Signal generator: dB referenced to what?

[tooold]

Hi -

Can anyone tell me what the "dB" used in Signal Generator is referenced to? No mention in the Digirack docs.

I though it would be dBm, but the output levels (measured with a Fluke) don't conform to any of the cross reference charts for dBm/mv I can find. Granted, the termination resistance can skew things, but this is way off. -20 dBm is supposed to be 22.5 mV, but I'm getting 9.3 mV out of my 002R. And the switches are set to +4.

This matters because I have to measure system gain to calibrate a pair of outboard compressors, and I need to give them -34dBm. If what's coming out of Signal Generator is off, the whole process won't work.

I'm using PT9, which is why I posted this here, but if it should be in the LE section because of the 002R, feel free to move it.

[JonesH]

I might be misunderstanding you, but the Signal Generator is referenced to 0 dB digital Full Scale. Usually FS is referenced as +18, 20 or 24 dBu, but if I recall correctly the 002r can only manage less than so. Guess you will have to measure and quite possibly apply use of the trim plugins.

[Rich Breen]

Originally Posted by tooold

Hi -

Can anyone tell me what the "dB" used in Signal Generator is referenced to? No mention in the Digirack docs.

I though it would be dBm, but the output levels (measured with a Fluke) don't conform to any of the cross reference charts for dBm/mv I can find. Granted, the termination resistance can skew things, but this is way off. -20 dBm is supposed to be 22.5 mV, but I'm getting 9.3 mV out of my 002R. And the switches are set to +4.

This matters because I have to measure system gain to calibrate a pair of outboard compressors, and I need to give them -34dBm. If what's coming out of Signal Generator is off, the whole process won't work.

I'm using PT9, which is why I posted this here, but if it should be in the LE section because of the 002R, feel free to move it.

dB is a relative measurement, and the reference needs to be stated, without it, it's meaningless in any absolute sense. In the case of the PT signal generator, it's dBFS (dB below full scale). Also when you say 9.3mV, you don't specify what kind of measurement; is it 9.3mV RMS, 9.3mV p-p? You need to specify. Are you measuring across the balanced pair? Across a half of the balanced pair? Across an unbalanced output? All these things are important.

What voltage you're getting out of any digital system at -20dBFS depends completely on how it's calibrated. Most common in the US is '0'VU = -18dBFS = +4dBu = 1.228Vrms = 3.5 voltsp-p (the dBm suffix refers to power dissipated across a 600 ohm load and is used far less these days as we're usually working with constant-voltage systems), but -16dBFS and -20dBFS calibrations are also common here.

I don't know how you're cal'ing your compressors, but I've never heard of a -34dBm cal; really you just want to typically cal for unity gain and so that your nominal operating level (what you would call 0VU) will correspond to a '0' meter indication on the compressor, and to optimize for roughly equal amounts of headroom throughout the system.

Sounds to me like you need to gain a deeper understanding of gain structure and level practices before you start mucking about with your voltmeter and a screwdriver.

There's a bunch of dBu to Volts calculators out there on the web, here's one: http://www.sengpielaudio.com/calculator-db-volt.htm

Good luck,
rich