What’s your Noise Floor?
« on: November 27, 2018, 06:45:31 AM »
I am curious as to others “noise floor”. Primarily with analog mix set up, “faders” at unity, master at unity, back into daw or other mix recording device.

Some info to include in this informal poll is :
# of channels
Type of D/A feeding mixer
Type of A/D capturing mix
Measurement method (meter from daw or hardware?  Other method? )
Type of D/A for monitoring
Grounding considerations in wiring.


I am reading -90ish with an avid hd, 16 channel summing box daw connected.  Same interface capturing mix.  I need to make sure of if analog master has gain in hand or not.  Using a dangerous st monitor.

Rough measurement by using internal daw meter, metering input of mix return normal “digital” type pro tools meter.


Does this seem acceptable?  It seems pretty dang quiet to me.


ruffrecords

Re: What’s your Noise Floor?
« Reply #1 on: November 27, 2018, 11:15:30 AM »
Depends what -90ish is relative to and how many bits you are using.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

squarewave

Re: What’s your Noise Floor?
« Reply #2 on: November 27, 2018, 11:35:19 AM »
I don't have a mixer but I have been a little psycho about measuring noise so I can tell you what I would do to evaluate honestly.

First, I would measure the noise of your converter by terminating an input with 50 ohms just to see what the absolute floor is. This is what I get with my MOTU Traveler mk3:



However, I always make noise measurements by using a tone to find the clipping point and then I turn off the tone, record silence and then plot that with GNU Octave. So 0dB in that plot is actually the clipping point and not 0dBu. I can always adjust for that later but I actually don't recall what the clipping point is on my MOTU. I think it's in the +4dBu range. Actually there is a control in the MOTU software to adjust the input level pad that you will want to try if you have it so that you can put as much signal through your desk as possible before clipping the converter. So assuming it is +4 then that plot should be adjusted upward +4. So it looks like the noise floor of conversion is still well below 120.

Then I would measure your mic pres but use 150 ohms term because that is more realistic for a mic. I have a DIY API pre that looks like this:



Again, this has to be shifted up ~+4dB at least because of how I make noise measurements.

You can see how it's useful to look at spectra because the mains hum will push meter readings into the 90s but thats not really the "noise floor".

I think -90ish is pretty good in practice but I would want to see high-res spectra to "see" for sure what's going on.

Re: What’s your Noise Floor?
« Reply #3 on: November 27, 2018, 04:58:19 PM »
Thank you for all the comments so far.

My personal measurements so far are under these conditions:

16 channel mixer with avid hd connected to inputs at unity, master fader at unity returning to avid hd....

My measurement of “-90db” is with 0db being +18dBu.... so the -90 is full scale digital....

I am mainly attempting to confirm that the summing setup is quiet as possible in a real world mixing situation.

Please add more thoughts of what good real world reading would be.

Thanks

squarewave

Re: What’s your Noise Floor?
« Reply #4 on: November 27, 2018, 05:27:08 PM »
What sort of level are you putting into the mixer? Presumably the avid hd output level is like +4dBu. So are you boosting something like +14dBu on each input?

I will assume that you know that getting levels and gain structure right are important.

Re: What’s your Noise Floor?
« Reply #5 on: November 27, 2018, 08:28:17 PM »
There is no signal being sent to the mixer during this observation, but the inputs are connected to the avid hd outputs which is powered on.

The “mixer” is an inward connections solid state summing amp that only has pans no levels, just unity inputs.

The avid hd operates and is calibrated at +4dBu with 0dBfs=+18dBu.   +4dBu therefore I guess equals -14dBFS.... correct me if this doesn’t make sense.

The avid hd says the dac should be -110 noise and input is -114...
« Last Edit: November 27, 2018, 08:56:41 PM by eastwoodsound »

squarewave

Re: What’s your Noise Floor?
« Reply #6 on: November 27, 2018, 09:24:39 PM »
So the Avid HD can put out +18dBu. Wow. That's not bad. If I knew better I would say it was designed to do precisely what you're doing. Carry on.

Re: What’s your Noise Floor?
« Reply #7 on: November 28, 2018, 12:28:49 AM »
I think that is the stock calibration +18dbu=0dBfs

Everything is fine and useable but I’m really just curious about my original intent of this post:

What is anyone else out there’s noise floor like with analog mixing???

Like what is typical real world measurement with channels un muted?? 

I can see that mine is down aroun -90dBfs on avid hardware.  This is with 16 line amps routed to stereo bus and connected back to interface for mix capture.

Is that good?  Bad? Average?  I know it’s not a precise or very informative measurement description but it’s something and it’s “real world”.

john12ax7

Re: What’s your Noise Floor?
« Reply #8 on: November 28, 2018, 02:58:59 AM »
What meters are you using? The ones in Pro Tools don't seem to go down super low.

Things will get worse 3dB for every doubling of signals.  So if you are at -110 dBFs with 1 output then 16 summed would be -98 dBFs, best case,  plus some additional noise from the mixer itself.

What does 1 output looped back measure? What does 1 output going through the mixer measure? What does the mixer by itself measure?

-80 dbu would be a reasonable number for a mixer,  with your setup -90 dBFs seems a bit on the high side.
« Last Edit: November 28, 2018, 04:03:52 AM by john12ax7 »

ruffrecords

Re: What’s your Noise Floor?
« Reply #9 on: November 28, 2018, 09:14:38 AM »
One thing you need to be very careful of is that the value of the level shown in a spectrum plot is not the rms noise floor over the full 20KHz bandwidth. What the spectrum shows you is the noise per unit bandwidth.  The rms noise is the sum of all these noises over the 20KHz bandwidth. Good spectrum analysers will have a function to convert this into rms noise over a given bandwidth.

Note that if your spectrum analyser shows the noise per root Hz then you need to add 43dB to this value to get the rms noise over 20Khz (which is what you actually hear). So if the spectrum bumps along on the -140dB line, the actual rms noise is about -97dB.

Cheers

Ian
« Last Edit: November 28, 2018, 03:56:16 PM by ruffrecords »
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'


Re: What’s your Noise Floor?
« Reply #10 on: November 28, 2018, 03:19:28 PM »
Thanks John and Ian,

This gives me some good perspective. 

John, from what you are saying this makes sense and I think I can assume I am in the ballpark of “good” operational conditions.

Ian,  I am simply “measuring” by reading the peak level on the pro tools input meter using avid hd a/d.  There are several different metering options in pro tools but I believe this is just a number that indicates peaks etc.  you can click to refresh this number displayed.

I’m not trying to get too tweaking with information from these observations, just mainly wanting to confirm improvements or bad changes in operation.

But..... back to the original topic..... I want to know what other people’s real world noise floor is to see if my set up is performing well.

I’d like to be able to perform a commissioning type measurement, like what people would do when delivering and installing an older large format console.... am I in the realm of “pro”. With regards to noise?

What are others analog mixing noise floor???

Re: What’s your Noise Floor?
« Reply #11 on: November 28, 2018, 03:23:52 PM »
Also to John:

Are you saying -90 seems too noisy for my set up or more quiet than it should be considering set up?

The math you presented makes some good sense to me.

ruffrecords

Re: What’s your Noise Floor?
« Reply #12 on: November 28, 2018, 04:19:20 PM »
You said earlier that +4dBu = -14dBFs and you were getting a noise reading of -90dBFS which translates to  -80dbu.

To put this into context, if you have a mic pre set to 40dB gain, even if it is perfect, the output noise will be close to -90dBu. Mix 16 of these together and the noise level will be 12dB higher at -78dBu.

The output of a good mixer with all the controls down should be better than -90dBu and could approach -100dBu. At this point you are measuring the noise of the output amplifier alone. Turn up the bus fader to zero but keep the channel faders down and you will hear the inherent bus noise. You would expect this to be higher than the previous measurement. A lot depends at this point exactly what is connected to the bus. Many mixers have an active stage after the fader which could be creating noise at -100dBu. 16 of these on the bus will raise the noise by 12dB all by themselves. The this you need to add the noise due to the bus impedance multiplied by the bus noise gain  (which with 16 inputs is 24dB). However, bus impedance noise is usually the least of your worries so If there is no post fader amp then all you will hear is the bus impedance noise.

Difficult to give figures because usually noise is dominated by the noise of the source signals but the above figures will give you an idea of what is normal.


Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

squarewave

Re: What’s your Noise Floor?
« Reply #13 on: November 28, 2018, 07:38:12 PM »
16 channel mixer with avid hd connected to inputs at unity, master fader at unity returning to avid hd....

My measurement of “-90db” is with 0db being +18dBu.... so the -90 is full scale digital....
You mean -90dBFS is with 0dBFS being +18dBu?

Quote from: eastwoodsound
The avid hd operates and is calibrated at +4dBu with 0dBfs=+18dBu.   +4dBu therefore I guess equals -14dBFS.... correct me if this doesn’t make sense.

The avid hd says the dac should be -110 noise and input is -114...
This might very well be a stupid question because I know nothing about Avid anything. But just to be perfectly clear, what does 0dB within the Avid HD correspond to? We know it can put out +18dBu and we know that corresponds to 0dBFS but what does 0dBFS correspond in the software view? Presumably there are some "faders". What are they set to when you play tracks into the desk? Is 0dB on the "fader" 0dBFS or is it less and you can make the fader go higher?

john12ax7

Re: What’s your Noise Floor?
« Reply #14 on: November 29, 2018, 08:12:06 PM »
Also to John:

Are you saying -90 seems too noisy for my set up or more quiet than it should be considering set up?

The math you presented makes some good sense to me.

-90 dBFs would equate to -72 dbu,  off hand this seems a bit noisy.

I have a 16 channel summing setup too, can check the noise levels next time am at the studio.


 

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