate to keep up with the changes in data. The DVDs that you're creating actually have only 4.37GB worth of files. This is due to the fact that there are 1024 kb in a MB. This adds up, so a 4.7 GB DVD-R that we're burning actually only story 4.37GB.

MPEG-2 video allows a maximum bit rate of 9.8 Mbps. But don’t ever set your bit rate higher than 8Mbps because you need to leave room for audio and subtitle streams. DVD data rate is 10.08 Mbps, meaning that all the DVD data rates at one point in time can only add up to 10.08 Mbps. Also, some DVD players won’t play MPEG-2 video encoded at bit rates higher than 8Mbps. Computers have an even harder time keeping up with bit rates greater than 6 or 7 Mbps. Most people suggest that a bit rate between 5.4 and 6 Mbps will offer the best quality with smooth playback on most devices. Using the new Quicktime MPEG 2-pass encoding, you can get away with a bit rate of 5.6 Mbps.

Bit-rate encoding:
Quicktime Pro encodes at a constant bit rate or a variable bit rate

Constant Bit Rate (CBR): In a constant-bit-rate encoding, picture quality fluctuates while the bit-rate remains constant. CBR compresses your video at a constant rate, say a 5.4Mbps. With fast motion, you will see a lower-quality picture as compared to a still image or talking head.

Variable Bit Rate encoding (VBR): Variable bit-rate encoding keeps the picture quality constant while changing the data rate. It allocates only as many bits as needed to low-motion scenes while high-motion scenes are given more bits.

< home    next>