Calibrating I/O

studiorat

Over on the Analogue Something thread the Trout mentioned this.

TroutMask wrote: »

I have a mil-spec 600 ohm precision resistor soldered to a F-XLR somewhere that I use for calibrating consoles. I even made a couple or artful loops with the excess resistor leads for the scope probes and DVM clips to grab onto - now that's dedication!

Since the analog meter is such a rarity these days ( I don't have any) I'm trying to figure out how I can check I/O trim pots of a 192 without with a VU or PPM.
Feels like they are trimmed down a little. (Pre Amps into A/D box)

Can this be done with a multi meter perhaps?

I have the following, Scope, Multi-Meter. No Oscillator except a laptop and a sound card...

I'm thinking do it like a tape machine, get -18dB from the return to read 0vu on the Trouts Magic XLR. Reading from each side of the resistor, no? Then do like wise for your Oscillator and plug that back into the inputs and adjust the 192 trims to read -18dB.

Would this work with a Multi-Meter? How would we set it up?

Any thoughts gents?

madtheory

If it's not a 600 ohm resistor I don't think a voltmeter would work because 0 dBu is power (I forget the figure) into a 600 ohm load. Need to check Sound Reinforcement Handbook... :eek:

I would use +4dBu source and set the meters to -18dBFS. The Proel/ Thomann meters can provide the tones at that level I think.

PaulBrewer

I suppose you could 'calibrate' your output by plugging it into a few things and see do you get the 'same' reading then use that 'calibrated' output to go to your ins?

madtheory

Here's the answer:
AC Voltmeter If the RG transducer or an oscilloscope is out of the question, another alternative is to use an AC voltmeter (preferably with a "dB" scale). Here, instead of relying on the OL indicator, you choose a very large output level, say, +20 dBu (7.75 Vrms) or whatever is your maximum, e.g., max consumer level is often only -10 dBV (316 mv) and define that as your maximum level. Now set everything to not clip at this level. This is a reasonable and accurate way to do it, but is it an appropriate maximum? Well, you already know (from the above discussion) that you need 12-20 dB of headroom above your average signal. It is normal pro audio practice to set your average level at +4 dBu (which, incidentally, registers as "0 dB" on a true VU meter). And since all high quality pro audio equipment can handle +20 dBu in and out, then this value becomes a safe maximum level for setting gains, giving you 16 dB of headroom -- plenty for most systems.

From Rane Notes- one can always rely on Rane!

TroutMask

hi Studiorat,

http://akmedia.digidesign.com/support/docs/192_IO_Guide_25670.pdf

this is the 192 calibration guide and it gives the steps for a self-contained calibration starting on Pg. 23. The only weird bit is the 'plug in an external VU' bit - and this function is simulated by my load resistor + DVM setup. If your DVM has a True RMS setting, you can dispense with the load resistor.
Calibrate the outputs using a tweaker to 1.228V = -18dBFS = 0 VU (or +4 dBU).

If you can afford it, get a Fluke meter they are the best and do a very 'true' RMS

studiorat

Cool thanks chaps. Using a buss assignment of the tone to the output tracks is a nice touch!!

Will have a go over the next few days and report back.