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In article , Charles wrote:
Flywheel sync isn't terribly modern - it's been around for at least 40
years.

I suppose it depends on what you call modern. Our first dual-standard TV
set didn't have it. You could clearly hear the line whistle drop in
pitch if you selected a blank channel, and it sounded very ragged indeed
if there was video noise. 405 line whistle was annoying enough already,
and one of the best reasons I thought for moving to 625.

That would have been around 1960. Seems like yesterday. :-)

Rod.

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In article , John Evans
wrote:
Surely synchronised means in step/time with. The difference between
triggered and free running is the latter "free runs" in the absence of
a pulse but the former doesn't. The are both kept in step by the
pulse. The phase comparator oscillators are free running - those that
I know of at least. The difference is that they are controlled by a
voltage derived from the pulse and not by the pulse directly.

Both types of TV timebase will free run in the absence of synchronising
pulses. A big difference is that one will free run (in principle) at the
correct frequency, whereas the other must run slow, because the pulses
bring them into synchronism in quite different ways.

Maybe I should be referring to "triggered synchronism" rather than a
"triggered timebase", as the pulse does in fact directly trigger an
event, unlike what it does in a flywheel or phase lock system, even if
it isn't required to make the oscillator oscillate in either case.

Rod.

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On Sat, 01 Mar 2008 17:23:19 -0000, Roderick Stewart
wrote:

In article , John Evans wrote:
err, no. *The flyback starts at the start of the blanking period, ie the
start of front porch. *Flyback happens when the scanning voltage reaches it
maximum and quickly discharges to be ready for the next trigger.

Err yes!

The flyback is initiated by the negative going edge of the sync pulse
that occurs at then end of the front porch. The pulse puts the
oscillator voltage over the threshold and the oscillator resets. It
then starts the scanning part of the cycle again. It is free running
in the absense of a sync pulse.

In a field timebase this is more or less what happens, but not in a modern
line timebase. Line pulses don't reset anything; they are compared in a phase
comparator with a sawtooth voltage derived from the oscillator, to produce a
control voltage which is applied to the oscillator to control its frequency.
This arrangement is known as a phase locked loop or flywheel sync.

Rod.


So? Read all of the threads


PPL aka Flywhell sync has been around for years and it is a different
type of oscillator - a votage controlled one. (and it is still free
running in the abscence of an incomimg signal).

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On Sat, 01 Mar 2008 21:23:39 -0000, Roderick Stewart
wrote:

In article , John Evans
wrote:
Surely synchronised means in step/time with. The difference between
triggered and free running is the latter "free runs" in the absence of
a pulse but the former doesn't. The are both kept in step by the
pulse. The phase comparator oscillators are free running - those that
I know of at least. The difference is that they are controlled by a
voltage derived from the pulse and not by the pulse directly.

Both types of TV timebase will free run in the absence of synchronising
pulses. A big difference is that one will free run (in principle) at the
correct frequency, whereas the other must run slow, because the pulses
bring them into synchronism in quite different ways.

Maybe I should be referring to "triggered synchronism" rather than a
"triggered timebase", as the pulse does in fact directly trigger an
event, unlike what it does in a flywheel or phase lock system, even if
it isn't required to make the oscillator oscillate in either case.

Rod.


No argument with that.

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In article , John Evans wrote:
Maybe I should be referring to "triggered synchronism" rather than a*
"triggered timebase", as the pulse does in fact directly trigger an*
event, unlike what it does in a flywheel or phase lock system, even if*
it isn't required to make the oscillator oscillate in either case.

Rod.

No argument with that.

Phew! Agreement at last.

Rod.

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In message , Roderick
Stewart writes
In article , John Evans wrote:
Maybe I should be referring to "triggered synchronism" rather than a*
"triggered timebase", as the pulse does in fact directly trigger an*
event, unlike what it does in a flywheel or phase lock system, even if*
it isn't required to make the oscillator oscillate in either case.

Rod.

No argument with that.

Phew! Agreement at last.

Rod.

Not finished yet!

Although it's still a long, long way from DVD regions, no one has yet
mentioned that flywheel sync (intended to improve picture jitter on
noisy signals) was a pain when VCRs appeared. Although the VCR output
signal was not noisy, it had inherent jitter on it (caused by the
mechanical imperfections of the tape and the tape transport mechanism).
The result was a wobbly picture. To overcome this, the then
state-of-the-art TV sets (with push buttons), normally had one of the
channel-selection buttons which also disabled the flywheel action.
--
Ian

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In article , Ian Jackson wrote:
Maybe I should be referring to "triggered synchronism" rather than a*
"triggered timebase", as the pulse does in fact directly trigger an*
event, unlike what it does in a flywheel or phase lock system, even if*
it isn't required to make the oscillator oscillate in either case.

Rod.

No argument with that.

Phew! Agreement at last.

Rod.

Not finished yet!

Although it's still a long, long way from DVD regions, no one has yet*
mentioned that flywheel sync (intended to improve picture jitter on*
noisy signals) was a pain when VCRs appeared. Although the VCR output*
signal was not noisy, it had inherent jitter on it (caused by the*
mechanical imperfections of the tape and the tape transport mechanism).*
The result was a wobbly picture. To overcome this, the then*
state-of-the-art TV sets (with push buttons), normally had one of the*
channel-selection buttons which also disabled the flywheel action.

Oh dear oh dear! It didn't actually disable the flywheel action, but just
shortened the time constant of the filter in the control path, so the
oscillator could respond more quickly to variations in the input frequency.

Rod.

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Oh dear oh dear! It didn't actually disable the flywheel action, but just
shortened the time constant of the filter in the control path, so the
oscillator could respond more quickly to variations in the input frequency.

Rod.



One of the Panasonic range used Channel 8 as the VCR channel. This
automitacally changed the time constants when selected.

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In message , Roderick
Stewart writes
In article , Ian Jackson wrote:
Maybe I should be referring to "triggered synchronism" rather than a*
"triggered timebase", as the pulse does in fact directly trigger an*
event, unlike what it does in a flywheel or phase lock system, even if*
it isn't required to make the oscillator oscillate in either case.

Rod.

No argument with that.

Phew! Agreement at last.

Rod.

Not finished yet!

Although it's still a long, long way from DVD regions, no one has yet*
mentioned that flywheel sync (intended to improve picture jitter on*
noisy signals) was a pain when VCRs appeared. Although the VCR output*
signal was not noisy, it had inherent jitter on it (caused by the*
mechanical imperfections of the tape and the tape transport mechanism).*
The result was a wobbly picture. To overcome this, the then*
state-of-the-art TV sets (with push buttons), normally had one of the*
channel-selection buttons which also disabled the flywheel action.

Oh dear oh dear! It didn't actually disable the flywheel action, but just
shortened the time constant of the filter in the control path, so the
oscillator could respond more quickly to variations in the input frequency.

Rod.

Well, you knew what I meant, didn't you? Shorten the TC sufficiently,
and there isn't any flywheel action.
--
Ian

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In article , Ian Jackson wrote:
The result was a wobbly picture. To overcome this, the then*
state-of-the-art TV sets (with push buttons), normally had one of the*
channel-selection buttons which also disabled the flywheel action.

Oh dear oh dear! It didn't actually disable the flywheel action, but just
shortened the time constant of the filter in the control path, so the
oscillator could respond more quickly to variations in the input frequency.

Rod.

Well, you knew what I meant, didn't you? Shorten the TC sufficiently,*
and there isn't any flywheel action.

Since this appears to be a nit-picking and hair-splitting thread, I feel
obliged to point out that shortening the time constant is more like changing
the flywheel for a smaller one, rather than disabling the mechanism
altogether. The oscillator would still free-run at the correct frequency.

I think we all know what we're talking about here really, but something has
put us in the mood for an argument....

Rod.