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720x576 1920x1080 pixels, aspect ratios
2steps
04-02-2009
Thought I'd try to pull this from other threads
SD is broadcast at 720x576 pixels, in largely 16x16 pixel blocks depending how the picture has changed from previous. As far as I know it doesnt know anything about interlacing

A number of control flags transmitted with the programme tell the display whether the image should be displayed 'as is' to give a 4:3 picture, or stretched widthways by a third to equivalent of 1024x576 a 16:9 picture or whether some intermediate such as 14:9 to show an original 4:3 image on a 16:9 display
Phew
However it's not quite as simple as that as of the 720 pixels only 704 should be visible, the remaining 8 each side are overscan.
I dont know if there are overscan lines vertically

Currently HD is broadcast as 1440x1080 pixels
As before: For 16:9 images the pixels are stretched widthways by a third or interpolated (upscaled) to fit on a 1920x1080 display.
I dont know how many pixels or lines are overscan

It's only the box and display that decide whether they need an interlaced or progressive scan connection

Is that right? & anyone fill in the blanks?
son_t
04-02-2009
You are better off sticking to that other thread or post this in the technical section if it has anything to do with satellite. Or another forum site if it is to do with display technology...
2steps
04-02-2009
It seemed to be running in one thread in freesat and another in freesat+
I thought it might help try explain some of the issues with display quality, which are more than about compression
derek500
04-02-2009
Originally Posted by 2steps:
“Currently HD is broadcast as 1440x1080 pixels”

I thought that BBC HD and ITV HD broadcast in 1440x1080, but all the Sky HD channels including C4HD were broadcast as 1920x1080.
2steps
04-02-2009
Originally Posted by derek500:
“I thought that BBC HD and ITV HD broadcast in 1440x1080, but all the Sky HD channels including C4HD were broadcast as 1920x1080.”

I don't know
Must admit I was thinking BBC HD & ITV HD when I wrote that
Anyone know?
jzee
04-02-2009
Originally Posted by derek500:
“I thought that BBC HD and ITV HD broadcast in 1440x1080, but all the Sky HD channels including C4HD were broadcast as 1920x1080.”

I don't think any HD is currently broadcast at 1080p, it's either 1080i 1440 x 1080 or 720p 1280 x 720.
sg_
04-02-2009
The majority of SD channels available on Sky as well are not even broadcast at a resolution of 720x576/704x576.
tellytart1
04-02-2009
Originally Posted by 2steps:
“However it's not quite as simple as that as of the 720 pixels only 704 should be visible, the remaining 8 each side are overscan.
I dont know if there are overscan lines vertically”

You've not quite got that bit right. The digital size of 720 pixels wide was chosen so that it could accurately sample the rising edge and trailing edge waveforms of an analogue picture, which would then be 704 pixels active picture, the other 8 pixels either side containing the rising and trailing edges.

However, in the digital domain it is valid to use all 720 pixels.

There is no defined overscan in the system, however a broadcast standard of 5% overscan is allowed all around and classed "Action Safe", and 10% from the edges is classed as "Title Safe"
2steps
04-02-2009
It's 702 not 704
http://www.bbc.co.uk/commissioning/t...turesize.shtml

I thought it was said to be too complicated to broadcast interlaced, so the compression algorithm only broadcasts parts of the picture at a time.
I'm not sure of the update rate or how that compares to 25 or 50 fps
2steps
04-02-2009
this threads been moved
I didnt even know this forum section existed
Nigel Goodwin
04-02-2009
Originally Posted by 2steps:
“Thought I'd try to pull this from other threads
SD is broadcast at 720x576 pixels, in largely 16x16 pixel blocks depending how the picture has changed from previous. As far as I know it doesnt know anything about interlacing

A number of control flags transmitted with the programme tell the display whether the image should be displayed 'as is' to give a 4:3 picture, or stretched widthways by a third to equivalent of 1024x576 a 16:9 picture or whether some intermediate such as 14:9 to show an original 4:3 image on a 16:9 display
Phew
However it's not quite as simple as that as of the 720 pixels only 704 should be visible, the remaining 8 each side are overscan.
I dont know if there are overscan lines vertically

Currently HD is broadcast as 1440x1080 pixels
As before: For 16:9 images the pixels are stretched widthways by a third or interpolated (upscaled) to fit on a 1920x1080 display.
I dont know how many pixels or lines are overscan

It's only the box and display that decide whether they need an interlaced or progressive scan connection

Is that right? & anyone fill in the blanks?
”

You seem to be confusing MPEG compression and TV standards?.

One of the many aspects of MPEG is to only transmit full frames every so many frames, in between only the changes per frame are sent. But it's a LOT more complicated than that.
2steps
04-02-2009
Originally Posted by Nigel Goodwin:
“You seem to be confusing MPEG compression and TV standards?.

One of the many aspects of MPEG is to only transmit full frames every so many frames, in between only the changes per frame are sent. But it's a LOT more complicated than that.”

I know - I've seen the book

I'm not even sure that it does transmit a full frame every so many frames - it's just that if it makes sure it transmits enough updates to a frame that after a while the whole frame will be new.

If it is a complete change, say a switch from continuity to the next programme, how long does that take 20 frames - almost a second?
Nigel Goodwin
05-02-2009
Originally Posted by 2steps:
“I know - I've seen the book

I'm not even sure that it does transmit a full frame every so many frames - it's just that if it makes sure it transmits enough updates to a frame that after a while the whole frame will be new.

If it is a complete change, say a switch from continuity to the next programme, how long does that take 20 frames - almost a second?”

No, they have to transmit a full frame every so many frames (I can't remember the specific number, the course was far too long ago) - essentially it's a 'reset' function - it gets everything back where it should be.
bobcar
05-02-2009
Originally Posted by jzee:
“I don't think any HD is currently broadcast at 1080p, it's either 1080i 1440 x 1080 or 720p 1280 x 720.”

No HD is broadcast at either 1080p or 720p it is all 1080i at 1920x1080 or 1440x1080.

Your statement that 1080i is 1440x1080 shows a lack of understanding of the difference between 1080p and 1080i.
2steps
05-02-2009
Originally Posted by bobcar:
“No HD is broadcast at either 1080p or 720p it is all 1080i at 1920x1080 or 1440x1080.

Your statement that 1080i is 1440x1080 shows a lack of understanding of the difference between 1080p and 1080i.”

strange
I'd come to the point of deciding it was all broadcast at 1080p or 720p but I guess that needs definition of terms
1080p or 720p meaning all the lines are broadcsat sequentially. However it's not broadcast like that at all but in encoded blocks of 16x16. I'm pretty sure those are not interlaced blocks or theyd have stripes in fault conditions?

As far as I know interlacing only occurs if required as the last phase when taking the data from the decoded transmission data buffer so as to drive a screen
Nigel Goodwin
05-02-2009
Originally Posted by 2steps:
“strange
I'd come to the point of deciding it was all broadcast at 1080p or 720p but I guess that needs definition of terms
1080p or 720p meaning all the lines are broadcsat sequentially. However it's not broadcast like that at all but in encoded blocks of 16x16. I'm pretty sure those are not interlaced blocks or theyd have stripes in fault conditions?

As far as I know interlacing only occurs if required as the last phase when taking the data from the decoded transmission data buffer so as to drive a screen”

As already explained, ALL HD transmissions in the UK are transmitted as 1080i - you still seem confused about compression and broadcasting standards, two completely different things.
2steps
05-02-2009
Originally Posted by Nigel Goodwin:
“As already explained, ALL HD transmissions in the UK are transmitted as 1080i - you still seem confused about compression and broadcasting standards, two completely different things.”

ok I havent seen an HD broadcast and pixellation yet so I'm at a disadvantage
I can only speak about 720 being pretty much 720p.

Definition of terms:
What do you mean by transmitted as 1080i?
It doesnt seem to send out the data line at a time , even frames followd by odd frames
Nigel Goodwin
06-02-2009
Originally Posted by 2steps:
“ok I havent seen an HD broadcast and pixellation yet so I'm at a disadvantage
I can only speak about 720 being pretty much 720p.
”

720P is the only version of 720, BUT it's not used, as endlessly repeated, all broadcasts are 1080i.

Quote:
“
Definition of terms:
What do you mean by transmitted as 1080i?
”

It's transmitted as an interlaced image, in two interlaced fields to make a single frame. The intervening compression is completely irrelevent.

Quote:
“
It doesnt seem to send out the data line at a time , even frames followd by odd frames”

You're still thinking of compression, not transmission - as it's not sent as an analogue picture it makes no difference, it's uncompressed to two interlaced fields, as it started out originally.
2steps
06-02-2009
I thought I'd read that they wouldnt compress the two fields separately as by the time theyd gone through compression decompression the two frames would be different enough that images would become very zebra like. I thought instead they send out compressed blocks of 16x16 which are from both odd and even frames. It agrees with when a corruption occurs as the whole 16x16 block goes blank rather than just the odd lines or just the even lines.

I'm not sure but does that still go along with what youre saying?
thanks
daveac
07-02-2009
Originally Posted by 2steps:
“Thought I'd try to pull this from other threads

SD is broadcast at 720x576 pixels,

Is that right? & anyone fill in the blanks?
”

Some SD channels are 540x576 as well

Cheers, daveac
2steps
07-02-2009
I forgot about those

Are they only 4:3 or can they be widescreen?
bobcar
07-02-2009
Originally Posted by 2steps:
“I thought I'd read that they wouldnt compress the two fields separately as by the time theyd gone through compression decompression the two frames would be different enough that images would become very zebra like. I thought instead they send out compressed blocks of 16x16 which are from both odd and even frames. It agrees with when a corruption occurs as the whole 16x16 block goes blank rather than just the odd lines or just the even lines.

I'm not sure but does that still go along with what youre saying?
thanks ”

You read wrongly, they take a 1080i stream, compress that and then the Sky box decompresses to 1080i. Yes there are interlacing artefacts that wouldn't be there if they used 1080p25 instead.

You also have to realise that frames are not compressed in isolation, the compression takes place over several frames which are then reassembled - a corruption can affect several frames not just halves of an interlaced image. You need to think of the transmission as a stream of data which is quite different from an uncompressed transmission.
2steps
07-02-2009
Yes when a corruption occurs it affects both frames and stays there for what might be a second or so. I'd assumed that was due to that part of the picture not being updated faster than that if say that part hadnt changed in that time
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