User Guide

Factors Affecting Video Compression 303
Image Quality
The encoder’s ultimate objective is compressed video that looks as close to the source as
possible. For DVD, this is full screen (720 × 480 for NTSC and 720 × 576 for PAL), full frame
rate video (24 or 30 frames per second), at 24 bits per pixel (16.7 million colors). Image
quality should be the factor that you consider most. Go with what looks the best and meets
the data storage and transmission requirements.
Factors Affecting Video Compression
Determining compression needs requires an understanding of how video factors (see
“Digital Video Components” on page 300) affect encoding results. Generally, there is a price
to pay for quality. With more colors, higher resolution, faster frame rates, and better quality,
the more horsepower you will need and the more storage space your video will require. By
adjusting these factors, you can dramatically change the digital video compression
requirements.
Filtering video noise and removing frame redundancy are two areas in which the source
video can be improved or modified to make compression more successful. This section
includes the following topics:
• “Prefiltering” on page 303
• “Inverse Telecine” on page 304
Prefiltering
Noise in video streams is basically random information. Since MPEG-2 is based on detecting
similar information between frames, noise represents a problem because it can be totally
random on a frame-by-frame basis. Noise can come in the form of grainy film, dust, snow, or
extremely detailed textures such as a white stucco wall or a waterfall. By applying a digital
noise reducer or low-pass filter prior to encoding, high-energy noise can be reduced,
resulting in a video stream with random information and is therefore easier to encode.