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From: sp_mclaugh on 1 Nov 2006 02:41
Hello,

I'd like to understand the "guts" of the deflection circuit inside a
fixed-frequency display. I know the most monitors today are either
tri/quad sync, or truly multi-synchronous, but for learning purposes,
I'd rather start with a fixed-frequency display. I'm just learning the
basics of VGA timing (doing a video project on an FPGA), and I feel
that these answers would give me a more rock-solid grasp on what I can
and can't do with VGA.

At first, I thought that the horizontal deflection circuit would be
even simpler than a normal sawtooth relaxation oscillator. My thought
was that rather than using a "threshold" type device to trigger the
draining of the capacitor in the oscillator, we could just use the sync
pulse directly to turn a drain transistor on/off. But all the
information I've read on the net seems to suggest that the sync pulses
in some way add/subtract only slightly to the "normal" frequency. Can
someone clarify what's actually going on here? Perhaps there isn't one
correct answer (different companies, different method), but I'm
guessing that there's a common method used by most.

As for the vertical deflection, it should look like a "staircase",
right? Of course, there would be a sharp downward line after each
"staircase" for the vertical retrace. How is this waveform generated?
My initial guess, again, would be a simple integrator whose input was
the horizontal sync pulses. It would reset itself on each vertical sync
pulse.

However, this type of a setup would mean that the *length* of the
horizontal sync pulses would directly affect how much the picture was
vertically stretched. I'm not sure whether this is true or not.
Question: So long as the horizontal sync pulses are at the correct
frequency and are close to the correct length, does their *exact*
length in fact have a great effect on the picture? I'm a bit more into
the digital circuit world, so excuse the terminology, but another way
of asking the question might be: Is there some part of the deflection
circuit which is "edge-triggered" by the sync signal, or does the
actual pulse length of the sync signal matter? (I know that's kind of a
crude analogy since not all CRT's use digital circuitry - sorry)

If anyone can answer those questions, the next will be:

Tri/quad frequency monitors seem like a fairly straightforward
extension of fixed-frequency monitors, but how does the deflection
circuitry in *true* multi-synchronous CRT's work? I know a PLL is
involved, but what is the supporting circuitry? I've only just started
to become familiar with PLL's in general, so please excuse me if this
is fairly obvious. We were using PLL's in the context of sinusoids,
please let me know how they would be used to generate the sawtooth
signal we desire.

Of course, the threshold voltage (after which we want each sawtooth
pulse to end) would still be the same as with the fixed-frequency case.
After all, this is directly related to the size of the CRT screen. So
determining when to "reset" isn't hard. It's determining how fast the
sawtooth should rise (ie, how fast the beam sweeps horizontally) that I
don't understand (circuit-wise).

Thank you for your help !

Sean

PS - If anyone thinks that it would be better to a different group, let
me know.

From: Ancient_Hacker on 1 Nov 2006 06:54
Traditionally, TV sets have to be designed to handle noisy sync pulses.
So they use some kind of locked oscillator, nowdays a full-blown PLL,
where an integrator or capacitor smooths out the average frequency.

On a computer the sync pulses are cleaner than in a off-the-airwaves Tv
signal, but it's still advantageous to use a PLL or some kind of
stabilized and locked oscillator.

Tis is true of both vertical and horizontal syncs and sweeps.

The actual sweep generator has to deliver a linear ramp, in current,
not voltage, to the CRT deflection coils. This requires an impossible
voltage waveform (an infinte pulse at the beginning to instantly start
the current rising, this is usually omitted and the start of the sweep
is put a little bit off the left of the screen to not show this
glitch). The rest of the waveform is a trapezoidal pulse (in voltage)
which in an inductor generates a current ramp.

For the retrace, that requires a huge voltage pulse to sweep the
current quickly back to the left or top edge.

These funny signals are always generated by a very clever resonant and
diode-damped circuit, which is optimized to minimize the huge power
dissipation it would take to generate the retrace using normal methods.

I would NOT suggest you try to do one of these circuits on your own,
they're tricky and require parts that can take the high voltages and
currents.

From: Jan Panteltje on 1 Nov 2006 12:45
On a sunny day (31 Oct 2006 23:41:26 -0800) it happened sp_mclaugh(a)yahoo.com
wrote in <1162366886.757045.320690(a)i42g2000cwa.googlegroups.com>:

>Hello,
>
>I'd like to understand the "guts" of the deflection circuit inside a
>fixed-frequency display.

When we talk CRT (not LCD), here you go:
A local oscillator is phase locked to the H sync pulse.
The oscillator drives a switch that basically puts a voltage
across the H defection coils for the duration of the line.
Because the coils are inductive, a linear rising current happens in the coils,
the resulting magnetic field sweeps the electron beam from left to right.
When the switch goes off at the end of the line, back EMF happens
and the current reverses (using a diode).
Use is made of resonance in this case.
To make a correction for the angle a capacitor is is series with
the H deflection coil, S correction, so angular speed is maximum in
the middle.

In the V deflection the inductance and frequency is so low that the coils
are driven by a normal totempole amp, the defection coil current is
measured by a small series resistor, and compared to a reference linear
sawtooth, also phase locked to the V sync.

In a VGA monitor there are many more corrections in the deflection system,
and perhaps the same system for H is used as I described for V here.
There is no 'staircase'.

When the frequency is higher, to get the same current in say half the time,
you would need double the voltage on the H deflection coil.

It is possible they use low inductance H deflection coils and a similar
drive as V, but I am not sure.
The days of a little diagram that comes with TVs or monitors are past..

My Samsung CRT is still working after being on 12/24 7 days/week, now for
5 years, so I have not looked at the circuit.
LCD is the future, but the picture is not as good as a good CRT.
Especially for video.
There are more better display systems in development.

From: sp_mclaugh on 1 Nov 2006 12:58
Ancient_Hacker wrote:
> Traditionally, TV sets have to be designed to handle noisy sync pulses.
> So they use some kind of locked oscillator, nowdays a full-blown PLL,
> where an integrator or capacitor smooths out the average frequency.
>
> On a computer the sync pulses are cleaner than in a off-the-airwaves Tv
> signal, but it's still advantageous to use a PLL or some kind of
> stabilized and locked oscillator.
>
> Tis is true of both vertical and horizontal syncs and sweeps.

I guess I'm still not 100% clear on the details of how the PLL is set
up for this task. Part of the problem might be that I'm only just
starting to learn about PLL's in general. But how would we take those
short square-pulse sync signals, feed it into a PLL, and then
eventually get a sawtooth output? And for the vertical deflection, how
would we use a PLL to eventually get that "staircase-like" output?

> The actual sweep generator has to deliver a linear ramp, in current,
> not voltage, to the CRT deflection coils.

Yes, I actually did realize that, but I guess I didn't word my post
quite right. If you like, add a voltage-controlled current source to
the output I described. Like you said down below, that might be very
impractical (ie, power-hungry, and stressful on components). But my
main interest is in generating the "shape" of the deflection waveform,
even if it wouldn't be a smart way to build a real CRT.

Thanks for the reply.
-Sean

From: sp_mclaugh on 1 Nov 2006 13:21
Perhaps there is some way to make a voltage-controlled relaxation
sawtooth oscillator (where the threshold voltage for "resets" is
somehow adjustable by a voltage) and use a Type 2 PLL ? ie, the type
which only looks at the *edges* of the oscillator and reference input?
Just a guess.

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