A reference for the rest of us...

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A reference for the rest of us...

Postby Steve Anderson » Mon Jul 02, 2007 8:51 pm

There have been repeated references to the 'club standard' of the video signal being 1.0V p/p which is (presumably) inherited from the FSTV standard (nothing wrong with that). But, in my investigations of CD audio players few of them are consistant.

Over the past two weeks I have taken measurements from the line-out of eight CD players which were of fixed level, i.e. there is no way to alter the output level.

They were all different, even from within the same brand! (S*%y take note!). They varied over a ratio of almost 5:1.

Sound cards of course have software and sometimes hardware controls of the output level so these don't really count.

So what to do? At first I recorded a simple NBTV waveform and measured the output on the oscilloscope. This was fine when I was at home, but when I wanted to test other machines at my relatives it meant I had to drag along my scope.

So I created a 50Hz sinewave and burnt it onto an audio CD with a p/p amplitude equal to the NBTV waveform. This simply allowed me to use a digital multimeter to measure the output amplitude using the same disc in every machine.

Why 50Hz? I have noticed that many digital multimeters have a very limited frequency response on the AC Volts range. Even good multimeters (Fluke) have a limited response, the cheap ones are far worse.

So it might be useful to have a reference 50Hz track/posting as a line-up tone for those that don't have access to a scope, or when at the convention where you don't want to drag along a bulky oscilloscope along with all the rest of the stuff.

A 50Hz sinewave at 1.0Vp/p should come out at 354mV RMS. What if it's not? A simple pot will attenuate an oversized signal, a simple amplifier will boost it.

Then there's the subject of polarity...

Steve A.
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Postby AncientBrit » Mon Jul 02, 2007 10:46 pm

Steve,

For polarity indication how about a variant of the 50Hz waveform with a clipped or dipped positive peak?

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