[opendtv] Re: HD vs. 625 vs. 525 vs. 405
- From: "Alan Roberts" <roberts.mugswell@xxxxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Sat, 28 Aug 2004 13:20:28 +0100
I don't like spoiling a good story, but there are mistakes in this one.
ITU standard A (405-line) was interlaced. Line frequency 10,125Hz and 25Hz
frame rate, makes it interlaced. (I know it was interlaced, I watched it for
years and frequently had to repair and tweak our old 12" Marconiphone tv
set). Incidentally, the strange line frequency meant that total line
duration was 98.765,432,1 microseconds, exactly. I don't know if that fact
was crucial in setting up the standard, I suspect that the easy relationship
between line and field rate had much to do with it since line counters used
valve amplifiers at that time.
The earlier competitor for standard A, from Baird, was 240-line progressive
at 50Hz. The bandwidth requirements were similar. The fight-out between
Baird and EMI to set the UK standard before the war must have been an
interesting time, since there were neither cameras nor displays that could
do full justice to 405, but 240 was easy.
The Kell factor is nothing to do with interlace, it's to do with sampling.
Kell worked on progressive sampling, not interlace. The "interlace factor"
happens to have approximately the same value for most purposes, but is a
separate issue.
----- Original Message -----
From: "Terry Harvey" <tjharvey@xxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>; <opendtv@xxxxxxxxxxxxx>
Sent: Saturday, August 28, 2004 2:46 AM
Subject: [opendtv] Re: HD vs. 625 vs. 525 vs. 405
> I would like to share a few thoughts on quality perception of broadcast
> television.
>
> The 405 system (System A) produced subjectively sharper pictures due to a
> number of factors. The original transmissions from London (until 1956)
were
> double sideband (instead of vestigial sideband for all other systems
> since). This would lead to somewhat better video phase response due to the
> presence of both sidebands. Also, the Kell factor was 1.0 instead of 0.6
> for 525 systems. This lead to better horizontal resolution. I was
impressed
> with off-air 405 years ago: the pictures looked good to the critical eye.
>
> The move to vestigial sideband and Kell factor of 0.6 were motivated by
the
> need to reduce transmission bandwidth. Like interlaced scanning and
> interleaved analog quadrature modulated chrominance systems (NTSC and PAL
> color), the analog bandwidth reduction methods were crude but of great
> benefit in the days before digital television signal processing became
> available. I constantly find amusement in the fact that we in the TV
> business still rely on these bygone methods and technology.
>
> I mentioned earlier that the BBC no longer accepts PAL format tapes for
> transmission. The move is slow in the US to abandon NTSC production and
> delivery and I feel this is slowing down the DTV transition in this
> country. Broadcasters and service providers are going to have to go the
> extra mile to ensure that production and delivery is cleaned up.
>
> Also, the public perception of HDTV is curious after experiencing the
> following incident. I was in a large major television showroom only last
> weekend and was pleasantly surprised to see an 'Olympics DTV' display.
> However, upon closer investigation, I discovered the 61" Wega display
> costing retail $19,000 was displaying the HD Olympics from our local NBC
> affiliate off-the-air with noticeable NTSC chroma crawl. I questioned the
> Sales Manager. He insisted it was HD....he pointed to a DTV set top box
and
> to the big display.
>
> I pointed out to him that the display was connected via the composite NTSC
> interface: I helped him set it set up and the difference was dramatic. It
> was like this for a week with the NTSC interface and why nobody noticed
> escapes me.
>
> Bravo to Sinclair for taking the lead as a broadcaster promoting over the
> air DTV. However, more consumer education is definitely required.
>
> Regards,
>
> Terry Harvey
> --------------------------------------------------------------------------
----
>
> At 06:32 PM 8/27/2004 -0400, Mark Schubin wrote:
> >Having just returned from a shoot in New Zealand, I thought I'd throw my
> >two cents in on this.
> >
> >Back in the early 1950s, a U.S. delegation toured UK TV facilities to
> >try to determine why UK pictures looked better. At the time, the UK was
> >405 lines, not 625, but it was still said to have better pictures. The
> >delegation determined it was because the UK equipment was better
> >maintained -- clean lenses, etc.
> >
> >I found the same in New Zealand. The equipment was in impeccable shape.
> >
> >Food for thought:
> >
> >- The resolution of human vision tops out at about 30 cycles per degree,
> >nominally about 22.
> >
> >- U.S. viewers sit about 9 feet from their TV sets (the Lechner
distance).
> >
> >- Those two facts combine to make it impossible for viewers to see more
> >than 480 lines on a 25-inch 4:3 TV at normal viewing distance. If it's
> >22 cpd and 16:9, it's no more than 480 lines on a 42-inch set.
> >
> >- Meanwhile, because the psychological sensation of "sharpness" is
> >proportional to the square of the area under an MTF curve and the use of
> >HD in production leads to a higher MTF curve at all detail resolutions,
> >a viewer watching a non-HD set will nevertheless see an improvement in
> >sharpness from HD-shot programming.
> >
> >TTFN,
> >Mark
> >
> >
> >
> >
> >
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>
>
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