Subject: Multimedia System

Topic: Video Compression Techniques

Difficulty: Medium

MPEG-1 adopts the digital TV format also known as SIF (Source Input Format).

1 to 1.5 Mbps data rate

200 to 250 Kbps for audio: 1.15 to 1.2 Mbps for video

Suitable for traditional CD-ROM throughput rate

MPEG-1 supports only non-interlaced video. Normally, its picture resolution is:

• 352 × 240 for NTSC video at 30 fps
• 352 × 288 for PAL video at 25 fps
• It uses 4:2:0 Chroma sub sampling

Group of Pictures GOP

The number of frames /Pictures between successive I- frames is known as GOP .It’s given the number N and typical value of N are from 3 through 12.

Here, size of GOP is 12 for frame sequence IBBPBBPBBPBBI.

The MPEG standard specifically defines three types of pictures (frames) :

• Intra
• Predicted
• Bidirectional

Let’s consider these in turn.

Intra Pictures

Intra (or I) pictures, are coded using information only found in the picture itself. I-frames provide potential random access points into the compressed video data. I-frames only use transform coding and provide moderate compression ratios, typically 2-bits per coded pixel.

Predicted Pictures

Predicted (or P) pictures are coded with respect to the nearest previous I or P-frame. This technique is called forward prediction. Like I-frames, P-frames serve as a prediction reference for B-frames and future P-frames. However, P-frames use motion compensation to get a higher compression ratio then is possible for I-frames. Unlike I-frames, P-frames can propagate coding errors, because P-frames are predicted from previous reference (P or I) frames.

Bidirectional (or B) pictures use both a past and future picture as a reference. This technique is called bidirectional prediction. B-frames provide the most compression and do not propagate errors because they are never used as a reference. Bidirectional prediction also reduces the effects of noise by averaging two pictures.