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From: Arfa Daily on 23 May 2010 11:53

"Caius" <briccus(a)yahoo.it> wrote in message
news:4bf8e661$0$40286$4fafbaef(a)reader2.news.tin.it...
>
> "Arfa Daily" <arfa.daily(a)ntlworld.com> ha scritto nel messaggio
> news:VK_Jn.39676$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..
>

Without a 'scope, you are at a huge disadvantage for trying to figure the
foibles of this sort of circuit, where potentially complex waveforms
predominate ...


>
> 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.
>

At the very least, there should be an additional decoupling cap of say 47 to
100uF, situated close to the supply pin of the sync separator chip, if you
are going to use a remote supply, connected to other electronics within the
system. This is important.


>
> 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?


It is if there is any residual HF noise on the signal that you are trying to
measure. It would normally be considered good practice to have a simple low
pass filter on such a feed.


>
> >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...

Yes, I understand that JAMMA is arcade stuff. As to whether vertical sync
rate and refresh rate are the same thing, it's a bit more complex than that.
It depends on whether you are talking interlaced or non interlaced. If you
see a stable 30Hz on the odd / even pin, then chances are it's a 30Hz
interlaced system, which is sort of equivalent to a 60Hz refresh rate

Arfa


From: stratus46 on 25 May 2010 01:38
On May 23, 12:05 am, who where <no...(a)home.net> wrote:
<snip>
> >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.

They'll deal with level variances but they fall apart when the LF
response of the video is bad. You need _good_ coupling caps in the
entire video chain. That's going to be very difficult to correct
without a scope. That lousy .1uF input to pin 2 cap is much better
when it's 1uF and the 680K resistor on pin 6 works better at 470K. I
was _not_ using this with consumer gear but in a broadcast/post
production environment where the video was generally excellent.

G²
From: who where on 26 May 2010 21:29
On Mon, 24 May 2010 22:38:13 -0700 (PDT), stratus46(a)yahoo.com wrote:

>On May 23, 12:05�am, who where <no...(a)home.net> wrote:
><snip>
> > >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.
>
>They'll deal with level variances but they fall apart when the LF
>response of the video is bad. You need _good_ coupling caps in the
>entire video chain. That's going to be very difficult to correct
>without a scope. That lousy .1uF input to pin 2 cap is much better
>when it's 1uF and the 680K resistor on pin 6 works better at 470K. I
>was _not_ using this with consumer gear but in a broadcast/post
>production environment where the video was generally excellent.

Not sure if that is a response to my post ...

We had tried the 1881 on a variety of signal sources with the same
issues presenting. The 4981 worked fine on all those sources. Go
figure.
From: Caius on 27 May 2010 11:49

<stratus46(a)yahoo.com> ha scritto nel messaggio
news:2709ccd5-9830-49f7-a95d-78179fc1eb3e(a)v37g2000vbv.googlegroups.com...
On May 23, 12:05 am, who where <no...(a)home.net> wrote:
>That lousy .1uF input to pin 2 cap is much better
when it's 1uF and the 680K resistor on pin 6 works better at 470K.

I tried a 1uf capacitor on pin 2 and nothing has changed (I didn't obtain
anymore the value of 15.575 KHz on pin 1..).A friend of mine told me that he
can measure correctly Vsync of 60 Hz with a LM1881 with exactly the same
circuit on NATIONAL datasheet (the same I'm trying).
So,I can conclude, it's a issue of mine frequency counter...

P.S.
My friend uses a DMM which can measure frequency up to 20 MHz, probably it
is better (more sensible..) than my "assembled from kit" counter


From: stratus46 on 28 May 2010 12:41
On May 27, 8:49 am, "Caius" <bric...(a)yahoo.it> wrote:
> <stratu...(a)yahoo.com> ha scritto nel messaggionews:2709ccd5-9830-49f7-a95d-78179fc1eb3e(a)v37g2000vbv.googlegroups.com...
> On May 23, 12:05 am, who where <no...(a)home.net> wrote:
>  >That lousy .1uF input  to pin 2 cap is much better
> when it's 1uF and the 680K resistor on pin 6 works better at 470K.
>
> I tried a 1uf capacitor on pin 2 and nothing has changed (I didn't
obtain
> anymore the value of 15.575 KHz on pin 1..).A friend of mine told
me that he
> can measure correctly Vsync of 60 Hz with a LM1881 with exactly the
same
> circuit on NATIONAL datasheet (the same I'm trying).
> So,I can conclude, it's a issue of mine frequency counter...
>
> P.S.
> My friend uses a DMM which can measure frequency up to 20 MHz,
probably it
> is better (more sensible..) than my "assembled from kit" counter

You cannot make that assumption without a scope which unfortunately
you don't have. I'm only guessing but I bet your video has bad low
frequency tilt which shows of as the 'DC' of the back porch wandering
all over the place. Cause: bad capacitors somewhere in the video
chain. IIRC the Gennum chip includes clamps to guarantee sampling at
50% of the sync pulse meaning it's more tolerant of poor video. The
scope would tell all.

G²
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