[opendtv] Re: HD vs. 625 vs. 525 vs. 405
- From: "Alan Roberts" <roberts.mugswell@xxxxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Sun, 29 Aug 2004 21:49:03 +0100
I very strongly suspect that decisions were taken based on the number of
channels needed for coverage and the total bandwidth available. When 405 was
planned, only Band I was available. That numbers game controlled it all.
It's very tempting to post-justify the decisions, but in practice, such
niceties weren't available at the time.
----- Original Message -----
From: "Terry Harvey" <tjharvey@xxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Sunday, August 29, 2004 6:56 PM
Subject: [opendtv] Re: HD vs. 625 vs. 525 vs. 405
> I always thought the Kell factor would be a consideration. If not, how
else
> would you define how wide the video channel was to be?
>
> The 6MHz channel had been defined in the US for double sideband 441 line
> 60Hz interlaced television in the late thirties. When in 1941 the move to
> vestigial sideband was planned, the designers had to balance vertical and
> horizontal resolution of the system. If you take into account the Kell
> factor and interlace losses for vertical resolution then the maximum
> resolution horizontally to match would be defined by the bandwidth.
>
> It was a balance at the time. Similarly when 625 appeared in Germany in
> 1952, to maintain a matched vertical against horizontal resolution, Kell
> and the interlace factor would be applied to define the ultimate video
> channel width of 5 MHz.
>
> The UK in 1964 chose 5.5 MHz for the video channel, for the little extra
> bandwidth/ horizontal resolution it would provide and also I believe to
> accommodate NTSC colour full-double sideband chroma channels. But that is
> another story!
>
> Terry Harvey
>
> At 06:06 PM 8/29/2004 +0100, you wrote:
> >Surely the Kell factor is NEVER applied in the design of a tv system? The
> >horizontal bandwidth defines horizontal resolution, and the raster
defines
> >the vertical resolution. Kell never comes into the design, Kell
calculates
> >the likely vertical resolution of the resulting system. In an ideal
world,
> >these two values are identical or at least similar in terms of elements
per
> >spatial unit, but I've never heard of any tv system being designed with
Kell
> >in mind. Neither 525 nor 625 are square, nor is 1080i or psf or 720p on a
> >crt. Only 1920/1080p and 1280/720p are truly square on pixellated
displays.
> >
> >----- Original Message -----
> >From: "Terry Harvey" <tjharvey@xxxxxxx>
> >To: <opendtv@xxxxxxxxxxxxx>
> >Sent: Sunday, August 29, 2004 4:04 PM
> >Subject: [opendtv] Re: HD vs. 625 vs. 525 vs. 405
> >
> >
> > > What I am saying is that the bandwidth allocated to the video channel
will
> > > determine horizontal resolution. When 405 was designed, 3MHz was
excessive
> > > in comparison with the later 525 and 625 systems. As a result, the 405
> > > picture element is narrow and not equally dimensioned vertically and
> > > horizontally.
> > >
> > > We would later acknowledge that the Kell Factor and interlace factor
would
> > > be applied: but it wasn't applied when system A was designed.
> > >
> > >
> > > Terry Harvey
> > >
> > >
> > >
> > > At 03:49 PM 8/29/2004 +0100, you wrote:
> > > >I really don't know what you mean by this. The bandwidth of System A
was
> > > >3MHz, Kell has nothing to do with that. It was an interlaced system
with
> >no
> > > >electrical vertical filters. The only vertical process done was in
the
> > > >tubes, where the target was clear each field because the tube spot
was
> >big
> > > >enough (although plastic) to clear two lines worth of it in each
sweep.
> > > >Again, Kell has nothing to do with this either.
> > > >
> > > >The lenses were good because they came almost directly from the film
> > > >industry where 35mm standards had been applied. So the horizontal
> >bandwidth
> > > >was nicely filled, as was the vertical bandwidth. Kell applies to the
> > > >vertical scan just as for all other scanned systems, as does the
> >interlace
> > > >factor. So you get a lower perceived vertical resolution than you
might
> >from
> > > >the number of lines, just as for any other interlaced tv system.
> > > >
> > > >Why do you say that Kell wasn't applied to it? Kell always applies to
> > > >scanned systems, that was his point. The interlace factor always
applies
> >to
> > > >interlaced systems as well. You can't get away from it, that's
physics.
> > > >
> > > >----- Original Message -----
> > > >From: "Terry Harvey" <tjharvey@xxxxxxx>
> > > >To: <opendtv@xxxxxxxxxxxxx>
> > > >Sent: Sunday, August 29, 2004 3:13 PM
> > > >Subject: [opendtv] Re: HD vs. 625 vs. 525 vs. 405
> > > >
> > > >
> > > > > Maybe I applied the term Kell too loosely. And there are other
facors
> > > >which
> > > > > made 405 pictures look sharp.
> > > > >
> > > > > Because the horizontal scan velocity was slower, it was more
easilly
> > > > > handled by an early amplifier chain in terms of frequency response
and
> > > > > phase response. Before the 405 closure, I made VHS recordings of
the
> > > > > signal and when displayed it looks almost 'broadcast' quality.
Also
> >note
> > > > > the extended bandwidth given to the video channel as the Kell
factor
> >was
> > > > > never applied to 405 systems.
> > > > >
> > > > > Terry Harvey
> > > > >
> > > > >
> > > > >
> > > > > At 01:08 PM 8/29/2004 +0100, you wrote:
> > > > > >405-line pictures looked sharp but the late 50s simply because
crt
> > > >displays
> > > > > >were being made with the spot too small for the raster. So you
could
> >see
> > > >the
> > > > > >lines. This became more and more true when we ran 405/625 dual
> >standard
> > > >tv
> > > > > >sets, where the spot profile was a reasonable match to 625 and
too
> >small
> > > >for
> > > > > >625, so you could see black between the lines of 405. That made
the
> > > >pictures
> > > > > >look artificially sharp. Kell never came into it.
> > > > > >
> > > > > >----- Original Message -----
> > > > > >From: "Terry Harvey" <tjharvey@xxxxxxx>
> > > > > >To: <opendtv@xxxxxxxxxxxxx>; <opendtv@xxxxxxxxxxxxx>
> > > > > >Sent: Sunday, August 29, 2004 1:42 AM
> > > > > >Subject: [opendtv] Re: HD vs. 625 vs. 525 vs. 405
> > > > > >
> > > > > >
> > > > > > > Okay I understand your definition and I was trying to say the
same
> > > >thing
> > > > > > > from a different perspective. In system A, the E.M.I.
engineers
> >did
> > > >not
> > > > > > > account for the loss of the vertical resolution. And I am
aware
> >of
> > > >the
> > > > > > > Kell, Bedford and Trainer "Experimental Television Station"
> > > >Proceedings of
> > > > > > > the I.R.E. Volume 22 - 1934!
> > > > > > >
> > > > > > > In system M as you indicate, the Kell factor was applied to
reduce
> >the
> > > > > > > horizontal resolution by the factor of 0.7 to account for the
> >apparent
> > > > > > > vertical resolution loss. In system A, the horizontal
resolution
> >is
> > > >higher
> > > > > > > relative to the vertical resolution because the apparent loss
was
> >not
> > > > > >taken
> > > > > > > account of.
> > > > > > >
> > > > > > > Maybe it would be more correct to say the Kell Factor was not
> >applied
> > > >to
> > > > > > > system A. That is what I meant by saying the Kell Factor is
> >unity. (I
> > > > > >will
> > > > > > > not bring interlace into this as it would further muddy the
> >issue.)
> > > > > > >
> > > > > > > Perhaps then the sharpness of 405 received pictures can be
> >accounted
> > > >for
> > > > > >by
> > > > > > > the excessive horizontal resolution applied.
> > > > > > >
> > > > > > > Terry Harvey
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > At 12:09 PM 8/28/2004 -0400, Mark Schubin wrote:
> > > > > > > >Terry Harvey wrote:
> > > > > > > >
> > > > > > > > >The definition of Kell Factor is the number obtained by
> >dividing
> > > >the
> > > > > >raster
> > > > > > > > >pitch distance by the width of the picture resolution
elements.
> > > > > > > > >
> > > > > > > >No, it is not. The common definition of the Kell factor is
the
> > > > > > > >reduction in vertical resolution from the number of scanning
> >lines
> > > > > > > >(although, in the age of fixed-pixel displays, it has also
been
> > > >applied
> > > > > > > >to the reduction in hirizontal resolution from the number of
> >active
> > > > > > > >samples per line).
> > > > > > > >
> > > > > > > > >Recall that system A was developed in 1935/36, before Kell
and
> > > >others
> > > > > > > > >discovered that the interlaced vertical resolution was not
> >ideal.
> > > > > > > > >
> > > > > > > >Kell did his work in the early 1930s, before System A was
> >broadcast.
> > > > > > > >
> > > > > > > >I have extensively researched Kell's work. I would suggest
that
> >you
> > > > > > > >look up the Proceedings of the IRE.
> > > > > > > >
> > > > > > > >TTFN,
> > > > > > > >Mark
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > >
> > > >
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