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576i
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576i is a standard-definition video mode used in (former) PAL and SECAM countries. In digital applications it's usually referred to as "576i", in analogue contexts it's often quoted as "625 lines".
The 576 identifies a vertical resolution of 576 lines (usually with a horizontal resolution of 720 or 704 pixels), and the i identifies it as an interlaced resolution. The field rate (not to be confused with the frame rate), which is 50 Hz, is sometimes included when identifying the video mode—i.e., 576i50.
Its basic parameters common to both analogue and digital implementations are: 575 scan lines of picture content, 25 frames (giving 50 fields) per second.
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Encyclopedia
576i is a standard-definition video mode used in (former) PAL and SECAM countries. In digital applications it's usually referred to as "576i", in analogue contexts it's often quoted as "625 lines".
The 576 identifies a vertical resolution of 576 lines (usually with a horizontal resolution of 720 or 704 pixels), and the i identifies it as an interlaced resolution. The field rate (not to be confused with the frame rate), which is 50 Hz, is sometimes included when identifying the video mode—i.e., 576i50.
Its basic parameters common to both analogue and digital implementations are: 575 scan lines of picture content, 25 frames (giving 50 fields) per second. Also in analogue, 50 additional blank lines for the synch pulse are added, resulting in 625 lines. 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 ca 500 columns 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)
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 with 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))
- FM or AM 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 subcarrers 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.
576i speed-up
Motion pictures are typically shot on film at 24 frames per second. When telecined and played back at 576i25'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 480i30's telecine system, which uses 3:2 pulldown to convert the 24 frames per second to the 480i30 frame rate, 576i results in the telecined video running 4% shorter than the original film as well as the equivalent 480i30 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.
There also exists conversion methods 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 is 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.
Some movie enthusiasts prefer 576i speed-up over 480i30's 3:2 pulldown, because the latter results in telecine judder, a visual distortion not present in 576i speed-up video. This is not an issue on modern upconverting DVD players and PCs, as they play back 23.97frame/s-encoded video at its true frame rate, without 3:2 pulldown.
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.
See also
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