Voltage/Frequency Equations
« on: December 30, 2005, 07:57:40 PM »
+4dbm (0vu) = 1.23volts or so.  How is that voltage derived and if different frequencies are inputed @ +4dbm, will the voltage be different, say at 10k or 50hz?  Or am I wrong here?

Trying to calibrate external gear using my Pro Tools signal generator and the different frequencies at the same output levels are giving different voltages, from 50hz (1.5 or so volts) to 20k (.750 volts or so), without changing any levels.   And I set the outputs to 1k @ 1.23volts.


PRR

Voltage/Frequency Equations
« Reply #1 on: December 30, 2005, 08:13:36 PM »
> +4dbm (0vu) = 1.23volts ... will the voltage be different, say at 10k or 50hz?

No. Voltage is voltage, from DC to THz.

> different frequencies at the same output levels are giving different voltages, from 50hz (1.5 or so volts) to 20k (.750 volts or so)

What meter are you using????? Many digital DVMs have steep 400Hz low-pass filters, useless for audio. Your numbers are not that bad, more like a lame-spec op-amp that just-barely-almost gets to 20KHz before collapsing. A mechanical meter's inductance tends to make the highs droop. A tape recorder's meters are sometimes pre-EQed.

I'm an old grump. I say you can NOT do audio measurements with DVMs, PCs, or DAWs unless you also have a stupid dumb oscillator and idiot wideband voltmeter to check them against. To an audio technician, these are as basic as a chisel is to a carpenter. And sadly, many modern carpenters don't use a chisel except to pry the safety guard off the power-saw, just as many audio-techs try to measure with too-smart machines.

rodabod

Voltage/Frequency Equations
« Reply #2 on: December 30, 2005, 08:31:48 PM »
Can anyone confirm whether or not this would be a reasonable audio signal generator for someone on a budget with very little room?

eBay listing

Also, for measuring audio signal voltages, it would seem that my DMM would not be suitable due to the poor high frequency response which you describe. What is my best option for budget price and small-ish size? A wide-band voltmeter or a 'scope?

Sorry for the hijack!

Roddy
Quote from: tv
punchy fat bastard chip

Voltage/Frequency Equations
« Reply #3 on: December 30, 2005, 09:21:19 PM »
I'm using a Fluke mult-meter to measure the voltage across the xlr 2 and 3 pins and a signal generator tdm plugin in pro tools.  I understand your analogy.  Hence the apprentice coming to the lab, with nothing more than a tack hammer.   I might get an external HP 204C, or something like that, signal generator, but wasn't planning on a full on digital oscillator with a screen and all that.  I'm only looking to align some things and have a tech come for the rest.

I'm just a little confused about why the voltage is changing with the frequency.  Maybe it's my meter.  I don't know.  I use it a lot, but not a lot for audio purposes.  Mainly just at work.

gyraf

Voltage/Frequency Equations
« Reply #4 on: December 31, 2005, 02:56:37 AM »
..voltage changing with frequency is called equalisation or filtering..

 :razz:
..note to self: don't let Harman run your company..

pstamler

Re: Voltage/Frequency Equations
« Reply #5 on: December 31, 2005, 04:48:59 AM »
Quote from: "audioedge"
+4dbm (0vu) = 1.23volts or so.  How is that voltage derived


Just to answer that first question: The dBm scale is a power scale; 0dBm is defined as 1 milliwatt, at a working impedance of 600 ohms (formerly the standard, derived from phone company practice). The relationship between power, voltage and impedance:

P = V^2 / Z

Rearranging to solve for voltage:

V = sqrt (P * Z)

If P = .001W and Z = 600 ohms then V is about .775V. That's 0dBm.

To find what +4dBm is, divide 4 by 20 (4/20 = 0.2) and take the antilog (10^0.2 = 1.58). That's the factor by which +4dBm is higher than 0dBm, so multiply the factor by the 0dBm voltage (1.58 * .775 = 1.228) and Bob's your uncle.

Peace,
Paul

Voltage/Frequency Equations
« Reply #6 on: December 31, 2005, 01:21:39 PM »
Thanks Uncle Bob.  Let me read that a few more times to make sense of it all.  So maybe there is some type of filtering or equalisation going on whithin the software itself.  Any good oscillators out there, under $200?

PRR

Voltage/Frequency Equations
« Reply #7 on: January 01, 2006, 02:15:14 AM »
> this would be a reasonable audio signal generator for someone on a budget with very little room?

For 15 bucks, it's terrific.

Listen to its 1KHz sine output, and then turn your treble control way down. Does the sound change? That type of function generator will have DEAD flat output over the audio band, but several percent sine distortion, a lot of little corners on the "smooth sine" because it is really a soft-clipped triangle. You can do a lot with a crummy sine, just remember that it has harmonics all up the audio band.

I would not like to see it bid-up to 50 bucks/pounds/euros/clams, because we "should" be able to do a lot better for $50-$100. However the next best thing, the Global Specialties 2001, seens to sell for $200 if you can find it in stock.

Nearly any of the good-old H-P, Heath, Philips oscillators 1950-1980 would be a better deal, especially since they can be had $5-$75 as-is, and usually work fine. But they run from big to BIG. And because they were built to last, if they do develop a fault it can be hard to figure out.

> A wide-band voltmeter or a 'scope?

IMHO, you must have a classic Vacuum Tube Volt Meter VTVM. The basic models were not accurate below a fraction of an AC Volt, and some were not flat to 20Hz, but most are dead-flat to far above the audio band (my Heath is perfect at 200KC and isn't falling at 1MC). The resolution does not compare to a DVM, but the dumb analog display will show trends and short peak/dips much better. And unlike these digi-meters, there is nothing to go screwy on high frequency AC: hollow-state rectifier, cap, DC meter.

Whatcha really need is BOTH an audio source and an audio meter that read flat against each other. Set at 1KHz, note the reading, sweep 20Hz to 20KHz, should hardly change. It is possible to live with un-flat gear: one of my 200ABs droops at the top of each band so I remember to add a db at 200, 2KC, 20KC to check general trends, but if you need db accuracy it is too tedious to be adding and subtracting 0.7dB here and 1.25dB there.

Voltage/Frequency Equations
« Reply #8 on: January 01, 2006, 03:01:35 PM »
Thanks PRR.  I'm looking around as we speek for a used oscillator.  Was recommended a HP 204C or HP 200.  I'll look for the voltmeter as well.  As far as my signal generator, if I sweep through the frequencies from 50hz to 20k, and first set the voltage to 1.23v at 1k sine, the vu meters don't have much of a change, but the voltage changes if I check the pinouts of my outputs.  That's where I am now.  So I'm trying to figure out if I have do or don't have a problem, since the vu meters don't really change, but the voltage changes.  

Thanks again for all the help here.

bovox

Voltage/Frequency Equations
« Reply #9 on: January 01, 2006, 04:59:54 PM »
To bring up a point. I´m sitting with an ill HP 209A OSC. that just brings out dc and no freq. at all. Is there perhaps someone who has a scheme of it? It´s very dear to me , but I´de like to see it in shape.
Cheers Bo


CJ

Voltage/Frequency Equations
« Reply #10 on: January 02, 2006, 05:52:34 PM »
Yes I bought an old HP VTVM for 5 dollars in perfect shape, great value and never needs batteries. Leave it on all the time.

You might be able to get away with hi-freq measurements if your DMM has a frequency counter in it. But these are usually high end models. Check the owners manual for specs.  If the freq readings go to zero, you know you are beyond the measuring abillity of the meter.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
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Val_r

Voltage/Frequency Equations
« Reply #11 on: January 04, 2006, 02:27:19 AM »
Quote from: "audioedge"
Thanks PRR.  I'm looking around as we speek for a used oscillator.  Was recommended a HP 204C or HP 200.  I'll look for the voltmeter as well.  As far as my signal generator, if I sweep through the frequencies from 50hz to 20k, and first set the voltage to 1.23v at 1k sine, the vu meters don't have much of a change, but the voltage changes if I check the pinouts of my outputs.  That's where I am now.  So I'm trying to figure out if I have do or don't have a problem, since the vu meters don't really change, but the voltage changes.


What about the HP 334A, a thd analyzer, wide band AC Volt meter and built-in wide band low distortion oscillator.
There are some listings on eBay.

 :?:
Ok, cool ! Jahmekya

PRR

Voltage/Frequency Equations
« Reply #12 on: January 04, 2006, 02:31:37 AM »
> What about the HP 334A

It is $100-$150, it is huge, and if you have to ask, you probably need the Manual.

Otherwise: a solid machine.

Brian Roth

Voltage/Frequency Equations
« Reply #13 on: January 04, 2006, 02:42:48 AM »
For aligning analog tape machines, and many other bench tasks, I LOVE having a function generator at hand.  Ultra-flat frequency response and instant "settling" time.  This kit is basically the "app notes" schematic published by Exar, and sells for $25 w/o case or power supply:

http://www.jameco.com/wcsstore/Jameco/Products/ProdDS/20685.PDF

Yes, the sine wave output of a function generator isn't totally pure, but good enough for many/most applications.  Hell, every MRL test tape made for the past 15 or 20 years used a computer controlled function generator.

If I need to make distortion measurements, then I fire up my Amber test set.

Bri
Brian Roth Technical Services
Salina Kansas, home of the best vinyl on the planet!

http://www.BrianRoth.com
recordingservicesandsupply.com/
www.qualityrecordpressings.com/
store.acousticsounds.com


 

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