HOW GOOD is VCD?

Basic Information

This document applies to PAL VCD. The data storage requirements pertaining to the PAL VCD coding scheme are outlined in this document.

Total data rate: 1150000 bits/sec=143750 bytes
Audio: 224000bits/sec=28000 bytes
Video: 143750-28000=115750 bytes
This means and average of 115750/25=4630 bytes per frame for video.

Compression comparison and data storage requirements

The PAL VCD resolution for video is 352 pixels wide by 288 high at 25 frames per second. Audio is encoded at 224 kbits/sec.

If video at that resolution is stored in as uncompressed 24-bit RGB values that would require
For video: 352*288*25*3=7603200 bytes/sec
For audio: 44.1kHz, stereo, 16bit = 44100*2*2=176400 bytes/sec
=7779600 bytes per second in total uncompressed.

This means that the compressed data on VCD takes up a fraction of the uncompressed data.

(143750/7779600)*100=1.84778%
That gives us a 98.152219% saving!

Analysis

It is unknown if this bitrate was chosen purely to allow the use of standard one speed CDROM players which were the cheapest available for consumer electronics in 1994 by Philips or if it is generally agreed that it provides acceptable audio and video quality.

The problems of VCD such as low capacity i.e. only 66 minutes of video per disc, no subtitles and no interactive capabilities (other than skip forward\backward and pause at the start of chapters) are all fixed by the higher capacity of DVD and extensions to the coding scheme. I think that sub-picture data is used in MPEG for interactive features on DVDs. NOTE: sub-picture data may be stored in some "ancillary data" part of the coding scheme. I'm not sure on the exact details though.

DVD is based on MPEG2 which only adds 16*8 macroblocks to the MPEG1 coding scheme. Better visual quality is available on DVDs because they are generally mastered at much higher resolutions and bitrates made possible because of the "much higher than CD" transfer rate of entry level DVD players.

Summary

In summary VCD is very good, particularly when transferred from digital sources. When used to store from VHS tapes an excellent video capture card is required or a lot of hard disk and time is required to convert uncompressed video to MPEG1. Real-time MPEG1 and 2 video capture cards that do not shear or drop frames are either very expensive or thin on the ground.

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