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Type of Decibel Units Used In Level Adjustments?

ucnick

Inspired
Simple question, but I cannot seem to find it definitively answered: are the dB (decibel) units used for the various level settings on the AX8 root-power quantities (20 log) or power quantities (10 log)? I assume power quantities, but would like to make certain. Probably doesn't mean a lot to many people, but it is helpful for me to better understand exactly how much I am changing a signal level when I change it by +6 dB, is it twice as loud, or four times as loud?

Thanks!
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
Decibels are decibels. There is no such thing as "root-power decibels".

By definition a decibel (dB) is a ratio of two powers. The formula is 10 * log10(P1/P2) where P1 and P2 are the power of two signals, respectively.

In electronics, however, we usually manipulate and measure voltage levels. It's convenient to represent the ratio of two voltage levels in dB. To do this you would need to square the voltage to get the power (since P = V^2 / R). We also assume R = 1 for convenience. With a little math you get dB = 20 * log10(V1/V2).

Therefore if we reduce the voltage level of a signal by a factor of 0.1 then the signal is now -20 dB relative to before.

dB is simply an easy-to-read logarithmic-to-linear mapping. Music, human perception, and many other things in nature typically have a logarithmic response. The decay of, for example, a cymbal is logarithmic. If you plot this on a linear axis it's hard to display because of the dynamic range. But if you use a logarithmic axis you "compress" the data into something that's easier to view. Decibels are just a widely accepted mapping. You could use any base for the log; log2, ln, etc but since we have 10 fingers log10 is nice.

The point is that X dB is X dB. If you reduce a signal by 20 dB you've reduced it's voltage to 10% of what it was previously. You also reduced it's power to 1% of what it was previously. These are the same things: 20 * log10(0.1) = 10 * log10(0.01).
 

mikeymike

New Member
Decibels are decibels. There is no such thing as "root-power decibels".

By definition a decibel (dB) is a ratio of two powers. The formula is 10 * log10(P1/P2) where P1 and P2 are the power of two signals, respectively.

In electronics, however, we usually manipulate and measure voltage levels. It's convenient to represent the ratio of two voltage levels in dB. To do this you would need to square the voltage to get the power (since P = V^2 / R). We also assume R = 1 for convenience. With a little math you get dB = 20 * log10(V1/V2).

Therefore if we reduce the voltage level of a signal by a factor of 0.1 then the signal is now -20 dB relative to before.

dB is simply an easy-to-read logarithmic-to-linear mapping. Music, human perception, and many other things in nature typically have a logarithmic response. The decay of, for example, a cymbal is logarithmic. If you plot this on a linear axis it's hard to display because of the dynamic range. But if you use a logarithmic axis you "compress" the data into something that's easier to view. Decibels are just a widely accepted mapping. You could use any base for the log; log2, ln, etc but since we have 10 fingers log10 is nice.

The point is that X dB is X dB. If you reduce a signal by 20 dB you've reduced it's voltage to 10% of what it was previously. You also reduced it's power to 1% of what it was previously. These are the same things: 20 * log10(0.1) = 10 * log10(0.01).
I've been having issues with the output level of my axe-fx 2 xl+ for a while now and I'd really love it if you could answer a few technical questions for me regarding the output and alignment levels FAS chose to go with.

When I place a synth block next to the output bar and set it to 0db (voices 1 and 2 disabled) and use voice 3 for a 1khz test tone the output meter in the UTILITY menu shows -1.1 dB. At this very same moment I'm measuring the output level from OUT 1 R (left is going into my mixer so I can keep an eye on the VU meter there) using a Fluke 87 V (True RMS) multimeter and my results are 0.7841 volts AC (calculated I'm getting a Vp of 1.109. Measured with the Fluke it's showing 1.113 to 1.119 volts peak).

What's up with the UTILITY meter not showing 0db despite the synth block being set to 0db? Is there something I'm not understanding about the meter alignment to the internal signal flow? Perhaps the attack rate of the VU meter is slow? I'm not sure this would matter with a sustained test tone though and more explanation would be really helpful in clearing this up for me.

Might I also ask why the output of the device isn't showing proper +4 dBu voltages (1.228) when the synth is set to 0db 1khz? If my understanding is correct this is giving closer to a 0 dBu signal of 0.775 volts. Most meters are aligned at +4 dBU and current SMPTE alignment level is -20 dBFS = +4 dBu.

Now comes my second issue. When I generate a test tone in adobe audition at -20 dBFS and use the AFX ASIO playback I'm not getting anywhere NEAR the voltages that I should be at -20 dBFS (aligned to +4 dBU). I'm getting around .627 volts. Now I understand that dBFS alignment is not set in stone and runs the gamut from -20 to -14 (more on -14 in a second). So I am curious, is my unit messed up? Are my test tone settings wrong? Or is there something I'm not understanding?

The most perplexing thing is the synth block being at 0db but showing up on the UTILITY vu meters as -1.1 dB. If I adjust the synth block to make it read 0 dB by setting the output level of the synth to 1.1dB I get a measurement of 0.89 v. This internal mismatch is driving me crazy and again, further explanation would be really welcomed (perhaps something is set wrong? The output block is set to 0.0 across the board and on the main slider so I know it's not that.

So I got curious, I set my adobe audition test tone generator to go up fractionally until I reached the "correct" voltage for +4 dBu line output (1.228) and it required an alignment of -14.05 dBFS. This shows up on the UTLILTY meter as 2.8 dB (the synth block had to be set to 3.9 dB to get the same results).

Honestly I am trying to figure all of this out so I can properly gain stage this into my mixer using the unity markings on the dials (yes I know they're not that accurate but I have an accurate DMM and I can adjust accordingly) but the whole thing is so far away from +4 dBu/1.228 volts that I don't know what's up and the internal mismatch with itself (synth at 0db but UTIL meter showing -1.1) is further cloudying my situation.

Basically I wanted to use the synth block set to 0db and get 1.228 volts out of the device and set my gains on my mixer using the synth as a test tone generator. This is very clearly not happening and I am confused as to why. If I'm understanding correctly, you want to set your mixer VU meter to read 0 (no change) at 1.228 volts.

In no way is this written from the point of view that I know what I'm talking about. I hope it didn't come off that way, I'm sincerely just very confused about these levels not matching up. Is it possible the AFX2 was set to align at 0dBu instead of +4? Is it also possible that the -1.1 dB mismatch was an attempt at maintaining some kind of headroom in the device? I honestly am baffled. Thank you for your time and yes I know this is a 5 year post from the dead but this thread seemed the most fitting considering the search results I came across when trying to figure all of this out. Hopefully someone else will find it as well. Of course, If necessary I don't mind this being moved to the AFX2 sub forum.

Thanks for your time and also thank you for making such an amazing device!
 

mr_fender

Axe-Master
Do you have your front panel Output knob turned all the way up? That's the setting for full unity gain output.

Also, USB playback level is adjustable in the I/O menu (USB Return Level). Make sure you don't have the level turned down any there.

Not sure why the synth block level is off like that.
 

mikeymike

New Member
Do you have your front panel Output knob turned all the way up? That's the setting for full unity gain output.

Also, USB playback level is adjustable in the I/O menu (USB Return Level). Make sure you don't have the level turned down any there.

Not sure why the synth block level is off like that.
Yep the front outputs are all the way up, I'm measuring the voltage coming out with a multimeter.

The USB playback level is set to maximum 0.0 dB

I'm using a sine waveform by the way, I don't think I clarified that but I just wanted to be thorough.

I'm going to make a screen recording video showing the axe-edit settings and video of the screen on the device showing the -1.1 dB as well as the multimeter showing the voltages soon, if you're interested.
 

Bakerman

Axe-Master
The horizontal level meters in the utility menu have the "0 dB" line somewhere that generally leaves 10 dB or so (IIRC) of headroom. They also have some kind of basic virtual head/ear algorithm where different content with the same peak dBFS reading won't necessarily give the same value on those meters.

See this for why the synth level might be lower than expected: https://forum.fractalaudio.com/threads/synth-block-output-level.168156/post-2020948

For USB playback level from computer being lower than expected, check for any pan law setting in the DAW and also verify Output 1 boost/pad (Axe I/O: Audio menu) is set to 0 dB.
 

mikeymike

New Member
The horizontal level meters in the utility menu have the "0 dB" line somewhere that generally leaves 10 dB or so (IIRC) of headroom. They also have some kind of basic virtual head/ear algorithm where different content with the same peak dBFS reading won't necessarily give the same value on those meters.

See this for why the synth level might be lower than expected: https://forum.fractalaudio.com/threads/synth-block-output-level.168156/post-2020948

For USB playback level from computer being lower than expected, check for any pan law setting in the DAW and also verify Output 1 boost/pad (Axe I/O: Audio menu) is set to 0 dB.
Yes but a sustained sine wave shouldn't be a problem for the meter I suspect?

To answer your question about the DAW, Adobe Audition 3 has a wave editor built in and doesn't have any sort of mixing in that part of the application ie there is no console or master controls and what's displayed is played at full scale. These test tones are rendered, not real time generated so I am able to see the dBFS values in the editor next to the wave form display on a graph.

OUT 1 boost/pad is set to 0 dB but I thought this doesn't matter because it's negated on the other end of the AD conversion? either way it's set to 0 dB as I've found I can't really go up very far on it without having some horrid sounding distortion when playing.

The thread you linked me isn't really relevant to my situation unless I'm missing something about what was said? If you could direct me to specific posts that may help me I would appreciate it. I do note that this quote from yourself doesn't seem to be true or accurate for me:

"I'd guess the synth was set to produce the stated "nominal output" of 4 dBu with one voice at 100% level, block level at 0 dB"

Being that +4 dBu is 1.228 volts and the measured voltage coming out of my AFX II XL+ OUT 1R 1/4" jack is not reading anywhere near 1.228 volts there seems to be an issue someplace else? I am certain my sine wave is set to 0 dB but the utility meter shows -1.1 dB. This doesn't make sense at all and has been perplexing me. Even IF it were to read 0 dB on the utility menu meter it still would NOT correspond to +4 dBu alignment voltage (1.228) until the synth is turned up to 3.9 dB and the meters in the utility menu show 2.8 dB to get that voltage to come out of the OUT 1R 1/4" jack.

As you can see this is quite frustrating. I have pretty much learned that the internal meter and synth dB level are not accurate at all and I have to use a multimeter to find +4 dBu. I think I'd be okay with it if I could understand the reasoning behind it but it seems so incredibly arbitrary I can't make any sense of it. But, then again I'm not very knowledgeable in this and am completely open to being corrected! I hope someone from FAS will see this and can shed some light on the issue I'm having.

By the way I do want to thank you for trying to help out. I am greatly appreciative of your effort. This whole thing is so baffling to me haha
 

Bakerman

Axe-Master
OUT 1 boost/pad is set to 0 dB but I thought this doesn't matter because it's negated on the other end of the AD conversion?
I remembered wrong; USB playback has the boost & pad applied. I was thinking only grid audio might get the boost, making USB audio quieter when padded.

The synth block (unfiltered voice at 100% level) produces 0 dBFS with block level at +18 dB. Assuming this gives the maximum +20 dBU in the specs (with Out 1 knob at 100%), you should see 1.228 V with synth block level at +2 dB. So, your result is 1.9 dB off from expected. Have you checked whether Out 1L produces the same result as Out 1R?
 

mikeymike

New Member
So at +18 dB on the SYNTH block output level I am getting 6.12 volts on OUT 1R and 6.11 volts on OUT 1L. Rounded that would be 17.939 and 17.954 dBu respectively.

At +2 dB on the SYNTH block output level I am getting 0.985 volts on OUT 1L and 0.987 volts on OUT 1R. Rounded that would be 2.087 and 2.105 dBu respectively.

I've noticed my unit has issues with squealing feedback even when I have only IN-EARS in. This feedback occurs when the cable is plugged into my guitar or unplugged (as long as the other end of the cable is still IN THE AFX) and only when I'm plugged into the front instrument input and only when the OUT 1 LEVEL knob is past a certain amount (somewhere before or around halfway). This leads me to believe that the headphones signal is somehow bleeding into the INSTR input but I could be wrong. This feedbacking also changes as I change the input impedance load value through the ranges, the ones that have capacitors seem to be more stable and squeal less or not at all. Often times if I have a drive pedal in front of the amp and I turn the tone knob up or down the pitch of the feedback will change. It's very resonsant-like.

I'm not sure if this is an issue but I've just been using the SPDIF output of my POD HD PRO rack unit. I just plug my guitar into the POD HD PRO guitar input and run dry out through SPDIF to the AFX 2XL+. Naturally, this avoids all feedback issues but I'm stuck with Line 6's input circuitry (not ideal when Fractal has spent so much time making their input circuitry with quality in mind). Something peculiar I've noticed is that this squealing feedback occurs on certain factory preset(s) (I forget the name but it has a Marshall JVM as the amp model and possibly the title) as well as ones I make.

I'm not sure that feedback issue is going to affect the synth output level consistency within the unit though. I could be wrong.. Perhaps I need to flash Ares on here and start from a clean firmware slate. I don't know.

I've attached pictures of my synth block and output block settings just so you can see that they're set properly.

This is driving me nuts. lol

Just as an aside, when I set it to 18 dB the red OUT 1 CLIP light came on so I reduced it by 10ths until it went away, the figure I got was 17.1 dB and the voltage measured on OUT 1R was 5.609 volts. This comes out to 17.196 dBu.


And of course when I set the out to 0 dB I'm still showing -1.1 dB on the internal meter in the UTILITY menu.
 

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