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Author Topic: digital level setting and bit depth  (Read 436 times)

lindsay Dean

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digital level setting and bit depth
« on: March 24, 2017, 02:25:18 pm »

 I was in a discussion with a well respected system designer
who stated to get the full bit depth of an audio signal it has to have a decent digital gain set.
to reduce s/n ratio.
 isn't the noise floor only in the a/d conversion ?
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Corey Scogin

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Re: digital level setting and bit depth
« Reply #1 on: March 24, 2017, 02:46:05 pm »

I was in a discussion with a well respected system designer
who stated to get the full bit depth of an audio signal it has to have a decent digital gain set.
to reduce s/n ratio.
 isn't the noise floor only in the a/d conversion ?

A dithered digital signal has dither noise at a very low level. You might call that the noise floor.

The dynamic range of a digital signal is defined by its bit depth. 16-bit ~ 96dB; 24-bit ~ 144dB. Dither noise will be near the bottom of that. The noise floor of analog electronics used to capture the signal is often higher than either of those.

The issue that the system designer is talking about is trying to maximize the S/N ratio and maximize the dynamic range in the digital domain by ensuring that signals are captured hot. If you record at 16-bit and your signal peaks are around -30dBFS, then your dynamic range is limited to (96-30) 66dB.  This is much less of an issue at 24-bit, where signals can peak at -40 and still have >100dB of dynamic range.

I'm sure JR or someone will come along and possibly explain that better or correct my off-the-top-of-my-head claims there.

« Last Edit: March 24, 2017, 02:48:35 pm by Corey Scogin »
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lindsay Dean

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Re: digital level setting and bit depth
« Reply #2 on: March 24, 2017, 02:59:40 pm »



  Dithering is white noise added during recording and or playback and depends on bit rate used.

   Sorry I should have been clearer
the signal level referenced would be a live mic/instrument input.
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TJ (Tom) Cornish

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Re: digital level setting and bit depth
« Reply #3 on: March 24, 2017, 03:13:04 pm »


  Dithering is white noise added during recording and or playback and depends on bit rate used.

   Sorry I should have been clearer
the signal level referenced would be a live mic/instrument input.
The bit depth of the device affects the digital headroom and noise floor.  The original white Yamaha 01v only had 20-bit DACs on the OMNI outputs.  Poor gain structure was indeed audible, and running the OMNIs hotter solved the problem.  This same board had 24-bit DACs on the L&R main outs and they didn't have this problem.

I don't know of any devices today with less than 24-bit depth, so while your friend's statement is true, it isn't as material as it was 20 years ago, and especially for live sound you need to balance the noise floor issue against the headroom issue.  Standard procedure is to set your input gains to -18dBFS which seems to be a good balance between a low noise floor but enough headroom to not overload with a dynamic live signal.
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Luke Geis

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Re: digital level setting and bit depth
« Reply #4 on: March 24, 2017, 08:01:52 pm »

The issues of dynamic range and S/N ratio as related to digital audio is pretty much a thing of the past. Most digital units even use floating point audio structures that further eliminate the issues of S/N and dynamic range. The weak link in most all modern digital units is the A/D & D/A converters. Most of the A/D - D/A converters will have the analog portion of it clip well before the digital section does.

The basic thing to remember is that the digital mixers are not designed to be digital mixers, they are designed to be more like analog mixers. They have similar gain structures and dynamic range, they have pretty much all the same everything except that the bulk of the audio exists in a digital format. The in's and out's are the only thing that is truly analog, but that portion of it is designed to be analog in every way. If you follow basic practices for HA mic gain, you really can't go wrong since the mixer will pretty much react the way an analog one would anyway. If you start to get tricky and run hot HA settings or really low HA settings, you may start to find the limitations of the mixer, but then you are running the mixer outside of standard practices.
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John Rutirasiri

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Re: digital level setting and bit depth
« Reply #5 on: March 25, 2017, 01:41:15 am »

isn't the noise floor only in the a/d conversion ?

If you define noise as an error in the signal (something that was not present in the original signal), you get errors on both ends (A/D and D/A) of the conversion.  You have quantization error (rounding off error) that is noise.  There's thermal noise.  There's nonlinearity errors, more so on chips that were not laser trimmed.  Modulation noise on higher order delta sigma ADCs.  And so forth.

Now if you do multiple A/D and D/A conversion such as digital mixer with its analog outputs feeding a DSP, and the DSP analog outputs feeding a processing amp (amp with built-in DSP), noise could be apparent depending on overall system gain.

Dither on the other hand is random, low-level noise added to increase apparent S/N and create an apparent lower noise floor on systems with lower bits such as 16 bits.  The analogy I use is how people cook rice.  You can just use plain rice and water, or you an add a bit of dithering agent such as butter, sticky rice (to increase apparent starchiness, or vinegar (as in sushi rice.)

Use of floating point versus fixed point arithmetic:  that's the architecture of the DSP, not the A/D or D/A converter.  Floating point does not mean better S/N or dynamic range.  Floating point can represent a larger value within given word length (e.g. 32 bits), but you can just as easily use wider words (e.g. 40 bits) fixed point.

I agree with what everyone said...with modern digital mixers using 24 or 32 bit converters, there's not much to worry about w/r/t noise floor, even with feeble input levels.

John R.
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John Roberts {JR}

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Re: digital level setting and bit depth
« Reply #6 on: March 25, 2017, 10:41:33 am »

I was in a discussion with a well respected system designer
who stated to get the full bit depth of an audio signal it has to have a decent digital gain set.
to reduce s/n ratio.
 isn't the noise floor only in the a/d conversion ?
The dominant noise floor is in the analog mic preamp, and then A/D conversion...

Modern convertors are good enough that their noise floor sounds and acts "analog"... so don't overthink this by trying to apply digital theory. Modern digital paths can be evaluated based on using the same analog specs as analog paths...

Good gain structure just like with an analog console matters.

JR 
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