VU and PPM

farss wrote on 12/1/2004, 2:29 AM
I know what the difference is and how they work but I have a rather technical question that I've so far been unable to find the answer to and my maths is not up to the task.
Given how the both work I believe it should be possible to come up with a test signal of the appropriate duty cycle that'll read "0' on both types of meter. This would seem very useful to me, we have a lot of vintage analogue video gear still in service most of which has VU meters but trying to work out if their calibration matches correctly the PPM meters on the digital gear is difficult. A simple test tone would be might useful.
Bob.

Comments

MrPhil wrote on 12/1/2004, 5:40 AM
Isn't the difference simply 6dB?
farss wrote on 12/1/2004, 12:55 PM
Not that simple!
One reads absolute instantaneous peak value (PPM) whereas VU meters have a defined response time so depending to some extent on the peak to RMS value of the material VU meters may read very differently or the same as PPMs.
Bob.
Youn wrote on 12/1/2004, 1:06 PM
It'll pretty much always vary from product to product. Most people just use a 1kHz sine wave to check the meter alignments, but keep in mind some things may look and even say "VU" or "PPM" on them, but really they just aren't - well, maybe not according to the 'standards'.
MJhig wrote on 12/1/2004, 4:57 PM
Bob,

PSP VintageMeter (free!)

It toggles between scalable VU and PPM.

MJ
farss wrote on 12/1/2004, 9:35 PM
Got that one already!

Thanks.
musicvid10 wrote on 12/1/2004, 9:44 PM
http://www.shure.com/support/technotes/app-meter.html
farss wrote on 12/2/2004, 3:32 AM
Yeah, read that one some time ago, doesn't answer the question.
I know the difference between the metering systems, what I'm trying to create is a test tone of tone burst that should read 0 VU when a PPM meter read 0dBFS. Sounds simple enough and it should be doable. Furthermore it would seem a very useful thing to have around. Given that modern PPM meters are usually accurate it'd be a good way to check that VU meters are actually ballisticaly within spec (that's much harder to check than the readings on a PPM). The only other method I know of to check a VU meter involves high speed photography.

Bob.
Youn wrote on 12/2/2004, 7:29 AM
Get a steady-tone signal (the 1kHz sine wave) goin so it reads 0 on the VU, and short spikes at 300ms intervals so they read 0 on the PPM.

Really though, just find a VU meter that you know is working to spec then compare the other's side-by-side. I guess I just don't understand why you'd bother with the PPM reading - seems like more trouble then is really worth.
musicvid10 wrote on 12/2/2004, 9:56 AM
Instinct tells me the only tone that could approximate both 0db VU and PPM is a square wave . . .

Here's a nifty tone generator that can save directly to .wav without the need to record.
http://nch.com.au/tonegen/index.html
farss wrote on 12/3/2004, 3:39 AM
Like I said my calculus isn't that good so it did it the brute force way.
Using SF and Vegas so I could easily fiddle with different tone bursts I found that a 1KHz tone burst with 10mS on, 34mS off yields both 0dBFS and 0VU. Metered using Vegas PPM and PSP VintageMeter.
Just how accurate PSP VU meters ballistics are I'm not certain, switched to PPM it reads 1dB lower than the Vegas meters.
SF Stats say that RMS value = -9.44 dB which doesn't seem to quite line up with what I'd thought the answer would be although maybe it's correct, the PSP VU meters ballistics would be having an effect on the reading.

So to further test this I'm going to try it on some Durrough meters next week.
Bob.