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Author Topic: 32Bit Float recording - The Technical view  (Read 5255 times)

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Offline aaronji

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Re: 32Bit Float recording
« Reply #15 on: September 30, 2019, 04:54:38 PM »
Yeah, or RX7 type cleanup.  If it's splattered with a bunch of distortion harmonics, it's a lot harder to clean.

This is why I have been using the limiter on my MixPre-6 first version. I set the levels pretty conservatively, so I tend not to hit them often, but occasionally there is that guy doing the "missile whistle" under the mics...

Offline Gutbucket

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Re: 32Bit Float recording
« Reply #16 on: September 30, 2019, 05:15:48 PM »
This thread, dedicated to 32bit Float recording in itself but not addressing the details of how it is implemented in specific recorders won't really be of much practical interest to tapers.. other than being useful to dispel some academic misunderstandings about what it can and can't do.  In other words it will be mostly academic because what really matters is how its implemented in each specific recorder in question. 

That's where the rubber meets the road and where all the current confusion lies!
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Offline Gutbucket

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Re: 32Bit Float recording
« Reply #17 on: September 30, 2019, 08:52:31 PM »
The constraint is not 32bit floating point storage (which is ridiculously immense), but real-world bottle-necks prior to it.

One needn't adjust input trim/gain on a recorder if:

1) The total dynamic range of the signal will fit within the available dynamic range of the storage format..
(check, 32-bit floating point does this easily)
       AND
2) The total dynamic range of the signal will fit within the available dynamic range through the recorder's signal path, up to the point where the signal is stored in that format..
(this reflects the most critical part of any specific implementation- the preamp and ADC performance of the specific recorder)
       AND
3) The actual upper and lower dynamic range values of the signal fit comfortably within the upper and lower dynamic range limits of the recorder's signal path, up to the point where the signal is stored.
(this is about shifting level of the source if necessary to comfortably fit requirement 2, by making sure the signal does not overlap or exceed either end of the available range)

32-bit float storage solves the first problem. The available storage range of 32-bit float is effectively unlimited for audio purposes.  Practically, it will depend on how well the other two things are handled in any particular recorder touting the benefit of 32-bit floating point storage. 

The problem is accommodating the total output range of a source to the available input range through the recorder (which has been steadily increasing as tech advances, yet remains limited).   Most of the hoopla from manufacturers and reviewers thus far has focused on the "wow factor" of the second part of that statement, while not talking much about the "real world" constraints of the first part.  Those of us calling for temperance understand that the first part is really what is going to matter.  And that will boil down to a performance measure of each individual recorder which is rarely specified clearly by manufacturers.
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Offline voltronic

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Re: 32Bit Float recording
« Reply #18 on: September 30, 2019, 08:52:50 PM »
This thread, dedicated to 32bit Float recording in itself but not addressing the details of how it is implemented in specific recorders won't really be of much practical interest to tapers.. other than being useful to dispel some academic misunderstandings about what it can and can't do.  In other words it will be mostly academic because what really matters is how its implemented in each specific recorder in question. 

That's where the rubber meets the road and where all the current confusion lies!

True.  I think that we will just have to figure it out ourselves.  I have an F6 on the way, and will get to put it through its paces very soon.  One difference in implementation right off the bat has to do with that max level.  The MixPre II units have that incredible +14 dBu max mic input level, whereas on the F6 it's only +4 dBu.  I don't record anything that would get my mics near that lower spec, but some others here might need to consider it if they don't have inline pads.
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Offline Gutbucket

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Re: 32Bit Float recording
« Reply #19 on: September 30, 2019, 09:09:01 PM »
^ Yes! Exactly the problem I have currently with the F8.
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Offline dallman

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Re: 32Bit Float recording
« Reply #20 on: September 30, 2019, 09:49:17 PM »
2) Not exceeding the +12dBv (+14dBu or 11 volts p-p) maximum input on the MixPre's mic input. Good luck with that!

^ This.

A lot of hot modern condensers will clip that, then 32 bit float does nothing for you.
That's never been an issue for me. I'm recording with the same exact gear that worked at 24 bit, I can just run much hotter without fear. I have never had any of my mics distort and the only variable for me is recording at 32bit float instead of 24bit.

I'm recording right now a little hotter than I did at Mule. I'll try to post some pictures tomorrow.
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Offline H₂O

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Re: 32Bit Float recording
« Reply #21 on: September 30, 2019, 09:52:47 PM »
I am pretty sure DSD recording allows for this same behavior as well just not as much of it - The Korg MR-1 could go 2 db over 0 before it clipped - you would just need to lower the levels after the recording to remove the clipping above 0db.
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Offline Gutbucket

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Re: 32Bit Float recording
« Reply #22 on: September 30, 2019, 09:53:35 PM »
Considering the SoundDevices MixPre-II implementation in this light-

Requirement 1) The total dynamic range of the signal will fit within the available dynamic range of the storage format..
(check, 32-bit floating point does this easily)

Requirement 2) The total dynamic range of the signal will fit within the available dynamic range through the recorder's signal path, up to the point where the signal is stored in that format..
Paul reports a total dynamic range through the SoundDevices MixpreII's of 142dB (difference between Mic EIN @ -130dBV and maximum mic input level before clipping @ +12dBV).  This is very impressive and reflects the clever design by SD using parallel preamps and ADC's stages of more limited range combined with a way of shifting between their outputs in real time.  That will also shift the noise floor simultaneously, so technically the recorder cannot represent the full 142dB range at any particular moment, instead it automatically adjusts constantly to provide that overall range in a way which is presumably audibly transparent (and I expect it is).  More on this in panatrope's post at GS. The noise-floor shifting thing is probably not consequential for music recording - we don't hear or care about the noise floor of current recorders when signal is nearing 0dBFS - but it would be consequential if this were a data recorder rather than an audio recorder, which actually needed that full range at any moment.  Anyway, this represents more than sufficient dynamic range for me.

Requirement 3) The actual upper and lower dynamic range values of the signal fit comfortably within the upper and lower dynamic range limits of the recorder's signal path, up to the point where the signal is stored.
This is gets to what paul and EmRR posted above:
The only things you need to worry about with the MixPre-II in 32-bit mode are:
1) The microphone itself clipping!
2) Not exceeding the +12dBv (+14dBu or 11 volts p-p) maximum input on the MixPre's mic input. Good luck with that!
^
A lot of hot modern condensers will clip that, then 32 bit float does nothing for you. 

One will need to attenuate any input hotter than +12dBv  (+14dBu) prior to the recorder.  This is probably the biggest real world limitation of this particular implementation.
« Last Edit: September 30, 2019, 10:14:17 PM by Gutbucket »
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Offline voltronic

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Re: 32Bit Float recording
« Reply #23 on: September 30, 2019, 09:55:39 PM »
^ Yes! Exactly the problem I have currently with the F8.

Hmm, specs on that say mic input will do +14 dBu but with limiters on.  I wonder what it is without limiters that they felt the need to pad that spec.  (Ha!)
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Re: 32Bit Float recording
« Reply #24 on: September 30, 2019, 10:00:18 PM »
One will need to attenuate any input hotter than +12dBv prior to the recorder.

Do any recorders have metering that accurately shows this?  You have to know at what point the meter is tapping the signal also.  It could be showing +14, but that could be post-preamp, at the ADC.  By then, the input has already been overloaded.
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Offline Gutbucket

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Re: 32Bit Float recording
« Reply #25 on: September 30, 2019, 10:13:55 PM »
Hmm, specs on that say mic input will do +14 dBu but with limiters on.  I wonder what it is without limiters that they felt the need to pad that spec.  (Ha!)

F8 with Advanced Limiter switched on (implemented post ADC) attenuates input by 10dB prior to the ADC, so presumably +4dBu (that's not enough!)

F8N allows for phantom power on line-input, effectively increasing that to +24dBu with limiters on instead of +14dBu with limiters on (which should be sufficient)
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Offline Gutbucket

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Re: 32Bit Float recording
« Reply #26 on: September 30, 2019, 10:22:37 PM »
One will need to attenuate any input hotter than +12dBv prior to the recorder.

Do any recorders have metering that accurately shows this?  You have to know at what point the meter is tapping the signal also.  It could be showing +14, but that could be post-preamp, at the ADC.  By then, the input has already been overloaded.

Any of them that meter the input stage.  Most probably meter the ADC though.  Certainly the small hand-helds we use do.

Consider the "brickwalling" problem with many hand-held recorders.  We post about the minimum input level setting which can be used with various recorders without clipping distortion occurring from the an overly hot input even though the meters are not indicating clipping.  This indicates a design problem within the recorder for which we are finding a work arounds and sharing them with each other.  If the recorder was designed correctly, brickwalling could not occur.  The recorder's available range of input trim would be set up such that it could not be adjusted to do so.  The minimum input-trim setting which could be used without brickwalling would equal "0" on the scale (equating to the maximum available input attenuation).

MixPre-II does appear correctly designed in that way:

Quote from: pauljisaacs over at GS
This is achieved with multiple ADCs with more than one preamp in front of each one.
The MixPre-II Mic EIN is -130dBV and its maximum mic input level before clipping is +12dBV thus you have a dynamic range of 142dB. The 142dB dynamic range is mapped such that the maximum mic input signal (+12dBV) does not exceed 0dBFS in the multistage ADC circuit. The 32-bit output of the ADC is then converted to 32-bit float..

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Offline Gutbucket

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Re: 32Bit Float recording
« Reply #27 on: September 30, 2019, 10:57:35 PM »
Here's something that's been nagging me from the start about all 32bit float audio recorders.

Taking the MixPre-II as example-  Ignoring the input overload limits, what is important is the wide 142dB dynamic input range without requiring user adjustment.  A 24 bit fixed file format can accommodate a 144dB signal which exceeds the throughput of this recorder..   

32-bit floating point maybe helpful to accommodate the internal mixing aspects with appropriate processing headroom, but for anyone recording ISO tracks for later mix-down (most tapers), storing the larger 32-bit floating point files is a pointless waste of storage space..  and of bandwidth, which might be a real problem given the recently reported SDcard issues.   

We should be able to harness the full power of that 142dB dynamic range when writing 24bit ISO files.
« Last Edit: September 30, 2019, 10:59:08 PM by Gutbucket »
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Offline jerryfreak

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Re: 32Bit Float recording
« Reply #28 on: September 30, 2019, 11:05:36 PM »
so what yore saying is with proper circuit design, the clipping point of the analog and digital stages are all well matched, so 0dB=0dB across the board and users can just set their levels anywhere above -40 dB and still have 100 dB of dynamic range and 40 more dB of headroom
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Offline aaronji

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Re: 32Bit Float recording
« Reply #29 on: October 01, 2019, 05:44:06 AM »
A 24 bit fixed file format can accommodate a 144dB signal which exceeds the throughput of this recorder.

I think that 144 dB is a theoretical maximum. In reality, no ADC comes close. As Dan Lavry put it, "There is no such thing as true 24-bit conversion and there won't be in my lifetime." The first iterations of the MixPre, as well as the new Scorpio and 833, for example, specify a dynamic range of 120 dB for their 32-bit ADCs. I suppose that is why the 32-bit float machines need to use multiple ADCs and combine the data with, as jerryfreak memorably termed it, a little "digital fuckery"...

 

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