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HELP:Measuring refresh rate of PCBs


caius

Question

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)

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I might be wrong but surely the vsync would be in the 15kHz range, but the screen refresh will be 50 or 60Hz (mains AC frequency) divided by two, historically the refresh rate was derived from the AC frequency hence the relationship, so either 25 or 30 frames per second. Vsync rate and fps are not the same thing. Edited by Womble
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I might be wrong but surely the vsync would be in the 15kHz range, but the screen refresh will be 50 or 60 frames per second, vsync rate and fps are not the same thing.

Posted via Mobile Device

 

Yes, infact I obtain values of 15KHz on pin 3 of the LM1881..so, vsync and refresh rate are two different things.Anyway, how can measure the refre rate in order to help MAME team to achieve a more acurate emulation?I thought my circuit was enough for (a person close to MAME Team has confirmed me too..)....

For example, let's take Double Dragon II.In his screen property there is 57.444853 Hz, I thought this was the Vsync and was possible to measure it with my circuit...

 

P.S.

Guru from MAMETeam, we need your precious help!I know you use some kind of circuit to measure the refresh rate of the PCBs..You also adviced a friend of mine (Corrado from Italy) on how to build this circuit, he told me.

Edited by caius
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What you want to measure is the output of pin 7, and divide it by 2 for your frame rate measurement.

 

According to the datasheet, Pin 7 is the Odd/Even Frame out, and because the video is interlaced, there are 2 frames per video refresh.

 

Cheers

Jacob

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