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Frame types and compression

You can reveal many fundamental concepts of compression by looking at the kinds of

frames in a file. Depending on the codec and options used, one, two, or three kinds of

frames may be in a file. A key frame, or I-frame, is a complete image that is

compressed using intraframe compression. An I-frame isn't based on any other frames

in the file. The first frame of a movie is always an I-frame. A delta frame is

compressed using interframe compression and contains only those portions of the

adjacent frames that are different. If two frames are mathematically identical, and the

second one is a delta frame, it will require only a few bits to encode. But if more than

half of the image is different, most codecs automatically generate a new I-frame. For

typical content, a delta frame might be only 10-20% the size of the same frame at the

same quality as an I-frame.

There are two main types of delta frames: P-frames and B-frames. The content of a P-

frame (predictive frame) is based on the previous frame; the content of a B-frame (bi-

directional frame) is based on the previous and subsequent frames.

Audio compression

While audio may use only a small fraction of the bits in a compressed file, it is

responsible for half of the experience.

Sampling rate

The most fundamental factor in audio is sampling rate, which is measured in Hertz

(Hz), or cycles per second. Audio CDs use 44100 Hz, or 44.1 kilohertz (KHz), which

provides excellent reproduction.

Sample

Used

8 KHz

Phone system

11 KHz

Old multimedia standard

22.050 KHz

Also old multimedia standard

32 KHz

Common in broadband video

44.1 KHz

Audio CD

48 KHz

DVD, DAT

96 KHz

High-end audio recording

Bit depth

Bit depth is the number of bits used for each sample in each channel—the higher the

bit depth, the better the audio quality.

• 8-bit sampling This bit depth, which was responsible for that harsh sound in the

early CD-ROM titles, produces fairly poor quality audio. There is no reason to

Sufficient for

Speech

Better speech

Recognizable music

Minimum for decent music

Music

Music mastering