[sdiy] How to measure VU meter properties

[rsdio at audiobanshee.com]

On Oct 17, 2014, at 9:25 PM, David Ingebretsen wrote:
> How can I measure the electrical properties of an "unknown" VU meter (and
> its light) for circuit design so I don't damage the meter?

To avoid damage, I'd say all you have to do is make sure that your test signals are in the audio range and within normal audio studio voltage levels (not much more than +4 dBu).

But it really matters whether you're talking about the visible response or the meter's load on the audio circuit.

For the former, there really are no measurements that you can automate without "eyeballing" it. VU standard says 300 ms from 0% to 99% of the distance to the correct reading, which should be what you want calibrated to 0 VU. But 0 VU is not necessarily 0 dBu or 0 dBV or 0 dBFS, but rather whatever a given studio wants as a standard. Then there's the allowed overshoot of 1.0% to 1.5% - a lot of a meter's response has to do with the physics or the spring, the mass of the needle, and various aspects that cannot be measured electrically.

For the latter, the load should be simple to measure. The VU standard says to measure at 1 kHz for the level needed to read 0 VU. Just don't apply more voltage than would be present with +4 dBu and you shouldn't damage the meter.

What circuit design are you trying to do? Are you trying to make an "unknown" VU meter perform precisely according to the VU specifications?

Also, what sorts of electrical properties are you trying to measure for the light?

Brian Willoughby
Sound Consulting