576i is a standard-definition video mode used in (former) PAL and SECAM countries. In digital applications it is usually referred to as "576i"; in analogue contexts it is often called "625 lines", and the aspect ratio is usually 4:3 unless otherwise specified. Its NTSC counterpart is 480i; these are the two common forms of standard-definition television.
The 576 identifies a vertical resolution of 576 lines, and the i identifies it as an interlaced resolution. The field rate, which is 50 Hz, is sometimes included when
identifying the video mode, i.e. 576i50; another notation, endorsed by EBU/SMPTE, includes the frame rate, as in 576i/25.
Its basic parameters common to both analogue and digital implementations are: 576 scan lines or vertical pixels of picture content, 25 frames (giving 50 fields) per second.
In analogue 49 additional lines without image content are added to the displayed frame of 576 lines to allow time for older CRT circuits to retrace for the next frame, giving 625 lines per frame. Digital information not to be displayed as part of the image can be transmitted in the non-displayed lines; teletext and other services and test signals are often implemented.
Analogue television signals have no pixels; they are rastered in scan lines, but along each line the signal is continuous. In digital applications, the number of pixels per line is an arbitrary choice as long as it fulfils the sampling theorem. Values above about 500 pixels per line are enough for conventional broadcast television; DVB-T, DVD and DV allow better values such as 704 or 720.
The video format can be transported by both major digital television formats, ATSC and DVB, and on DVD, and it supports aspect ratios of standard 4:3 and anamorphic 16:9.
Baseband interoperability (analogue)
Spectrum of a System I (bands IV and V) television channel with PAL or SECAM colour ( see above)
When 576i video is transmitted via baseband (i.e., via consumer device cables, not via RF), most of the differences between the "one-letter" systems are no longer significant, other than vertical resolution and frame rate.
In this context, unqualified 576i invariably means
- 625 lines per frame, of which 576 carry picture content
- 25 frames per second interlaced yielding 50 fields per second
- Two interlaced video fields per frame
- With PAL or SECAM colour (4.43 MHz or 3.58 MHz (576i-N & 576i-NC))
- Frequency modulated or amplitude modulated audio (mono)
- Mono or stereo audio, if sent via connector cables between devices
Modulation for TVRO transmission
576i when it is transmitted for TVRO viewing is transmitted substantially differently from terrestrial transmission.
Full transponder mode (e.g., 72 MHz)
- Luma signal is FM modulated, but with a 50 Hz dithering signal to spread out energy over the transponder
- Chroma is phase modulated
- An FM subcarrier of 4,50, 5.50, 6.0, 6.50 or 6.65 MHz is added for mono sound
- Other FM subcarriers (usually 7.02, 7.20, 7.38, 7.56, 7.74 and 7.92 MHz) are added for a true stereo service and can also carry multi-lingual sound and radio services. These additional subcarriers are normally narrower bandwidth than the main mono subcarrier and are companded using Panda 1 or similar to preserve the signal to noise ratio
- Data subcarriers may also be added
Half transponder mode (e.g., 36 MHz)
- All of the above is done, but signal is bandwidth limited to 18 MHz
- The bandwidth limiting does not affect audio subcarriers
Baseband interoperability (digital)
In digital video applications, such as DVDs and digital broadcasting, colour encoding is no longer significant; in that context, 576i means only
- 576 frame lines
- 25 frames or 50 fields per second
- Interlaced video
- PCM audio (baseband)
There is no longer any difference (in the digital domain) between PAL and SECAM. Digital video uses its own separate colour space, so even the minor colour space differences between PAL and SECAM become moot in the digital domain.
Use with progressive sources
When 576i is used to transmit content that was originally composed of 25 full progressive frames per second, the odd field of the frame is transmitted first. This is the opposite of NTSC. Systems which recover progressive frames, or transcode video should ensure that this field order is obeyed, otherwise the recovered frame will consist of a field from one frame and a field from an adjacent frame, resulting in 'comb' interlacing artifacts.
PAL speed-up
Motion pictures are typically shot on film at 24 frames per second. When telecined and played back at PAL's standard of 25 frames per second, films run 4% faster.
This also applies to most TV series that are shot on film or digital 24p. Unlike NTSC's telecine system, which uses 3:2 pulldown to convert the 24 frames per second to the 30 fps frame rate, PAL speed-up results in the telecined video running 4% shorter than the original film as well as the equivalent NTSC telecined video.
Depending on the sound system in use, it also slightly increases the pitch of the soundtrack by 70.67 cents (0.7067 of a semitone). More recently, digital conversion methods have used algorithms which preserve the original pitch of the soundtrack, although the frame rate conversion still results in faster playback.
Conversion methods exist that can convert 24 frames per second video to 25 frames per second with no speed increase, however image quality suffers when conversions of this type are used. This method is most commonly employed through conversions done digitally (i.e. using a computer and software like VirtualDub), and is employed in situations where the importance of preserving the speed of the video outweighs the need for image quality.
A majority of movie enthusiasts prefer PAL over NTSC despite the former's speed-up, because the latter results in telecine judder, a visual distortion not present in PAL sped-up video. This is not an issue on modern upconverting DVD players and PCs, as they play back 23.97 frame/s–encoded video at its true frame rate, without 3:2 pulldown.
Naturally, PAL speed-up does not occur on native 25 fps video, such as European TV-series or movies that are shot on video instead of film.
Software which corrects the speed-up is available for those viewing 576i DVD films on their computers, WinDVD's "PAL TruSpeed" being the most ubiquitous. However, this method involves resampling the soundtrack(s), which results in a slight decrease in audio quality. The echo/audio balance issue can be resolved by re-adjusting the playback pitch (located in the Audio Effect tab) from normal to low and back to normal again.
582i
The original implementation of the version of the analogue 625 line standard used in Ireland, the United Kingdom and South Africa (System I) specified 582 active lines rather than the 576 used in all other implementations of the 625 line system. However most present day analogue broadcasting (and all non-HD digital bradcasting) in these countries is based on 576i exclusively.
