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HD/Filmic Effect

(June 2008)

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CW
cwathen Founding member
Quote:
On a similar note is there any way to lessen the blocky picture which I assume is from digital compression?

If it's an LCD panel you've got a scaling issue aswell - LCDs can only display an unaliased (i.e.'not blocky') picture at their native resolution. In other words, if you have a HD panel with 1920x1080 or 1280x720 resolution, but display an SD 720x576 source on it, it will always appear blocky to some extent regardless of the source quality.

This gets compounded even further when 720x576 can no longer be relied upon as a base SD broadcast standard - dubious resolutions like 704x576 and 544x576 are also in widespread use (which IMO should never have been allowed by OFCOM - digital is hardly progression if it's being broadcast to an inferior technical standard to the analogue system)
NG
noggin Founding member
Mr-Stabby posted:
My parents Samsung 50 inch plasma set also has this strange feature which smoothes the motion on such content making it look like interlaced video.


Though strictly speaking the look isn't that of interlaced video - it is that of a 50Hz rather than 25Hz motion sampling system.

It isn't the interlace bit that it looks like - it is the 50Hz motion. After all 50Hz progressive video has a very similar motion character.
NG
noggin Founding member
cwathen posted:
Quote:
On a similar note is there any way to lessen the blocky picture which I assume is from digital compression?

If it's an LCD panel you've got a scaling issue aswell - LCDs can only display an unaliased (i.e.'not blocky') picture at their native resolution.


That's a bit of a gross oversimplification.

It is entirely possible to scale a source image to a higher resolution without introducing aliasing artefacts. Aliasing artefacts are most commonly introduced when you poorly scale a high resolution image to a lower resolution without effectively scaling.

In fact if you watch the Euro 2008 coverage you see far more aliasing artefacts on the SD feeds than the HD - because the SD downconversion is not implementing that great an antialiasing filter...

It is also possible to reduce aliasing artefacts to very low, near negligible levels, by properly implemented filtering algorithms. This may reduce the maximum resolution that a panel can deliver, but it reduces aliasing artefacts as well.

Quote:

In other words, if you have a HD panel with 1920x1080 or 1280x720 resolution, but display an SD 720x576 source on it, it will always appear blocky to some extent regardless of the source quality.


Not if the resampling is done effectively it shouldn't. There should be no major blockiness resulting from scaling.

Blockiness may be REVEALED by scaling but is not usually CREATED by it. The scaling being revealied is mainly a function of the greater resolution of the HD panel being able to show the HF artefacts that high frequency edges, introduced by MPEG block edges (which contain frequencies not really possible to create in the source image), introduced by a lack of high frequency resolution in the compressed image (MPEG blocks with mainly DC components emphasise block edges, as the only HF content that results in the image is that introduced by the block structure of the MPEG2 compression, not from the source image) The higher the resolution of the panel they are scaled to, the more visible these block artefacts of compression become. On an SD screen they are usually filtered away, because just as the SD source can't sample HF artefacts of that frequency effectively (as they disappear into the HF filtering/optical antialising filters etc) SD analogue displays also implement an HF rolloff function that reduces their visibility.

This blockiness being revealed on SD content on an HD display isn't aliasing per se - as the HF detail is not part of the source scene, it is an artefact introduced by the compression scheme, in the SD domain.

Quote:

This gets compounded even further when 720x576 can no longer be relied upon as a base SD broadcast standard - dubious resolutions like 704x576 and 544x576 are also in widespread use (which IMO should never have been allowed by OFCOM - digital is hardly progression if it's being broadcast to an inferior technical standard to the analogue system)


Err - nothing dubious about 704x576 sampling...

702x576 is the exact equivalent to a 4:3 or 16:9 analogue active line - and is the definition of the active video content in an SD image. 720x576 is a slightly wider aspect ratio system introduced with digital sampling to avoid truncation and clipping of overshoot and undershoot - the 9 samples either side of the 702x576 frame are purely there to preserve analogue anomalies rather than truncating them and introducing more artefacts, like ringing. The 704x576 sampling scheme involves no scaling it is just the nearest MPEG2 multiple to the 702x576 analogue equivalent source, effectively 8 samples either side are not carried - but these are effectively blanking. During production the 720x576 aperture should be preserved (though DVEs have interesting things to decide when they scale down an SD picture and reduce its size - do they crop the 18 samples or do they allow the black lines either side that most sources will have to be visible?), but on transmission nothing active should be present outside the 702x576 frame...

Of course loads of people incorrectly assume that 720x576 is the 16:9 or 4:3 active frame... It isn't...

When it comes to 544x576 carrying 16:9 content I'm kind of with you - though when it is 4:3 content, there is no difference in resolution to this and a 4:3 pillarbox in a 16:9 frame.

Ofcom DO mandate 720x576/704x576 (effectively the same resolution in active video terms) for ITV1, C4 and Five on DTT...
PE
Pete Founding member
sorry noggin, that last paragraph just went right over. Embarassed any chance of getting the subtitled version as i do want to understand it
NG
noggin Founding member
Hymagumba posted:
sorry noggin, that last paragraph just went right over. Embarassed any chance of getting the subtitled version as i do want to understand it


A standard 50Hz SD TV line contains 52us of active video, with a total line duration including blanking and syncs of 64us. The standard luminance sampling rate for digital TV, as defined in ITU 601, is 13.5MHz.

Thus the 52us active line that contains the 4:3 or 16:9 image is contained within 702 samples.

The ITU 601 spec defines a wider digital active line of 720 samples, to avoid cropping overshoot/undershoot etc. when moving between analogue and digital systems. (If you crop overshoots or undershoots on analogue transitions you will potentially create ringing issues) There are thus 9 samples either side of the 702 sample active line which ensure that no cropping takes place.

This leaves you with a digital "active" line of 720 samples. However this frame of 720x576 is not 4:3 or 16:9 - in both cases it is slightly wider - as you have added 18 samples horizontally. The digital spec states that these samples should be preserved in the digital domain, but they can't be if the signal goes in and out of the analogue domain (if the A/D and D/A converters are properly in-spec)

Thus the analogue (RGB, Composite etc.) output of a properly compliant digital set-top box should only really ever be the central 702x576 samples - so whether you send 720x576 or 704x576 (i.e. the full - slightly wider than 4:3/16:9 - digital line or just the nearest MPEG multiple to the full analogue 4:3/16:9 active line) the box should, if it is properly implemented, only output the central 702x576 samples across the full active analogue output.

The bottom line is 704vs720 transmission doesn't involve rescaling a digital source, as 544x576 does, rather it just involves cropping 8 samples either side of a 720x576 source, and these 8 samples (+1 each side retained) are not part of the central 4:3 or 16:9 frame and should not be output via analogue outputs anyway.

However they may be retained when viewed on a PC.
DB
dbl
Hymagumba posted:
sorry noggin, that last paragraph just went right over. Embarassed any chance of getting the subtitled version as i do want to understand it

<tries to subtitle it>
Basically 704 and 720 are full res, it's not actually reducing the picture quality because 720 contains some inactive lines and 704 kinda crops it. On analogue it would be 702.
NG
noggin Founding member
dbl posted:
Hymagumba posted:
sorry noggin, that last paragraph just went right over. Embarassed any chance of getting the subtitled version as i do want to understand it

<tries to subtitle it>
Basically 704 and 720 are full res, it's not actually reducing the picture quality because 720 contains some inactive lines and 704 kinda crops it. On analogue it would be 702.


Not quite.

It is horizontal samples, not vertical lines that are cropped, and they don't have to be inactive. In digital-land they can contain active picture information - you just shouldn't expect it to be displayed when the signal is full-screen.

Annoyingly - a lot of video software - and even now, some hardware, assumes incorrectly that 720x576 is exactly 4:3 or 16:9 - which can cause real problems. (Sony's HD cameras output a 16:9 image in 720x576 when downconverting - which can cause real problems in virtual studio applications apparently)

When you shrink the signal you have an interesting decision to make - do you crop the 9 samples left and right - on the assumption that an analogue source will have black there probably, or do you leave them on the assumption that a digital source MAY have picture information there? (
SN
SN2005
wells posted:

Blu Ray is a format that can store HD content and is a rival to the HD DVD format a disc that can store HD content. Ultimately neither will win out right as the technology is moving along to fast IMO.


Too late. Toshiba shelved HD DVD back in Feb. Where have you been?
BA
bilky asko
SN2005 posted:
wells posted:

Blu Ray is a format that can store HD content and is a rival to the HD DVD format a disc that can store HD content. Ultimately neither will win out right as the technology is moving along to fast IMO.


Too late. Toshiba shelved HD DVD back in Feb. Where have you been?


On here talking about TV, presumably.

And to everyone who has got it wrong on here - it's called "Blu-ray" - not "Blu Ray" nor "Blue ray" (although I don't blame 623058) nor "Blu-Tack" - nor anything else. Only Mr Rothwell got it right.
RO
roo
bilky asko posted:
Only Mr Rothwell got it right.

It's really not the done thing to make much of this.
BA
bilky asko
Barney Boo posted:
bilky asko posted:
Only Mr Rothwell got it right.

It's really not the done thing to make much of this.


You're eyesight's amazing! Wink

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