Re: Why interlaced HDTV?

On Wed, 24 Aug 2005 10:24:09 +0100, Kennedy McEwen
<rkm@xxxxxxxxxxxxxxxxxx> wrote:

>>After watching an LCD i'm now like this even with some 100 Hz TVs.
>
>You need to eat more vegetables then, especially carrots!

Sorry, watching TV and healthy eating seem to be mutually exclusive.
;-)

>The fact that they sell to the unwashed masses means very little in
>terms of image quality - slow rise time LCD panels without any colour
>management have been selling in volume too, but the picture quality is
>complete crap.

I think we've already established that the great unwashed will buy
substandard products. What I'm saying is that one of the advantages of
LCD and 100 Hz TVs IS a noticeable reduction in flicker. If there was
no benefit at all I don't think 100 Hz TVs would have been on sale for
the last 15 years. I notice the difference.

>Yes, if you ignore the fact that interlace is already 50% data
>compression to start with! This has been debated extensively on this
>group.

Not compression, removal. 576i and 1080i have half the temporal
resolution of 720p.

>>Coupled with
>>the fact that 720p has double the temporal resolution of 1080i
>
>No it doesn't! 720p has exactly the same temporal resolution of 1080i,
>they both have 50Hz refresh rates. The fact is that 1080i provides
>additional information in that frame time, in the form of significantly
>enhanced horizontal resolution and additional vertical resolution
>through the interlace.

720p will only have the same apparent temporal resolution as 1080i if
the 1080i source and display are none progressive. The 1080i field
resolution will only be 540 lines though compared to 720 with 720p.
1080i provides half the FIELD rate of 720p.

>>making
>>it the only real choice for any material with movement

So all those sports stations that have adopted 720p for it's better
motion rendering have got things the wrong way round?

>What crap - interlace sources have been the format of choice for
>movement for over half a century.

Because it's a convenient form of lossey compression in the analogue
world. It's the technology of the 1930's. We've moved on from there.

>>>That is exactly the point: nobody would need them - if high quality
>>>backwards compatibility were delivered. Whilst that is certainly
>>>possible, indeed just as simple to achieve, it isn't what most flat
>>>panels provide. Consequently the push for progressive standards
>>>alienates about half a century of existing video heritage.
>>
>>I don't follow this.
>
>That is clear, from your previous posts. Perhaps you need to consider
>it more.

1080p production converts nicely to all current and proposed
interlaced and progressive standards. Choosing a progressive format
for HD broadcast is a natural progression of this. I don't follow how
this is in any way less backwards compatible than any other new HD
standard.

>> The push for progressive standards is as much
>>about moving forward with technology as the move to high definition.
>>
>But progressive is NOT a move forward with technology in itself.
>particularly when the option is between 720p and 1080i. Both formats
>provide similar vertical resolution in the presence of motion, but the
>interlaced option provides much higher horizontal resolution in all

You keep repeating this but it doesn't make it true. 720p has twice
the number of FRAMES as 1080i.

>This is a flawed logic. 25frames per second, but 50 fields per second,
>the temporal resolution is exactly the same as a 50 frame per second
>system. Yes, the full *spatial* resolution is not available
>simultaneously with the full vertical resolution, but that is no worse
>than most digital codecs, which drop horizontal and vertical resolution
>when full temporal resolution is required.

25 frames displayed as 50 half fields. In future both camera and
display will be natively progressive. This will show the true limits
of a 25 FRAME interlaced system.

>>This no more alienates existing
>>video heritage than any other part of HD. HD may consign much of what
>>you and I are familiar with to a museum, but that's progress for you.
>>
>If it means that it cannot be rebroadcast without introduction of
>artefacts than it is not progress, it is anarchy.

1080p production allows easy conversion to all lower standards. To fix
on 1080i as a broadcast standard would be to immediately throw away
half the information while introducing significant display artifacts.
720p may not have the static resolution of 1080i but wipes the floor
with it in all other respects.

>>I really agree that we should be looking to 1080p but as production is
>>moving in that area anyway it would appear logical to adopt the
>>broadcast standard that's the closest technical match.
>>
>Which is 1080i - simply drop every other line in alternate fields: no
>interpolative downsampling (with the consequential loss of resolution
>inherent in all interpolation techniques) required.

....and then try and reconstruct them in the displays memory from the
two time different half fields, while the nature of the interlace also
causes more artifacts for a given bandwidth at the digital compression
stage. 1080i is 25 fields per second. I do not believe this to be
enough for a modern broadcast system.

>>With 720p you have a down sampling of the source to from 1080 to 720
>>at the broadcaster
>
>Losing vertical resolution in the process - unlike sampling a 1080i
>field from a 1080p frame, getting a 720p frame is not an integer spatial
>division!

So how does a 540 line half field system beat a 720 line full field
system when the two half fields are taken effectively taken at
different points in time, resulting in major interlace artifacts when
combined to create 1080 line full frame?

>>With 1080i you first have to throw away half the temporal information,
>
>Wrong - and this is the mistake that seems to underpin most of the
>"progressive is superior to interlace" logic! You do NOT throw away
>half of the temporal information. What you throw away is the
>information which requires *BOTH* the full temporal and the full spatial
>resolution. That is very much *LESS* than a quarter of the information
>in a real world image - and, given the temporal response of the eye, is
>even further reduced at the point of viewing. This is why interlace was
>adopted in the first place - halving the transmission bandwidth resulted
>in a very small amount of the perceivable information being lost.

Half the frames are removed and those that remain are split in to two
time separated fields. This not only throws away half the available
information but introduces significant interlace artifacts on the what
remains.

As i've said. Interlace was a 1930's lossy compressions system to
solve a problem that not only no longer exists, but actually causes
more problems with modern equipment.

>>transmit this at 1080i and then the domestic TV has to reconstruct a
>>1080 line progressive frame in memory from two 540 line halves of the
>>interlace each taken 1/50 of a second apart.
>
>As already explained, that is trivial to accomplish if done correctly.

You can't create information that's not there. Some of the best
systems apparently average between the two fields and try to guess
what should have been present, but at the end of the day, if you have
two half FRAMES taken 1/50 sec apart with different content, there is
no accurate way to guess what was in the other half of each field.

>>Since these frames may
>>have completely different information (depending on the amount of
>>movement etc) this then leads to an apparent quality reduction.
>
>Back to your error - these frames (fields actually) do not have
>*completely* different information. Even in a high motion content
>scene, the fast majority of the information is identical in both fields.

If the scene is static they'll have the same information. Where there
is the slightest movement they will differ. I regularly see this on
material that has been captured from conventional TV. There are some
very good software de-interlacing routines around but they always add
other artifacts.

>The spatial resolution of a 1080 format is more than double that of a
>720 format. Your logic appears to argue that since the temporal
>resolution of interlace is half that of progressive this cancels out the
>spatial advantage - but the temporal resolution of progressive is NOT
>twice that of progressive. The difference between the two lies only in

When the interlaced material is converted back to progressive 1080i,
effectively contains half the temporal resolution of 720p. Yes in the
original 1080i signal there are 50 time separate half fields, but when
merged to create the 1080 line picture, this is lost and causes a
reduction of visual resolution.

>Which domestic display is currently available that has a 1080 line
>vertical resolution?

Plenty. The Samsung LW46G15W springs to mind as being one of the newer
larger sets:

http://www.samsung.com/he/presscenter/pressrelease/pressrelease_20040517_0000042331.asp


Rgds
Jonathan

.

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