Editing Video CD (section)

== Technical specifications ==

===Structure===
Video CDs comply with the [[CD-i Bridge]] format, and are [[Optical disc authoring|authored]] using tracks in [[CD-ROM#CD-ROM XA extension|CD-ROM XA]] mode. The first track of a VCD is in CD-ROM XA Mode 2 Form 1, and stores metadata and menu information inside an [[ISO 9660]] [[filesystem]]. This track may also contain other non-essential files, and is shown by operating systems when loading the disc. This track can be absent from a VCD, which would still work but would not allow it to be properly displayed in computers.<ref name="mplayer" />

The rest of the tracks are usually in CD-ROM XA Mode 2 Form 2 and contain video and audio [[Multiplexing|multiplexed]] in an [[MPEG program stream]] (MPEG-PS) container, but [[Compact Disc Digital Audio|CD audio]] tracks are also allowed.<ref name="mplayer" /> Using Mode 2 Form 2 allows roughly 800 megabytes of VCD data to be stored on one 80 minute CD (versus 700 megabytes when using [[CD-ROM]] Mode 1).  This is achieved by sacrificing the error correction redundancy present in Mode 1.  It was considered that small errors in the video and audio stream pass largely unnoticed. This, combined with the net bitrate of VCD video and audio, means that almost exactly 80 minutes of VCD content can be stored on an 80-minute CD, 74 minutes of VCD content on a 74-minute CD, and so on. This was done in part to ensure compatibility with existing CD drive technology, specifically the earliest "1x" speed CD drives.

===Video===
[[File:Digital video resolutions (VCD to 4K).svg|thumb|VCD resolution compared to other formats]]
Video specifications<ref name="icdia.co.uk">{{citation|url=http://www.icdia.co.uk/cdprosupport/encoding/pink/mpeg1_specs.htm|title=MPEG1 Specifications|work=icdia.co.uk}}</ref>
* Compression: [[MPEG-1]]
* [[Aspect Ratio]]: 4:3 
* Resolution:
** analog [[NTSC]] compatible: 352×240 ([[240p]])
** analog [[PAL]]/[[SECAM]] compatible: 352×288 ([[288p]])
* Framerate:
** analog [[NTSC]] compatible : 29.97 ''or'' 23.976 frames per second
** analog [[PAL]]/[[SECAM]] compatible : 25 frames per second
* Bitrate: 1,150 [[kilobit]]s per second ([[constant bitrate]])

Although many DVD video players support playback of VCDs, VCD video is only compatible with the [[DVD-Video]] standard if encoded at 29.97 frames per second or 25 frames per second. <!-- This statement refers to the video ONLY. Incompatibilities caused by the 44,100 Hz audio are discussed in the audio section -->

The 352×240 and 352×288 (or [[Source Input Format|SIF]]) resolutions, when compared to the [[CCIR 601]] specifications (defining the appropriate parameters for digital encoding of NTSC and PAL/SECAM TV signals), are reduced by half in all aspects: height, width, frame-rate, and chrominance.

===Audio===

Audio specifications<ref name="icdia.co.uk" />
* Compression: [[MPEG-1 Audio Layer II]] (MP2)
* Sample Frequency: 44,100 [[hertz]] (44.1&nbsp;kHz)
* Output: Dual channel, [[stereo]], or Dolby Surround
* Bitrate: 224 [[kilobit]]s per second ([[constant bitrate]])

As with most CD-based formats, VCD audio is incompatible with the [[DVD-Video]] standard due to a difference in sampling frequency; DVDs require 48&nbsp;kHz, whereas VCDs use 44.1&nbsp;kHz.

===Advantages of compression===
By compressing both the video and audio streams, a VCD is able to hold 74 minutes of picture and sound information, the same duration as a standard 74 minute audio CD. The MPEG-1 compression used records mostly the differences between successive video frames, rather than write out each frame individually. Similarly, the audio frequency range is limited to those sounds most clearly heard by the human ear.

===Other features===
[[File:Return button on vcd player.JPG|thumb|PlayBack Control (PBC) added in VCD 2.0 requires a special 'Return' button]]
The VCD standard also features the option of DVD-quality still images/slide shows with audio, at resolutions of 704×480 ([[480i]], analog NTSC compatible) or 704×576 ([[576i]], analog PAL/SECAM compatible). Version 2.0 also adds the playback control (PBC), featuring a simple menu like DVD-Video.