Old 06-07-2019, 12:11 PM   #1
Baggage
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Default Testing preamp with Tone Generator?

I've been skeptical of the accuracy of my preamp's gain. The manufacturer recommended I send a 1kh tone at -30 with specific faceplate settings, and report back what I find.

I'm trying to send the test tone out via reinsert to the preamp and back, but I'm not noticing any change when I adjust the gain levels on the pre.

Set up: I'm sending track 1 to track 2. Track 2 has the Reinsert plug working.

Do I put the Tone generator plug on Track 1 and send to Track 2?

Or do I put the Tone generator plug on Track 2 with the Reinsert plug?

Thanks!
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Old 06-07-2019, 12:30 PM   #2
karbomusic
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I'd dump ReaInsert and just send the tone from track 1 via hardware out into the preamp, then the preamp out to a hardware input and set track 2 to that input. All ReaInsert does is try to auto-adjust latency, that doesn't really matter here.
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Old 06-07-2019, 01:05 PM   #3
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Originally Posted by karbomusic View Post
I'd dump ReaInsert and just send the tone from track 1 via hardware out into the preamp, then the preamp out to a hardware input and set track 2 to that input. All ReaInsert does is try to auto-adjust latency, that doesn't really matter here.
Got it. Trying now-thanks!
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Old 06-07-2019, 01:33 PM   #4
DVDdoug
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I'm not noticing any change when I adjust the gain levels on the pre.
You mean the gain/level knob doesn't do anything? That would be obvious with your ears... (and REAPER's meters)...

Quote:
The manufacturer recommended I send a 1kh tone at -30 with specific faceplate settings, and report back what I find.

I'm trying to send the test tone out via reinsert
I assume that's -30dBm or dBV (analog). That's not necessarily going to calibrate to 0dBFS (digital) so you need a calibrated hardware signal generator and/or you need to measure and adjust the voltage out of your soundcard/interface.


And since preamps are analog, you'd to measure input and output voltages. Then you can calculate the (analog) gain.
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Old 06-07-2019, 02:03 PM   #5
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Originally Posted by DVDdoug View Post
You mean the gain/level knob doesn't do anything? That would be obvious with your ears... (and REAPER's meters)...
Yes, I didn't have the signal sent properly so when I ajusted the gain level knob there was no change in volume of the tone. (But I squared that away and I'm now hearing the change in level)

[/QUOTE]I assume that's -30dBm or dBV (analog). That's not necessarily going to calibrate to 0dBFS (digital) so you need a calibrated hardware signal generator and/or you need to measure and adjust the voltage out of your soundcard/interface.[/QUOTE]

Sorry I didn't specify what they asked for: it was -30dbu (but this voltage terminology is new to me so I'm trying to figure out what the dbu is when my meter says -30db. I'm assuming they are different units of measurement) Does reaper have toggle that allows for the meter to read in dbu?


[/QUOTE]And since preamps are analog, you'd to measure input and output voltages. Then you can calculate the (analog) gain.[/QUOTE]

Once I got the tone sent properly I used my Big Meter to get a visual of each stepped db value on the faceplate. I noticed as I increased it in increments of 6db, the increments (according to the meter) were not accurate to what was marked on the faceplate. From 28db to 34db on the stepped pot, the meter indicated only a 4db increase. From 34db to 40db was only 2db increase. 40 to 46 was >2 and so on.

Last edited by Baggage; 06-07-2019 at 02:25 PM. Reason: clarification
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Old 06-07-2019, 02:32 PM   #6
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I'm assuming they are different units of measurement) Does reaper have toggle that allows for the meter to read in dbu?
They are different references. In REAPER you're dealing with dBFS where 0dBFS (zero decibels full scale) represents a numerical value of 1.0.* in the computer's memory or in a file. 0dBu is 0.775 Volts RMS or 1.08V peak. (I had to look that up. )


If you play a 0dB test tone and I play a 0dB test tone we will get different voltages (and different dBu) out of our different soundcards/interfaces.


Quote:
Once I got the tone sent properly I used my Big Meter to get a visual of each stepped db value. I noticed as I increased it in increments of 6db, the increments (according to the meter) were not accurate. From 28db to 34db on the stepped pot, the meter indicated a 4db increase. From 34db to 40db was only 2db increase. 40 to 46 was >2 and so on.
OK... That's wrong! The dB change should be the same... If you reduce the analog input (or the acoustic level into the microphone) by -3dB, the digital level should go-down by -3dB etc. (That's assuming you're not clipping or the signals are not so-low that you're measuring noise.)

However, are the markings on the pot actually marked in "dB"? For example, many volume knobs are marked 0-10 (or 0-11 ) but those are just meaningless numbers.









* In floating-point 0dBFS is +/-1.0. With integer formats 0dB is represented by the maximum number you can "count to" with a given number of bits. With 16-bits you can count from -32,768 to +32,767 and if your peaks are hitting those numbers your peaks are hitting 0dBFS.

Everything is automatically scaled by the software & drivers so a 0dB file plays back at the same volume even though the raw numbers are drastically different.

Last edited by DVDdoug; 06-07-2019 at 02:57 PM.
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Old 06-07-2019, 04:12 PM   #7
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Originally Posted by DVDdoug View Post
They are different references. In REAPER you're dealing with dBFS where 0dBFS (zero decibels full scale) represents a numerical value of 1.0.* in the computer's memory or in a file. 0dBu is 0.775 Volts RMS or 1.08V peak. (I had to look that up. )


If you play a 0dB test tone and I play a 0dB test tone we will get different voltages (and different dBu) out of our different soundcards/interfaces.


OK... That's wrong! The dB change should be the same... If you reduce the analog input (or the acoustic level into the microphone) by -3dB, the digital level should go-down by -3dB etc. (That's assuming you're not clipping or the signals are not so-low that you're measuring noise.)

However, are the markings on the pot actually marked in "dB"? For example, many volume knobs are marked 0-10 (or 0-11 ) but those are just meaningless numbers.




* In floating-point 0dBFS is +/-1.0. With integer formats 0dB is represented by the maximum number you can "count to" with a given number of bits. With 16-bits you can count from -32,768 to +32,767 and if your peaks are hitting those numbers your peaks are hitting 0dBFS.

Everything is automatically scaled by the software & drivers so a 0dB file plays back at the same volume even though the raw numbers are drastically different.
Thanks Doug, Yes they are marked db.
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Old 06-07-2019, 07:27 PM   #8
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Originally Posted by DVDdoug View Post
That's assuming you're not clipping...
...or at least distorting the preamp. It sure does look to be the kind of exponential curve we might expect from an analog stage being overdriven, and those get to be some pretty big gains there.

What preamp and/or interface are we talking about?
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