Signal Generator - Calibrate Level - strange behaviour

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Phil Pope

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Hi

I am using latest version 5.31.1 but seem to have replicated this problem with v5.19.
generating a sine wave with the Generator at 50Hz, switch it to Volts scale and press play to start the signal.
connecting multimeter to speaker outputs of my amplifier I get a reading of 1.7V.
Press the calibrate level button, type in 1.7 and press enter. the generator now says the level is 1.7V RMS
Shouldn't the level in the generator now track the level on the multimeter?
If I turn the generator up to 3V the multimeter only reads 2.83V - nearly 10% error.
Is this normal? Have I fundamentally misunderstood how this should work?
I've tried two different laptops, two different amps, headphone output and USB interface.
 
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DanDan

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Hi Phil. Puzzling indeed. I don't see a Volts option on my REW 5.19 on an old iMac here. Also given the computers Volume Control I don't see how the Generator could generate a known output voltage? Presumably your laptops and interface and amplifier ALL have volume controls too. Also I don't see a Calibrate button on the Generator.
 
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Phil Pope

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the calibrate function and volt function are only on the newer version. but on version 5.19 increasing the generator level by 10dB doesn't give 10dB rise in multimeter voltage.
 
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DanDan

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I just tried Line out to Line In on this old iMac. SPL Meter Tracked the Generator dB for dB. With a foggy brain I am trying to get it around Minus dBFS!
 
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DanDan

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OK, Volume up full, Generator at max (-3dB) Left output only. 200Hz. I measure 1.4VRMS
Taking it down 6dB to -9dBFS I measure 0.7VRMS.
All seems as it should be here Phil.
 
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sam_adams

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Phil Pope said:
If I turn the generator up to 3V the multimeter only reads 2.83V - nearly 10% error.
Click to expand...

If the multimeter is not a true RMS design with at least a 20 Hz to 20 kHz bandwidth, you will not be able to measure audio signal levels accurately.
 
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Phil Pope

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the better one I have is a UNI-T UT60E multimeter. says it is trueRMS with a bandwidth of 40-400Hz. The signal I am generating for calibration is a 50Hz sine wave so it should be able to measure that accurately.
I have ordered a Multicomp Pro MP730678 to check whether my UNI-T is faulty but again it has 40-400Hz bandwidth. Surely I don't need to spend hundreds on a 20Hz-20kHz meter just to calibrate the voltage level?

nb just checked the UNI-T meter and it exceeds spec - consistent voltage reading from 20Hz-1kHz
 
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John Mulcahy

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There isn't a way for REW to get the levels wrong, it is only multiplication. If you are hitting a limit or the output is not increasing linearly that is outside REW.
 
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DanDan

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Phil, may I suggest you make a new post or question regarding what you are trying to do. If you are troubleshooting your REW rig, is suggest you start with the bare minimum. The onboard Line In and Out if possible. Adjust the OS levels to nominal or 0dB. Any old RMS AC Voltmeter should be reasonably accurate at a single sine frequency within it's range. You mentioned speaker outputs of your amplifier....... I would omit that from the chain of events.
 
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Phil Pope

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DanDan said:
Phil, may I suggest you make a new post or question regarding what you are trying to do. If you are troubleshooting your REW rig, is suggest you start with the bare minimum. The onboard Line In and Out if possible. Adjust the OS levels to nominal or 0dB. Any old RMS AC Voltmeter should be reasonably accurate at a single sine frequency within it's range. You mentioned speaker outputs of your amplifier....... I would omit that from the chain of events.
Click to expand...
measuring the voltage on the headphone jack output I get exactly the same issue - the output voltage doesn't track the voltage in the generator. I have tried two different laptops, disabled every audio setting I can find that might be interfering, reinstalled the audio drivers. Waiting for a third multi-meter to arrive see if that solves it.
 
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Phil Pope

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tried it at 400Hz and it seems a lot more accurate. calibrating then doubling the generator voltage i'm getting about 2% error rather than 10% or more. new multi meter is on its way so will try same with that one. Always assumed multimeters were most accurate at mains frequency 50Hz
 
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DanDan

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I used 200Hz for no particular reason. Fluke 87 Meter. It tracks precisely. Have you tried using the Meter in REW? Again mine tracked precisely. Given the change in performance with frequency It seems likely that the REW Meter can perform way better than the Hardware Meters you are buying.
 
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Phil Pope

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I'm trying to measure the output voltage from my amplifier so I can do calibrated loudspeaker sensitivity measurements - so I need to use an external meter don't I? what do you mean by 'the Meter in REW'?
 
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DanDan

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Ah, good to know. John Mulcahy many confirm this, but I suspect that the SPL Meter in REW is probably very very good when used with any half decent sound card or interface. I guess if you use Line Input and feed it a precise RMS Voltage Level, you would then have a very precise RMS dBVolt Meter. So if you could get a loan of a precise Generator or Audio RMS Voltmeter for an hour you should be able to Calibrate your rig.
 
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