Re: 1080i & 720p HDTV Resolution

From: DAB sounds worse than FM (dab_is_at_low.quality)
Date: 02/15/05 

Date: Tue, 15 Feb 2005 15:19:19 GMT

davidrobinson@postmaster.co.uk wrote:
> [snip]
>
> The reason that we examine the worst case scenario is because this
> sets the limit - so it's an obvious case to go for when trying to
> figure out what the limit is!

In my years of watching TV I have never seen the whole screen flash.
In my years of watching TV I have never seen a screen full of alternate
black and white lines with a pitch equal to the scan-line pitch on TV.

Basically, you appeal to the worst case scenario, but that worst case
scenario never happens.

In each and every post you mention a flashing screen. Personally I think
your telly is on the blink.

> Also, your suggestion that the human visual system's inherent low pass
> reduces the need for a pre-interlace vertical filter in video is about
> as valid as an argument that the human auditory system's inherent low
> pass reduces the need for a pre-sampling anti-alias filter in audio.

I've never said that the *need* is reduced; you always *need* a filter.

What I did say is that the 0.7*Fs factor can be *relaxed* for HD
compared to SD.

> In audio, without correct filtering, the (usually inaudible) high
> frequencies will be aliased right down into the audible band, where
> they will be very audible and objectionable. (Thanks to Kevin for this
> obvious, excellent example btw!)

I've just answered this in reply to Kevin's post.

> Likewise, as you've been shown at least 10 times, without correcting
> filtering, the (usually invisible) high vertical frequency detail will
> be transformed into highly visible flashing by the interlacing
> process.

AIUI, all TV uses a vertical (anti-aliasing) lowpass filter that does
not obey Nyquist's criterion. But when watching DTT have you ever seen
"highly visible flashing by the interlacing process"? I haven't.

> btw, whether you watch on an interlaced or progressive display is
> irrelevant - at that limit, even the progressive display can't "know"
> whether the original image contained temporally-static spatially-high
> frequency components, or temporally-flashing spatially-low frequency
> ones. The two domains are entirely confused by interlacing, and this
> area of confusion must be avoided in interlaced signals by the use of
> appropriate filters. Hence (to bring us back to where we started) you
> do not and cannot have 0.9x1080 effective pixels of resolution in a
> 1080i system.

To correct you: I originally said that you should be able to increase
the 0.7 factor. IIRC, it was you that introduced the 0.9 factor, and I
merely said that you might be able to. In hindsight I doubt 0.9 would be
feasible, but I am sticking with my assertion that the 0.7 factor could
be increased. 

-- 
Steve - www.digitalradiotech.co.uk - Digital Radio News & Info
Find the cheapest Freeview, DAB & MP3 Player Prices:
http://www.digitalradiotech.co.uk/freeview_receivers.htm
http://www.digitalradiotech.co.uk/dab_digital_radios.htm
http://www.digitalradiotech.co.uk/mp3_players_1GB-5GB.htm
http://www.digitalradiotech.co.uk/mp3_players_large_capacity.htm

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