HELP:Measuring refresh rate of PCBs

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

der

ut

of

ter

.

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).

G=B2

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.

"Arfa Daily" ha scritto nel messaggio news:VK_Jn.39676$ snipped-for-privacy@newsfe14.ams... >

I don't have a scope, sorry..

Is the power supply clean ? No hash or high

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

No, I didn't.Is it useful?

I use this counter:

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...

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 ...

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.

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.

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

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.

G=B2

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.

ha scritto nel messaggio news: snipped-for-privacy@v37g2000vbv.googlegroups.com... On May 23, 12:05 am, who where 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

snipped-for-privacy@v37g2000vbv.googlegroups.com...

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.

G=B2

messaggi> >That lousy .1uF input to pin 2 cap is much better

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

Absolutely agreed on all points

Arfa

ha scritto nel messaggio news: snipped-for-privacy@o12g2000vba.googlegroups.com... On May 27, 8:49 am, 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

Ok, thanks.Sadly the video (R,G,B ground + SYNC) is from arcade videogame pcb, so I assume they have all poor video signal??? I 'd wan to try the Gennum GS4981 but it seems unfindable...

?? have you tried their website? Google "gennum gs4981" and first hit. Then go to the "buy from distributor" or "get free sample" (sounds better) link at

"who where" ha scritto nel messaggio news: snipped-for-privacy@4ax.com... > ?? have you tried their website? Google "gennum gs4981" and first

Ok, thanks.I will try with the GS4981 instead of LM1881. But, are you sure I will solve my problem with this IC?

Not at all. But (a) it did for us, and (b) it won't cost a fortune to try - especially if you can get a "sample".

Be aware though that although the DS1881 is a drop-in replacment for the LM1881, the GS4981 has a different pinout.

"who where" ha scritto nel messaggio news: snipped-for-privacy@4ax.com... especially if you can get a "sample".

To get the sample is it necessary to have a company (it's a field required)?

Yes, I see the pinout from there:

but it changes only in PIN 7 which is horizontal sync output instead of ODD/EVEN circuit. So, I have to use the same circuit (two 0.1 uf capacitors and a 680 KOhm resistor) like in the LM1881?

On May 31, 1:23=A0am, "Caiusfabricius" wrote: > "who where" ha scritto nel messaggionews: snipped-for-privacy@4ax.com... > =A0 especially if you can get a "sample". >

You should read the data sheet and not just look at the pinout. They explain why you might want smaller vs larger coupling caps and do show the same resistors as the LM1881 -- logical if you're trying to make a drop-in replacement. For your purposes the 4981 vs 4881 in not important, just a reassignment of the odd/even pin which you're not interested in.

G=B2