Calibrating Input Level for Plugins

[James Freeman]

Yeah, that's exactly 17.5 dBu for the FM9.

I re-measured my fm9 via both usb and spdif
View attachment 7637View attachment 7636

View attachment 7638

 

[James Freeman]

What's the noise floor with guitar plugged and volume at zero?
RMS please.

 

[DLC86]

What's the noise floor with guitar plugged and volume at zero?
RMS please.

PS: it's in Italian but I hope it's understandable. For an input 1 A/D sensitivity setting of 0%, 50% and 100% respectively

 

[Orvillain]

At the default 50% A/D sensitivity, a 0.5v RMS sine (generated by an external synth) measures at -16.5dBFS in the DAW, reading directly from Input 1/Inst (front panel) input.

When I adjust A/D sensitivity down to the 6% figure required for my guitar pickups, it is the same measurement.

I did it again, 0.5v RMS using 1kHz sine-wave from my Summit synth. Plugged into the front of the Axe III with input 1 A/D sensitivity set to 50% and I get -17dBFS in the DAW. Set to 6% A/D sensitivity, I get the same, so there seems to be a 0.5dB window of error with my test, but more or less the same figure.

Axe III is set as the soundcard, and the input I am measuring is definitely the front instrument input.

If it helps, this is an Axe III Mark II Turbo.

 

[James Freeman]

Thank you DLC86, that's great.

PS: it's in Italian but I hope it's understandable. For an input 1 A/D sensitivity setting of 0%, 50% and 100% respectively

Yes it's understandable.
RMS medio, AES-17 is the value.


@Orvillain
Can you create a 100Hz sine tone, and measure Milli-volts in your DVM?

 

[DLC86]

What's the noise floor with guitar plugged and volume at zero?
RMS please.

For the record, I measured that with nothing plugged in the input jack, so basically shunted to ground. And I think this is the correct way to measure it, cuz even if the guitar volume knob turned to 0 shunts the signal to ground as well, there might be interference picked up by the cable which will throw off the measurement.

 

[DLC86]

I did it again, 0.5v RMS using 1kHz sine-wave from my Summit synth. Plugged into the front of the Axe III with input 1 A/D sensitivity set to 50% and I get -170dBFS in the DAW. Set to 6% A/D sensitivity, I get the same, so there seems to be a 0.5dB window of error with my test, but more or less the same figure.

Axe III is set as the soundcard, and the input I am measuring is definitely the front instrument input.

If it helps, this is an Axe III Mark II Turbo.

I can't recall if that affects the DI signal via USB or SPDIF, but I'll ask anyway... did you turn off the input gate?

 

[Orvillain]

Can you create a 100Hz sine tone, and measure Milli-volts in your DVM?

What do you want to know specifically??

 

[James Freeman]

For the record, I measured that with nothing plugged in the input jack, so basically shunted to ground. And I think this is the correct way to measure it, cuz even if the guitar volume knob turned to 0 shunts the signal to ground as well, there might be interference picked up by the cable which will throw off the measurement.

The Helix reads -Inf with no cable.
Probably a digital 'mute' on the TRS jack.

 

[Orvillain]

I can't recall if that affects the DI signal via USB or SPDIF, but I'll ask anyway... did you turn off the input gate?

No, it was active. But the signal level is high enough that it passes the default threshold. A constant sine tone effectively opens the gate fully. But I also don't recall if it affects the DI signal, I suspect not.

In fact, the patch was completely empty, so no access to the gate settings anyway.

 

[James Freeman]

What do you want to know specifically??

Well, maybe the 0.5v is too small value for the 1000v scale.
Millivolt range will be more accurate for smaller values, it should read 500mV.

 

[DLC86]

The Helix reads -Inf with no cable.
Probably a digital 'mute' on the TRS jack.

In that case better to use a plug with a jumper on the lugs.

PS: btw, cubase's mixer meters showed -inf as well for me, but that's cuz they have a limited range of measurement, I had to analyze the audio of a recorded track to reveal those values

 

[Orvillain]

Well, maybe the 0.5v is too small value for the 1000v scale.
Millivolt range will be more accurate for smaller values, it should read 500mV.

The way my meter works, it doesn't have a dedicated mV setting. Only volt. So xx.x readings:

10.0 = 10 volts
01.0 = 1 volt
00.5 = 0.5v, or 500mV.

When I set the level output, I slowly ramped from silence until I just broke past 00.4 into 00.5 territory. So there maybe a small difference, it might've been 505mV for example. But close enough.

 

[DLC86]

The way my meter works, it doesn't have a dedicated mV setting. Only volt. So xx.x readings:

10.0 = 10 volts
01.0 = 1 volt
00.5 = 0.5v, or 500mV.

When I set the level output, I slowly ramped from silence until I just broke past 00.4 into 00.5 territory. So there maybe a small difference, it might've been 505mV for example. But close enough.

if it shows just 1 decimal it could even be 0.451 or 0.549, still far from a 5dB error though, that's why I suspect your DMM can't correctly read the voltage at 1 kHz. I'd try with a 50 or 60 Hz sine.

PS: btw, I used the fm9 synth block to generate the sine wave and just connected an output to input 1

 

[Orvillain]

To be fair, it does say in the manual for my meter that there is an 1.5%+8 window of error on the meter for AC voltage measurements. It's just a cheap thing I got off Amazon, so it probably isn't super accurate. I did measure a battery and got 9.6v from it, so at least for the 1v range, it seems reasonably accurate.

But when measuring the output of a synth generating a 1000hz sine-tone, I set the output level of the synth until the meter straddles 00.4 and 00.5 - like.. the meter is literally oscillating between the two, so we know we're within the window of 500mV - and I'm getting the same kind of signal level in the DAW.

Via SPDIF from Axe III into Quantum: -16.8dB.
Via USB from Axe III: -16.8dB.

So at least we know there's no difference between USB and SPDIF and how loud the DI signal is from the front input.

Either my meter is total pony, or the Axe III has a different level. I'm willing to bet that it's the meter.... which begs the question... how do you know you can trust ANY of the measurements taken in the entire 17 pages of this thread???

 

[James Freeman]

how do you know you can trust ANY of the measurements taken in the entire 17 pages of this thread???

First page, last post, picture of 3 different DVM's reading the same value, DVM's are quite accurate.

Unless you got this POS:
1% error in the 200v AC range is 2v...

Might as well be reading 1.5v instead of 0.5v.

 

[Orvillain]

First page, last post, picture of 3 different DVM's reading the same value, DVM's are quite accurate.

Unless you got this POS:
1% error in the 200v AC range is 2v...

Might as well be reading 1.5v instead of 0.5v.

I have the Ultrics multimeter. It's not too disimilar to this one in features, don't know about the inside.

Aren't accuracy figures a percentage of the reading, and not the range??

IE: If you're reading 200v, then your 1% error would make the true reading somewhere between 198v - 202v ????

 

[James Freeman]

Aren't accuracy figures a percentage of the reading, and not the range??
IE: If you're reading 200v, then your 1% error would make the true reading somewhere between 198v - 202v ????

Yes, you're right.
Still, I don't think these generic DVM's are accurate for reading low AC voltage or frequencies above 60Hz, they are okay for testing AC mains.

 

[DLC86]

The problem is the frequency. Try 50 or 60 hz and see if the voltage stays the same.
I had one very similar to the yellow one above and as I increased the frequency on the fm9 synth block the voltage reading went initially up and then significantly down, almost as if there was a resonant high-cut filter on the detector. I knew that couldn't be right cuz the dB at the fm9 output stayed at the same level no matter where the frequency was set, and in fact with my new DMM the voltage reading stays constant too.

 

[Orvillain]

The problem is the frequency. Try 50 or 60 hz and see if the voltage stays the same.
I had one very similar to the yellow one above and as I increased the frequency on the fm9 synth block the voltage reading went initially up and then significantly down, almost as if there was a resonant high-cut filter on the detector. I knew that couldn't be right cuz the dB at the fm9 output stayed at the same level no matter where the frequency was set, and in fact with my new DMM the voltage reading stays constant too.

Using 60hz as my frequency, dialing in the output of the synth so the multimeter measures 0.5v... then plugging that signal into the front of the Axe FX III, I am still getting a -16.5dB signal.

Same thing for 50hz.