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Old 13th August 2017, 19:40   #54  |  Link
hello_hello
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Join Date: Mar 2011
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If you read some of what I quoted from the ABC pdf earlier (and as far as I know the ABC do everything the same as the BBC):

2.2.3 All SDTV programs produced using modern digital equipment shall have narrow horizontal blanking, ie. active vision shall be 720 horizontal pixels wide. Legacy programs with 702 pixel wide vision (wide horizontal blanking) will also be acceptable. A single program shall have consistent blanking throughout.

And combine it with the 788 square pixel width Ghitulescu's mentioned, to me it still adds up to an mpeg4/ITU pixel aspect ratio but there's very little to no black at the sides.

I did a little more searching and I'm still a bit confused, but some anecdotal (almost) evidence the picture is supposed to be 720 pixels wide, although wider than 4:3.

EBU Technical Recommendation R92-1999 (pdf)
Active picture area and picture centring in analogue and digital 625/50 television systems
The EBU is aware of a certain amount of confusion about the active picture area in the implementation and use of digital signals conforming to the ITU-R Recommendation BT.601 [1].
Recommendation BT.601 specifies a line length of 720 luminance pixels (13.5 MHz sampling). ITU-R Recommendation BT.470[2], specifies a line length of 52 µs for 625 line analogue signals. This corresponds to 702 luminance pixels.
This apparent difference can lead to difficulties in conversion, especially if users wish to maintain the correct aspect ratio of the pictures.
Recommendation BT.601 accommodates modest variations in the position and length of analogue blanking which arise before a signal is digitised or when digital signals pass through any subsequent analogue process.
The EBU recommends that:
In 625-line television systems sampled to ITU-R Rec. BT.601 part A, only the central 702 luminance samples of the digital active line (samples 9-710 inclusive) and their associated chrominance samples should be used to carry the active picture. The remaining 18 luminance samples and their associated chrominance samples may be used to carry picture information only but for no other purpose. It cannot be guaranteed that picture information in these samples will be displayed in either 4:3 or 16:9 aspect ratio images.


BBC Technical Standards for Network Television Delivery
4.1.4 Video Signal Timings.
Digitally delivered pictures are considered to have a nominal active width of 702 pixels (52us) starting on the 10th pixel and ending on the 711th pixel in a standard REC 601(720 sample) width. A minimum width of 699 pixels (51.75us) within these limits must be achieved. Additional active pixels outside the above limits must be an extension of the main picture.

There's information in this ITU PDF explaining the relationship between digital and analogue.
Studio encoding parameters of digital television for standard 4:3 and wide-screen 16:9 aspect ratios
Encoding parameter values for the 4:2:2, 13.5 MHz member of the family
The specification (see Table 3) applies to the 4:2:2 member of the family, to be used for the standard digital interface between main digital studio equipment and for international programme exchange of 4:3 aspect ratio digital television or wide-screen 16:9 aspect ratio digital television when it is necessary to keep the same analogue signal bandwidth and digital rates.

It refers to a chart on page 7 that lists the total number of samples per line for PAL as 864, with the number of samples per digital active line being 720.
On page 10 there's a diagram showing how the "digital blanking" interval is from pixel 720 to pixel 864.

My take on all that, and hopefully someone will correct me if I'm wrong, is the original BT.470 analogue spec is equivalent to 702 pixels wide in digital-speak, which is exactly 4:3, and BT.601 doesn't change that, only adding some bonus picture on each side.
If that's correct, it makes sense that a 4:3 DVD containing a video capture would have lashings of black down the sides, while a digital source (and those old Doctor Who episodes have been digitally process every which way), would be 720 pixels wide, although I assume still only 704 of the width would be 4:3. I'm still reasonably sure most 4:3 DVDs I've come across have an ITU/mpeg4 pixel aspect ratio, be they of BBC origins or otherwise..

I haven't seen enough of them to be certain, but I strongly suspect the few 16:9 BBC DVDs I've seen are ITU, despite the 720 width being almost entirely picture, but as a rule the rest of the world seems to be sticking to the 720 width being part of the total 16:9 aspect ratio.... mostly. I still don't fully understand why.

Some interesting reading here. (pdf)
The origins of the 4:2:2 DTV standard

Last edited by hello_hello; 23rd August 2017 at 23:36.
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