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G8MNY > HDTV 02.02.08 03:01l 201 Lines 10883 Bytes #999 (0) @ WW
BID : 45336_GB7CIP
Read: GUEST
Subj: TV systems & displays
Path: IZ3LSV<IW2OHX<I0TVL<ED1ZAC<GB7CIP
Sent: 080202/0004Z @:GB7CIP.#32.GBR.EU #:45336 [Caterham] $:45336_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : HDTV@WW
By G8MNY (Updated Oct 07)
Some time ago I went to another HDTV lecture held at a local ham club (Sutton
& Cheam), the previous one was 20 years ago an IEEE @ Kingswood BBC research
establishment.
FORMATS
In 20 years has not seen a much change really, it used to be 1250 line (1150
visible) @ 100Hz frame rate 16:9 format, to be sort of comparable with 625 line
PAL (575 visible lines) being 8x that data rate. But the new versions (4 of
them) made to be USA friendly, are worse. The best is only 1080 visible lines @
25 F/S & the worst 720 lines at 25/50 F/S there are (P) progressive update
formats of both types to improve visible motion by keeping interlaced frames
separate, this overcomes most of the smooth motion problems but keep the high
compression systems.
For HDTV large displays capable of full resolution are needed, as the normal
eye can only perceive 1 min of arc. So for a screen of 1080 visible lines you
need to see this as 18ø tall by the eye. If your sight is worse than perfect
then you well need to be even closer than the distances below.
SCREEN SIZE
Here are the MAXIMUM viewing distance to "just" see the detail of a pixel..
Display Diagonal 9" 14" 21" 28" 30" 33" 37" 40" 42" 50" Factor
380 Line 4:3 405 Old 48" 75" 112" 5.35
480 Line 4:3 525 USA 60" 86" 90" 120" 4.29
575 Line 4:3 625 32" 50" 75" 100" 107" 118" 132" 143" 3.57
575 Line 16:9 625 26" 41" 61" 82" 88" 97" 108" 117" 123" 146" 2.92
720 Line 16:9 47" 65" 70" 78" 86" 93" 98" 117" 2.33
1080 Line 16:9 33" 43" 46" 51" 57" 62" 65" 77" 1.55
4:3 16:9
Ú Ä Ä Ä Ä Ä Ä ¿
\ Ú Ä Ä Ä Ä Ä Ä Ä Ä ¿ Note what your eye appreciates
³ \ ³ \ is not the picture width, but
\ ³ \ ³ the total picture area! So 16:9
³ diagonal ³ diagonal gives big screen diagonals with
\ ³ \ ³ smaller actual picture area!
³ \ ³ \ Also is the central area is
\ À Ä Ä Ä Ä Ä Ä Ä Ä Ù smaller, you will feel cheated.
À Ä Ä Ä Ä Ä Ä Ù
So to appreciate what HD offers, you really do need a big screen & sit close
up to it, otherwise you're wasting your time & money.
Example: If your upgrading from say a 21" 4:3 TV you view at 6ft, to a 16:9
HDTV at the same distance you will need a huge 50" screen!
The original HD trial CRT TVs of 20 years ago were a maximum of 30" & weighed
in at a heavy 185kg, & unless you had double door access on the ground floor,
you weren't getting one!
Note that many modern displays can't actually reproduce the definition, even
though they are marked/sold as "HD ready"! That just means they have a
compatible connector & software that can either degrade definition or zoom in
missing the edges, that is all!
SCREEN/TV TYPES
There are 5 types all with their problems...
1/ CRT. Heavy & bulky. A fluorescent phosphor determines the colour per dot,
illuminate by 3 fast scanned electron guns via a shadow mask that remove
75% of the beam (& power) & generally half of the phosphor light is lost
into the tube giving over all transfer efficiency less than 12% !
Phosphor dots & line structure often cause moire patters on well focused
tubes!
Gamma correction is transmitted for CRT characteristics so the maker does
not have to bother with the correction curve.
When new they give very good definition, but age to give blurred highlights
& sometimes burnt screen areas. Sharp focus often variable over screen.
Lifetime good 20,000 hrs, if the QRO 16:9 LOPTY stage does not fail!
Colours can change with age/magnets.
Good viewing angle, but normally a curved screen!
Some tubes are blackened to help see blacks strong room lighting.
Quite high QRM @ multiples of line freq & from SMPSU.
2/ LCD Panel. Has very high levels of back fluorescent lights needed all the
time, as only about 10% of light entering the panel can get through to the
front. The light is polarised (60% loss) the LCD cell were (10% more is
lost), then another polariser & then the colour filter (75% loss) /pixel.
LCDs are well known for slow response (motion drag) due to slow cells some
are faster nowadays but not all!
Imperfect polarisation nulling results in poor blacks this is viewing angle
related, the good contrast figures often quoted are only for 1 viewer!
The gamma curve is often poor due to the non linear polarisation nulling
process in the LCD cells, which can effect true colour rendition.
Pixilated screens are always sharp! LCDs can't normally get picture burn
ins.
Standard conversion to the screen format may produce poorer moving pictures
than expected. Also picture delay resulting in lip sync problem on some
sources.
Susceptible to random pixel /addressing (stripe) failure. Tube failure & UV
degradation of the polarisers.
QRM fairly high from large screen address wires radiating, & lamp SMPS.
3/ PLASMA Panel. Like CRT, a fluorescent phosphor determines the colour,
illuminate by a UV from a Mercury arc as in a fluorescent tube lamp but per
pixel. Most of the light produced leaves the front of the screen, but
drivers can be inefficient..
Some artifacts can be visible if you scan your eyes fast as the pixels are
not on continuously.
Lifetime dependent on brilliance but around the life of the average CRT
nowadays.
Viewing angle very good, good real contrast figures.
Pixilated screens are always sharp!
Like CRTs it can easily get picture burn in if bright static picture is
left on for hours. Displays have anti burn strategies to reduce the chance
of this.
Colours are very true, as there is no gamma correction needed & brightness
much higher the LCD displays.
Screen is quite delicate, much more so than a heavy CRT, also must be kept
vertical for transport because of this!
Standard conversion to the screen format may produce poorer moving pictures
than expected. Also picture delay resulting in lip sync problem on some
sources.
Susceptible to random addressing (stripe) failure.
QRM high from power in large screen address wires radiating & SMPSU.
4/ LCD Projection. These need very bright white light from expensive Xenon arc
lamp lifetime up to 6000 hrs for under run lamps. (2000 on some projectors)
N.B. these lamps can explode.
The light is split into 3 colours with mirrors, feed the a polariser, LCD
tile & a 2nd polariser per colour, before being recombined in a colour
splitting prism (optical block) & then to the projection lens.
Incident light a real problem for black on all projection screen systems.
The gamma curve is often poor due to the non linear polarisation nulling
process in the LCD cells, which can effect true colour rendition.
Standard conversion to the screen format may produce poorer moving pictures
than expected. Also picture delay resulting in lip sync problem on some
sources.
Susceptible to random pixel /addressing (stripe) failure. Also due to heat
& UV failure/damage of LCD tiles & polarising filters in the optical block.
Due to optics focus etc. there will be some aberration/fuzziness.
Dirt in air degrades optics difficult to maintain spotless!!
Easy to get a large screen, with rear projection system or ceiling mount.
Fan cooled, must be cooled off after use. QRM from lamp SMPS.
5/ DLP Projection. Rotating coloured wheel & reflected pixel tile system.
These give very bright pictures, also using expensive Xenon arc lamps of
limited lifetime. N.B. these lamps can explode.
Main problem is the colour flicker due to the wheel, making reading text &
moving pictures quite eye aching.
Standard conversion to the screen format may produce poorer moving pictures
than expected. Also picture delay resulting in lip sync problem on some
sources.
Susceptible to random pixel /addressing (stripe) failure. Also due to heat
& UV failure of mirror tile block with age will occur.
Due to optics focus etc. there will be some aberration/fuzziness.
Less optics than a LCD, but dirt in air degrades optics difficult to
maintain spotless!!
Easy to get a large screen, with rear projection system or ceiling mount.
Fan cooled, must be cooled off after use. QRM from lamp SMPS.
The broadcasters are still deciding what standard to adopt to give the best
pictures, as each type of display ideally needs different production system /
technique to give its best!
DEMO
At the lecture, the BBC chaps had a SONY 30" 720 x 2000 pixel LCD that looked
very good, but it produced a blue for back! They were able to show the same
pictures that I originally saw 16ft x 9ft from a huge œ1M LCD RGB projection
system 20 years ago, but not as clear on the Sony TV! Some of the early stuff
was made with a modified 1" vidicon camera producing 40MHz bandwidth per
colour, this was an extreme a challenge for 4 linked up digital recorders at
the time!
Split picture comparisons of normal & HD were appreciated & showed the
difference could be seen by many, but by no means all could see a difference.
With a standard aerial installation they also tuned into the current London HD
test & compared off air HD to off air normal freeview.
COMPRESSION
TV starts out at the camera at a HD studio camera at an amazing 1.2Giga B/S,
but it is compressed down & broadcast at only 19MB/S, normal widescreen TV is
between 10MB/S to as low as 1 MB/S on some channels! Which is why it is often
full of annoying MPEG artifacts.
Better compression systems are around, but to compress at all, will always
produce some artifacts when doing something unusual with the picture.
e.g. cross fade or shake camera, need 25/50 new frames per second, not the 1 or
2 digital give you!
Most channels (except BBC1) use a shared statistical multiplexer, this allows
for more channels most of the time, but with the odd few seconds where there is
not enough bandwidth to cope with demand & a channel picture will then freeze
awaiting some spectrum to become available for an new I frame for example. This
is nothing to do with errors!
SPECTRUM
When the UK analogue TV is switched off in 2012, the spectrum freed up will not
be available for HDTV! The broadcasters are expect to cope with what they have
& encourage more/better compression (new Set Top Boxes needed for everyone?),
or to buy back commercial spectrum for HDTV.
Y Don't U send an interesting bul>?
73 de John G8MNY @ GB7CIP
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