Re: "PC World" magazine on TV formats etc.

On 16 Apr, 02:04, "Stephen" <step...@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
"Mortimer" <m...@xxxxxxxxxxx> wrote in message

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"Tony Quinn" <t...@xxxxxxxxxxxxx> wrote in message
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Yes, don't the 720p and 1080p formats have more pixels in the horizontal
direction as well - so 720p would be (720/576) ^2 (ie 1.6) times the
number
pixels for 768x576 or 720x576 and 1080 will be (1080/576) ^2 (ie 3.5).

What a load of old bollocks

576i is 720 x 576 = 414720 pixels
720p/i is 1280 x 720 = 921600 pixels (2.2 x 576i)
1080p/i is 1920 x 1080 = 2073600 pixels (5 x 576i)

Ah, so the 720p and 1080p formats are true widescreen, not 4:3 stretched
to a 16:9 frame as for 576i (eg "PAL"), as I was assuming.

1920 x 1080 and 1280 x 720 are true widescreen, 1440 x 1080 is 4:3 HD
stretched to a 16:9 frame, but the reason for 720 x 576 is more complicated.

576i was never designed to be deinterlaced. The resolution was worked out
using the Kell Factor (0.7) to be 625 lines x 5 MHz analogue bandwidth. The
equivalent in digital TV terms is 520 resolution horizontal and 576xKell
Factor vertically, which works out to 520 x 400.

The vertical resolution is lower than 576 in the ratio of the Kell factor
because interlaced displays are assumed to have that much less effective
resolution than progressive or deinterlaced ones.

Against this background ITV's 544 x 576 resolution (effectively 544 x 400
when not deinterlaced) is perfectly valid because it is just as good as
analogue, although only for narrowscreen.

In the same way the BBC's 720 x 576 is enough for widescreen by analogue
standards. If you take a 4 by 3 centre cut out from the BBC widescreen
picture it's 540 x 576 (effectively 540 x 400 when not deinterlaced), which
again is just as good as analogue.

It is often said that analogue TV does not have square pixels, however this
is only the case if you use modern equipment to take a freeze frame or
freeze field. The original choice of 5 MHz analogue bandwidth was made on
the basis of having equal vertical and horizontal resolutions on an
interlaced CRT display, the equivalent of square pixels. You might say that
there are "virtual" pixels when the Kell Factor is taken into account. They
would be 1/0.7 times taller than the real pixels in a freeze frame, and this
would make them square.

You make it sound more reasonable / logical than it is.

Different analogue standards have/had different bandwidths - 5MHz,
5.5MHz, and 6MHz being nominal values for various flavours of 625-line
PAL broadcasts across the world. The general aim was as you suggest,
but different countries took different interpretations. It kind of
flies out of the window with (early-ish) colour TV because of cross
colour artefacts on fine detail which older sets had no way of
removing.

720 pixels is the active line of an SD 4x3 image sampled at 13.5MHz,
plus a little to spare to avoid ringing edges and to ensure the
picture doesn't get cropped if the timing is slightly off. 13.5MHz was
chosen because it gives an integer number of pixels per line for both
625/25 and 525/~30 systems, and satisfies Nyquist for all broadcast SD
video formats.

This pre-dates 16x9 (widescreen) and rigid square pixels. There was
talk of higher sampling rates for 16x9 to yield squarer pixels; it's
in the standard, but rarely (never?) used.

Cheers,
David.
.

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