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From: Caiusfabricius on 22 May 2010 15:16
Hi all,
I built a simple circuit using te famous "sync separator" IC LM1881 in order
to measure the refresh rate (Vsync) of JAMMA PCBs.I follow this scheme (but
also the one form NATIONAL manufacturer is fine):

http://www.gamoover.net/tuto/schema-de-montage-du-lm1881-s%C3%A9paration-de-la-synchro-du-signal-composite

The problem is that when I try to measure with a frequency counter the
vertical sync output on PIN 3 of the LM1818 I obtain bad values (in order of
Khz not in Hz as it should be.Usually it should be about 60Hz)
To say that I check the circuit and is OK as well as my frequency counter
that works fine when I measure, for example, the clock of the various CPU.

Any suggestions?I'm going crazy...

P.S.
I use the VIDEO Sync (pin P of solder side of the jamma connector) as
COMPOSITE VIDEO IN (pin 2 of the LM1881)


From: Arfa Daily on 22 May 2010 20:34

"Caiusfabricius" <briccus(a)yahoo.it> wrote in message
news:4bf82d86$0$18994$4fafbaef(a)reader5.news.tin.it...
> Hi all,
> I built a simple circuit using te famous "sync separator" IC LM1881 in
> order to measure the refresh rate (Vsync) of JAMMA PCBs.I follow this
> scheme (but also the one form NATIONAL manufacturer is fine):
>
> http://www.gamoover.net/tuto/schema-de-montage-du-lm1881-s%C3%A9paration-de-la-synchro-du-signal-composite
>
> The problem is that when I try to measure with a frequency counter the
> vertical sync output on PIN 3 of the LM1818 I obtain bad values (in order
> of Khz not in Hz as it should be.Usually it should be about 60Hz)
> To say that I check the circuit and is OK as well as my frequency counter
> that works fine when I measure, for example, the clock of the various CPU.
>
> Any suggestions?I'm going crazy...
>
> P.S.
> I use the VIDEO Sync (pin P of solder side of the jamma connector) as
> COMPOSITE VIDEO IN (pin 2 of the LM1881)
>

Have you looked at the output of your sync separator chip on a 'scope to
verify that *only* field sync pulses are appearing at the pin that you are
connecting to your counter ? Is the power supply clean ? No hash or high
frequency ripple that might be appearing on the output ? Have you tried a
simple R-C low pass filter between the chip output and the frequency counter
input ? (pure off the top of my head guess, try 10k series resistor followed
by 1000pF shunt capacitor). Does your counter have appropriate sensitivity
for the level coming out of the chip ? Too little level (under-drive) or too
much level (overload) can cause counters to produce erratic and erroneous
readings. Also remember that a field sync pulse train is *highly*
asymmetric, being a series of short pulses with long gaps between. Can your
frequency counter handle that sort of waveform, and interpret it as a
'frequency' ? If the counter has a "time" function on it, you may do better
to try using this, and just work out the apparent field frequency from the
time between pulses.

Have you tried measuring the odd / even output to at least see if that gets
you in the right ballpark ? If it is an interlaced sync pulse train, then
the vertical rate should be twice the odd / even rate.


You probably need to do a bit more research as to what exactly the problem
is, before reporting back so that we can see if we can help you figure how
to make it work as you want it to.

Arfa


From: stratus46 on 22 May 2010 22:15
On May 22, 12:16 pm, "Caiusfabricius" <bric...(a)yahoo.it> wrote:
> Hi all,
> I built a simple circuit using te famous "sync separator" IC LM1881 in order
> to measure the refresh rate (Vsync) of JAMMA PCBs.I follow this scheme (but
> also the one form NATIONAL manufacturer is fine):
>
> http://www.gamoover.net/tuto/schema-de-montage-du-lm1881-s%C3%A9parat...
>
> The problem is that when I try to measure with a frequency counter the
> vertical sync output on PIN 3 of the LM1818 I obtain bad values (in order of
> Khz not in Hz as it should be.Usually it should be about 60Hz)
> To say that I check the circuit and is OK  as well as my frequency counter
> that works fine when I measure, for example, the clock of the various CPU..
>
> Any suggestions?I'm going crazy...
>
> P.S.
> I use the VIDEO Sync (pin P of solder side of the jamma connector) as
> COMPOSITE VIDEO IN (pin 2 of the LM1881)

Do you have a scope? Can you verify the sync and burst gate outputs?
LM1881s are fairly forgiving of poor quality / wrong level video. Did
you try measuring the odd/even pulse? It should be 30 Hz (OK,
29.97002616 for the purists).

G²
From: who where on 23 May 2010 03:05
On Sat, 22 May 2010 21:16:18 +0200, "Caiusfabricius"
<briccus(a)yahoo.it> wrote:

>Hi all,
>I built a simple circuit using te famous "sync separator" IC LM1881 in order
>to measure the refresh rate (Vsync) of JAMMA PCBs.I follow this scheme (but
>also the one form NATIONAL manufacturer is fine):
>
>http://www.gamoover.net/tuto/schema-de-montage-du-lm1881-s%C3%A9paration-de-la-synchro-du-signal-composite
>
>The problem is that when I try to measure with a frequency counter the
>vertical sync output on PIN 3 of the LM1818 I obtain bad values (in order of
>Khz not in Hz as it should be.Usually it should be about 60Hz)
>To say that I check the circuit and is OK as well as my frequency counter
>that works fine when I measure, for example, the clock of the various CPU.
>
>Any suggestions?I'm going crazy...
>
>P.S.
>I use the VIDEO Sync (pin P of solder side of the jamma connector) as
>COMPOSITE VIDEO IN (pin 2 of the LM1881)

The LM1881 is notorious for giving undesired output sync waveforms.
They were used in the old (pre-GPS) days for deriving a reference
frequency from TV signals, and caused enormous grief to many.

We had such a frequency reference system using a widely published
schematic, and it would not provide the correct output frequency due
to an extra pulse in the train. Eventually we switched to the Gennum
4981sync separator and the problem disappeared. YMMV.
From: Caius on 23 May 2010 04:24

"Arfa Daily" <arfa.daily(a)ntlworld.com> ha scritto nel messaggio
news:VK_Jn.39676$eM2.37344(a)newsfe14.ams2...
>
> Have you looked at the output of your sync separator chip on a 'scope to
> verify that *only* field sync pulses are appearing at the pin that you are
> connecting to your counter ?

I don't have a scope, sorry..


Is the power supply clean ? No hash or high
> frequency ripple that might be appearing on the output ?

Ground and power pins of the LM1881 are directly connected to the ground and
+5 Volt
of my JAMMA test rig.


Have you tried a
> simple R-C low pass filter between the chip output and the frequency
> counter input ? (pure off the top of my head guess, try 10k series
> resistor followed by 1000pF shunt capacitor).

No, I didn't.Is it useful?

>Does your counter have appropriate sensitivity
> for the level coming out of the chip ? Too little level (under-drive) or
> too much level (overload) can cause counters to produce erratic and
> erroneous readings. Also remember that a field sync pulse train is
> *highly* asymmetric, being a series of short pulses with long gaps
> between. Can your frequency counter handle that sort of waveform, and
> interpret it as a 'frequency' ? If the counter has a "time" function on
> it, you may do better to try using this, and just work out the apparent
> field frequency from the time between pulses.

I use this counter:

http://cgi.ebay.it/0Hz-to-50Mhz-Frequency-Counter-Module-Kit-LCD-display-/140353600315?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item20adb9fb3b#ht_1974wt_939




>
> Have you tried measuring the odd / even output to at least see if that
> gets you in the right ballpark ? If it is an interlaced sync pulse train,
> then the vertical rate should be twice the odd / even rate.

Yes, on some PCBs I have tested I obtain a value of 30 Hz on the odd / even
output.Surely I ALWAYS obtain the value of 15.675 KHz (PAL signal) on pin 1
of the LM1881.
But is the Vsync and refresh rate the same thing?I'm speaking about arcade
PCBs...

>
>
> You probably need to do a bit more research as to what exactly the problem
> is, before reporting back so that we can see if we can help you figure how
> to make it work as you want it to.
>
> Arfa
>


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