OT: discussion on TV frequency history, was RE: [MPlayer-users] R e: which deinterlace filter

[Karl Ewald]

Hi Rich,

I must say Reimar's comments do sound sensible to me.
While in digital circuitry, it is easy to divide frequency by using a
counter, this cannot be done in the same way in analog circuitry. So instead
you will happily use different sources for the horizontal and the vertical
timebase, since it doesn't really matter how the vertical rescan relates to
the horizontal position, you have a fairly large number of horizontal lines
that are blanked out between successive pictures, during which you need to
move the electron beam back up vertically, it doesn't really matter how this
is aligned with the horizontal oscillation.

I wouldn't be surprised if you had in fact trouble hooking up a VCR even to
a 60s made TV set, and the vertical frequency generator issues discussed are
stemming from the 40s or early 50s!

I recall reading in an operating manual for a 1975 made PAL color TV set
(WEGA brand, if anyone cares, built from a mix of tubes and transistors)
that program button 8 (the highest one) is intended for attaching a VCR or
video player and has a shorter timebase (can't remember the exact term, and
I'm translating it to English now anyway, so please don't be too picky about
the expression), so apparently it was known that the video signal modulated
(there was no video-in, so the attached device had to have an HF modulator
in it to merge the signal as an additional channel onto the antenna cable)
from video playing devices was slightly different from the one received from
live broadcast.

I recall that we had late 50s or 60s made b&w TV sets which had adjustment
knobs called "catch" (German: Fang) which as I recall were used to fine-tune
the frequency generators to the broadcast signal to avoid the picture from
"running through" (or, indeed force it to by misadjusting). This would
indicate that in these TVs, the vertical frequency was not taken from the
power supply, at least not directly, since this wouldn't have allowed
adjusting it in any way. But that was already at least a decade or two after
the invention of TV broadcast so the first devices may well have done it
that way.
It also indicates that back then it was certainly not easy to "lock onto"
sync signals without already having a timebase that is very close to that of
the signal, that's why your idea "the actual sync pulses can drive the
refresh" doesn't suffice without having already a suitable frequency
generator to match them.

Karl

-----Original Message-----
From: D Richard Felker III [mailto:dalias at aerifal.cx]
Sent: Dienstag, 21. Oktober 2003 16:06
To: MPlayer user's list.
Subject: Re: [MPlayer-users] Re: which deinterlace filter


[Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
On Tue, Oct 21, 2003 at 03:01:19PM +0200, Reimar Döffinger wrote:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> Hi,
> > But you have the numbers upside-down. :)) I don't see how having a
> > 60Hz clock source helps you get a clock source for horizontal refresh
> > (much higher). You're dividing time, not multiplying it, when you
> > multiply clock by a "nice integer"...
> 
> First of all I want to say that I don't know anymore where I read that 
> the framerates have to do with the frequency of the power supply, so I'm 
> not absolutely sure if it's true, but it seems to make sense to me.
> First of all you must think of the fact that at the time the first TV 
> sets appeared, you didn't have a quartz to create frequencies, but you 
> had to use a capacitor and a coil (hope that's the right word...).
> For high frequency you need only very small and simple components, so 
> it's easy to get a (more or less) exact frequency.
> For low frequencies (like 60Hz) you would need such a high capacity, 
> that you would probably have to use for example electrolytic 
> capactitors, which have a tolerance of more than 10%, which means your 
> resulting frequency will be at least +- 10% and temperature dependant 
> (and those old TVs got really hot because of the tubes in them), and 
> also aren't supposed to be used with alternating currents.
> Now suppose you use that already not too stable frequency generator in 
> an environment where there is everywhere a frequency only 20% apart.
> I think you would be lucky if it would continue working at all, but then 
> probably at something like 52Hz instead of 60Hz...
> And last but not least, I think you can use a low, exact frequency very 
> easily to improve the accuracy of a high-frequency resonant circuit (if 
> it's accurate enough to begin with) by simply plugging them together.
> The other way round you will have to either use a counter or frequncy 
> divider, with I think both couldn't support that high frequencies at 
> that time (at least today they work digitally, a technology that was int 
> its beginnings at that time).
> 
> Hope I could explain why I think I am right.. ;-)

I just don't understand how a 60Hz clock source is useful to begin
with in a television. It seems to me that all you should need is the
tuner clock, and then the actual sync pulses in the signal can drive
the refresh. Perhaps old B&W TV's somehow aligned the electron beam
vertically using the phase of the A/C current to control the magnets?

...BUT this shouldn't be possible, because if they did, the picture
wouldn't have stabilized when watching new 59.94 Hz color signals on a
B&W TV... So I'm still at a loss regarding how the 60 Hz from the
power supply is useful.

Rich

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