It’ll be the way your TV is set up. On SD, the picture shape changes when 4:3 programmes are broadcast, whereas on HD it doesn’t; they just keep the picture shape 16:9 and use black bars for 4:3 shows.
That is platform dependant too. It's the case on the BBC and I assume on C4 too that 4:3 programmes are pillarboxed (black vertical bars) on terrestrial but were stretched on satellite. It's part of the Sky spec
That's not actually the case. There is a difference between the two platforms, but 4:3 SD isn't 'stretched on satellite'
Here's the current status quo for SD.
The UK Freeview spec mandates AFD support for SD platforms. The Sky SD and HD receiver specs DON'T support AFDs, and use MPEG2 header switching.
AFD = Active Format Descriptor. This is a frame-accurate (I believe) descriptor that signals the active portion of the video signal (i.e. the bit that contains active picture content). AFDs can be used in an environment where video is broadcast permanently in one aspect ratio (16:9 in the case of BBC DVB-T SD) with 4:3 SD content pillarboxed and flagged as such. SD receivers can then, under AFD control, decide what to do with content that is only in the central 4:3 section.
So with AFDs 12F12 contention be broadcast as 12P16, 16F16 content is broadcast as 16F16. AFDs on STBs configured for 4:3 output can then trigger a 12P126->12F12 centre cut for 12P16 content, 16F16->12F12/14L12/16L12 based on AFDs for 16:9 content. (Movies were usually flagged for 16L12, general for 14L12 and sport for 12F12)
For the Sky platform, where MPEG2 header switching is in use, MPEG2 headers are GOP accurate only (I think they can only switch aspect ratio on an I-frame?) and actively signal an aspect ratio change of the underlying video format. This means you can get flashes of incorrectly handled content on transitions (if they aren't aligned with a GOP) - which is why the BBC used to ensure 4:3/16:9 switches were on black (and happened on the transition to the ident, not the show)
MPEG2 header switching means that 4:3 content is broadcast 12F12 full-width (not 12P16), flagged as 4:3 aspect ratio video, 16F16 continue is broadcast 16F16 full-width flagged as 16:9 aspect ratio video. The MPEG2 headers signal the two different underlying video aspect ratios (via Display Aspect Ratio flags in the MPEG2 stream headers). This means the broadcaster can't flag how 16F16 content should be output, as there are no AFDs, and instead it's just a receiver setting for 4:3 output of 16:9 full-width content (12F12 centre cut or 16L12 deep letterbox on Sky boxes) When a Sky receiver is configured for 16:9 output it outputs 16F16 full-width and 12F12 full-width video, but uses SCART Pin 8 aspect ratio signalling to tell the TV it is connected to whether the video is 16F16 or 12F12, and the TV changes its display handling accordingly (with 4:3 signalled content converted to 16:9 by the TV, with the display mode - Zoom, Panorama, Pillarbox etc. - selected by the user)
The way the BBC traditionally handle creating a feed for the Sky platform is to take their permanent 16:9 signal that is either 16F16 or 12P16 (and which feeds the DVB-T encoders), and under AFD control an ARC
drops a 12P16->12F12 conversion into the transmission chain for 4:3 content, along with accompanying data to trigger the downstream MPEG2 encoder into 12F12 encode mode.
So far - so clear. Both platforms handle 12F12 (i.e. 4:3) full-frame sources correctly, but in different ways. On Satellite BBC Services 4:3 is broadcast as clean 4:3 video, on Terrestrial BBC services it's broadcast as 4:3 pillarboxed into a 16:9 frame. Sky SD receivers handle this 4:3/16:9 mixed economy stream with no major issues if you have a TV and a correct SCART pin 8 connection.
Where it gets annoying is on Sky HD and Sky Q receivers.
For some reason unknown to anyone (but probably based on Sky getting a lot of complaints about 'my picture doesn't fill the screen') Sky didn't include 12F12->12P16 pillarbox as an output option for 1080i and 720p output on their HD boxes. 1080i and 720 are permanent 16:9 standards and don't support aspect ratio signalling in the way SD does. This means that SD channels carrying 4:3 content on Sky are output permanently in 16F16 IF you have your Sky HD box configured for 1080i or 720p permanent output. HOWEVER if you have you Sky HD box configured for AUTO then SD content is output at 576p (the Sky box deinterlaces from 576i) AND it carries aspect ratio signalling over HDMI - so your TV then knows whether the output is 12F12 576p or 16F16 576p on SD channels, and will aspect ratio switch accordingly. I don't have Sky Q - but believe this AUTO option isn't available and Sky Q boxes have no way of correctly outputting 4:3 SD broadcasts - even though they are being correctly flagged as 4:3. It's a design 'feature' (=fault) with the Sky Q platform's handling of 4:3 content.
You can easily demonstrate that the underlying broadcasts are correct if you watch FTA channels that are on the Sky platform, but can be tuned by third party receivers that DO pillarbox 12F12 output to 12P16 for 1080i and 720p modes. if you watch BBC SD services (or ITV etc.) you will get 'right shaped pictures' when 4:3 shows are broadcast.
The broadcasts are correct - the fault is in Sky's receivers and how they handle 12F12 SD.
Does that make sense...
Last edited by noggin on 6 August 2019 12:36pm - 2 times in total
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