DVDs, PAL and NTSC etc.

[Dave2002]

I was recently corrected when I referred to PAL as a standard for media, and I wondered how I'd made this mistake. Of course PAL is - or was - a standard for analogue TV which I think kept the colour stable across altenate scans of the screen using a CRT, and as such differeed from NTSC and SECAM which was used in France.

Puzzled by this - and my incorrect perceptions - I thought again, and then realised that I had been thinking of DVDs. DVDs are clearly digital media - noughts and ones and all that stuff, so why then are there different DVDs which play NTSC or PAL (or whatever ...)?
If the data is purely digital, then surely it would be up to the DVD player to sort out how it gets played. Perhaps there is some extra meta coding which enables the player device to output the correct video signal to the TV when a DVD is inserted. Nowadays, with even the display data being digital too, as well as the displays being essentially 2D matrixes whcih can be driven digitally and not necessarily in a regular scanning sequence (though probably most are), there might be little need for such concerns, and also whether the formats are interlaced or progressive - as these concepts are perhaps now somewhat outdated.

This is currently of some slight interest to me, as I'm trying to burn DVDs from digital video, and have realised that there are some quite significant problems in so doing. Currently my software is reporting transcoding errors, though I hope to win in the next few days.

[Dave2002]

Since posting msg 1 it has occurred to me that there could have been advantages, at least in the early days of DVD, for having different types of disk for different display technologies. As DVDs are played back in a rotating system, there might have been reasons for structuring the digital data to take advantage of the rotation, or to reduce buffering delays. Also, there might have been a lack of general high speed affordable memory for the DVD players, so that using computational means to reorder the data for the display scans might have been appropriate at the time. This is speculative on my part though. I still don't quite see why PAL should be singled out for special treatment. Interlacing might have been improved by some disk data layouts.

[Eine Alpensinfonie]

DISC

[Anastasius]

DVDs
The encoded video (MPEG2) on a DVD is stored in digital format, but it's formatted NTSC or PAL. Some players play only NTSC discs, others play PAL and NTSC discs. All DVD players sold in PAL countries play both kinds of discs. Most NTSC players can't play PAL discs.
The three differences between NTSC discs and PAL discs are:
1) Picture size and pixel aspect ratio (720x480 vs 720x576),
2) Display frame rate (30 vs 25),
3) Video from film is usually encoded at 24 frames/sec. but is preformatted for one of the two display rates. Movies formatted for PAL display are usually sped up by 4% at playback, so the audio must be adjusted accordingly before being encoded.

[Dave2002]

Indeed, I read that too. There's insufficient explanation though. Maybe the format on disc is in TV format frames - digitised, so that an on the fly conversion would still be quick and not need much buffering in the players. Maybe early players didn't even do buffering, but converted straight to analogue! However that wouldn't fit in with the philosophy of "perfect" reproduction. What size of data would be/have been used for error correction? Was error correction done at the video frame level (megabits) or scan line level (perhaps 216 bytes or thereabouts) or some other maybe ad hoc block size, from the DVD.

[Eine Alpensinfonie]

DVDs
The encoded video (MPEG2) on a DVD is stored in digital format, but it's formatted NTSC or PAL. Some players play only NTSC discs, others play PAL and NTSC discs. All DVD players sold in PAL countries play both kinds of discs. Most NTSC players can't play PAL discs.

But do all TVs in PAL countries play NTSC discs?

[Ferretfancy]

NTSC = Never Twice The Same Colour

PAL = Perfection At Last

[Gordon]

DVDs use MPEG2 to code the video material in component [ie RGB or derivative YUV/YIQ] and this is purely digital and has no direct connection whatever with "PAL", "SECAM" or "NTSC" which are now obsolete composite formats. The players that render PAL, SECAM or NTSC make those formats internally for any attached "analogue" displays. Those composite signals are then made available externally on a YC connector or even as an RF signal to go to the aerial input of a TV. Nowadays this is not so much a restriction with HDMI etc. and although old PAL TVs will not be able to render an NTSC source video a modern digital flat panel TV may well do so.

BTW SECAM is very like PAL except the colour signals are carried differently and non-compatibly even though a SECAM set can display the black and white part of a PAL signal and vice versa but you would not want to look for long.

On the disc the MPEG2 video file will take on the source parameters of one of PAL, SECAM or NTSC as required - see #4 above. Thus eg a US DVD will have a footprint of NTSC parameters in its MPEG2 video which has metadata to tell the player. Restrictions in format in regions is partly technical and partly copyright protection and anti-piracy.

To play a US DVD on a PAL player [or vice versa - a "universal" player] needs the right silicon with enough MIPS attached to perform a standards conversion which is complicated if done properly - in the early 1970s a pioneering wholly digital machine - DICE* - took 2 19" racks!! It did work reversibly between analogue PAL and NTSC and it was use of the then standard composite formats that made much of the complication. A component machine would be simpler and a tolerably good one for domestic use would be practical today.

https://en.wikipedia.org/wiki/Televi...rds_conversion which does not mention DICE* indeed although much of the article is good it seems ignorant of UK pioneering work in digital video processing. The RTE converter for 625 to 405 lines B&W mentioned was in fact designed by the ITA labs and used at all UK commercial TV 405 transmitters until the 405 service terminated at the end of 1984.

*: see here for a set of IBA Technical Reviews and find No8 that describes how DICE worked;

http://www.ntlpa.org.uk/memorabilia#