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Author Topic: 32Bit Float recording - The Technical view  (Read 6083 times)

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Offline voltronic

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Re: 32Bit Float recording
« Reply #30 on: October 01, 2019, 06:19:06 AM »
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)

I didn't notice it until you pointed it out, but the F6 does phantom with line-in also, specifying +24 dBu max level but no mention of limiter.  I would have never thought to connect a mic directly to a line input.  I wonder how the additional 2000 ohms of input impedance changes things on the low end of the signal spectrum.  In other words, I question whether running straight to phantom-powered line in really gains you 20 dB dynamic range (probably not).  You would only do that for constantly loud stuff.
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Offline voltronic

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Re: 32Bit Float recording
« Reply #31 on: October 01, 2019, 06:20:11 AM »
This thread on JWSound gets a bit into the implementation of SD MixPre II vs. Zoom F6:

https://jwsoundgroup.net/index.php?/topic/34205-zoom-f6-vs-sound-devices-mixpre-ii-series-dynamic-range/
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Re: 32Bit Float recording
« Reply #32 on: October 01, 2019, 07:26:56 AM »
One question I have is would a 32 bit float recording sound better than a properly captured 24 bit recording with all other things being equal?

My sense is that the answer is no, but that doesn't take away from the benefits of having a larger margin of error in the field.

Dolby B and later Dolby C expanded the margin of error for tapers using compact cassette in the early 1980s. But skill and experience continued to be a requirement to make the best recordings. PCM and later DAT reduced the need to run hot to reduce noise since tape saturation was no longer a factor. 20 bit, and later 24 bit A/D chips that dithered to 16 bit improved the lower level signal quality by reducing the impact of quantization noise, slightly increasing the margin of error setting levels a little more. Still, precision in level setting was a key factor separating the best recordings from the rest, exposing the less skilled and experienced unless they got lucky.

Moving from 16 bit to 24 bit recording offered the improvement of allowing for higher sampling rates, but primarily freed the taper from having to "nail" a recording in 16 bit. The skill level and amount of experience needed plummeted with 24 bit.

From everything I've read, 32 bit float recording is just another step in this evolution, making it almost stupid easy to make a good recording in the field. The notion of "set it and forget it" is realized to the point that only operator errors like forgetting to hit the record button can get in the way.

With all of these advancements, isn't it ironic that there were far more active tapers back when it was harder and required more skill to get a worthy result than are active now?
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Offline EmRR

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Re: 32Bit Float recording
« Reply #33 on: October 01, 2019, 08:35:37 AM »
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)

I didn't notice it until you pointed it out, but the F6 does phantom with line-in also, specifying +24 dBu max level but no mention of limiter.  I would have never thought to connect a mic directly to a line input.  I wonder how the additional 2000 ohms of input impedance changes things on the low end of the signal spectrum.  In other words, I question whether running straight to phantom-powered line in really gains you 20 dB dynamic range (probably not).  You would only do that for constantly loud stuff.

It does not gain you 20dB unless the limiter is on, and it's not entirely clear what's happening with the limiter.  As I reported in the F8 thread, you can overdrive the F8n line input (set to lowest gain) into square waves with it metering -5.5dBFS.   If you get a board feed, you may clip the input without an inline pad; I had a board feed drive it into clipping and ran the limiter just last week).  I frequently run MKH mics at shows with it set to line rather than mic, lowest mic gain setting not low enough. I don't run the limiter, I run gain low enough to avoid it. 
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Offline dallman

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Re: 32Bit Float recording, and a bunch of other ramblings
« Reply #34 on: October 01, 2019, 11:20:31 AM »
Well I have had the night to cool down and I am glad I did not post last night. I support everybody's right to an opinion and their ability to share it, but this thread was to be all about recording with 32bit float. It is new, it is different, the approach is different, and it would be great to have a place here to discuss as we use and learn this new feature on a few existing decks and more on the way. That is what I asked for in the thread, so I was a bit (or more) annoyed to see that after a nice start, the thread was totally hijacked with the same "stuff" that to me has been stated and restated. There are plenty of long rambling threads where people have already decided what they like or do not like about 32bit recordings or the company's that so far are selling them. I have no interest in that debate and I know I am not alone. I had a MixPre6 and loved it, but when the version II was announced, I sold that deck to a very good friend and decided to dive in to the new deck because when you record on the fly in different venues with different mics and different types of music to me what could be bad about not needing to worry about your gain settings? Again, everyone has a right to their opinions, but jeez, there are pages and pages of whining about cards and talking about why 32bit is no good. I just have no interest in thumbing through pages and pages of that to learn what those who are using 32bit are doing or learning or discovering. I just want a spot for that to be the focus as I think it is a pretty big deal. I have been doing audience recordings for 48 years in every conceivable form and deck in every conceivable format and I just only rely on my ears. I love the technical stuff, but audience recording has its own set of variables and rules. The only thing that never changes is that everything changes, and this concept, 32bit float on a portable deck is pretty amazing so far in my 2 outings. Whether I run super-hot or barely a bit hotter, I feel we are onto something here and those who do embrace it or those who want to learn more about how it is working in the field deserve one thread that talks about it. So, I hope that we can let the next thread not be the thread of anything more than what was originally stated.

I orginally locked this, but all of those that want to discuss what this thread became are welcome to.  8)
« Last Edit: October 01, 2019, 12:16:49 PM by dallman »
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Offline Gutbucket

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Re: 32Bit Float recording
« Reply #35 on: October 01, 2019, 12:32:00 PM »
Thanks dallman,

Apologies for my part in taking this thread in a direction contrary to your intent.  Your idea to open another dedicated to hands-on user reports and experience with 32-bit float recorders is a good one.  I also think it wise to continue the overview and technical discussion concerning 32bit floating-point recording in this thread, where the technical aspects have been laid out pretty up to this point. The other threads where this has been discussed thus far are more broadly dedicated to specific recorders and other aspects about them.  Hopefully keeping this overview and technical discussion about 32bit-float going here will keep it from inevitably spilling over into your new thread.  I don't intend to let that happen on my part, but I can easily see it happening otherwise. 

~regards
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Offline Gutbucket

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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"...

I'm referring to the file storage format written to memory, not the analog to digital conversion.  The multiple-ADC switching scheme is a clever work-around of the real-world practical limits imposed by traditional single-ADC conversion which Lavry was referring to, extending DR to 142dB. 

My point is that 32-bit float is useful internally for summing and DSP (just as it is in a DAW), yet is unnecessary as a storage format because 24bit fixed provides sufficient storage capacity (just as it does in a DAW). The 142dB of actual throughput will fit within the 144dB of dynamic range storage capacity of a 24bit PCM file. Storing in 24bit fixed files loses nothing useful, reduces the throughput burden of writing data to the SDcard (a currently reported problem), is more universally usable and playable, and significantly reduces file size with high channel counts.

In the current MixPre-II implementation there is some quantization noise way down there due to SD not applying dither in the conversion from 32bit-float to 24bit-fixed (Discussion about that here).  Paul mentions SD has up to this point not seen the need to do so, which indicates how insignificant the problem is (even though they are now highlighting it as example of superiority for 32-bit float recording - see here - erroneously in my opinion, for the reasons stated in the link above). If SD were to apply dither in the conversion from 32bit-float to 24bit-fixed there would be no quantization noise, yet the noise floor would increase by a few dB.  So maybe we'd get ~140dB total range or something instead of 142dB, which is still more than enough.  And if someone really needed to amplify so much that the noise-floor of the recorder became evident (and it isn't completely buried deeply beneath microphone self-noise and the environmental noise-floor, which is almost certain) they'd find a dither noise-floor rather than quantization artifacts.

For those reasons, I'd like to see Sound Devices implement dithering to 24-bit in combination with the new 32bit-floating point conversion, in addition to direct 32bit floating point storage.

[edit- SD page link fixed, thanks voltronic]
« Last Edit: October 01, 2019, 02:28:36 PM by Gutbucket »
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Offline voltronic

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Gutbucket - your embedded link to SD's site is broken:
https://www.sounddevices.com/low-signal-32-bit-float/

Here is another relevant JWSound thread:
https://jwsoundgroup.net/index.php?/topic/34174-floating-point-v-fixed-point-wav-files/
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Offline jerryfreak

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Re: 32Bit Float recording
« Reply #38 on: October 01, 2019, 02:36:55 PM »
One question I have is would a 32 bit float recording sound better than a properly captured 24 bit recording with all other things being equal?

not for concert recording purposes. same for 16 bit. if you are careful with levels and make a recording that peaks near zero your room noise is still 30-40 dB above the noise limit of the digital data
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Re: 32Bit Float recording
« Reply #39 on: October 01, 2019, 03:32:30 PM »
On the F8/F8N overload point and Advanced limiter-

It does not gain you 20dB unless the limiter is on, and it's not entirely clear what's happening with the limiter.

Not sure about the 20dB figure.  I understand the Advance limiter to reduce sensitivity by 10dB, providing that much additional headroom before clipping should otherwise occur, but that 10dB includes the action of the limiter and presumably increases the noise-floor by the same 10dB.  Threshold is not set directly nor clearly defined, with only the not to exceed level being set by the user.   I've speculated that the Advanced limiter increases ratio as the signal rises higher above the threshold, reaching the stated infinity:1 ratio just below the do not exceed setting - basically a 10dB soft knee.  It doesn't sound like inf:1 when lightly engaged.  It has worked to keeep the F8 out of clipping for me, but I'd rather it be just a safety measure rather than the only way I can avoid overload when using high-sensitivity mics - and that's basically how I'm currently running it.

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When the limiter of the F8/F8n is activated for a channel it automatically cuts 10 dB of gain from the analog preamp. That 10 dB is automatically added back after the AD converter. This is a way we build an extra 10 dB of headroom to avoid clipping over the already high dynamic range (A/D Converter has 120 dB dynamic range, with limiter effective dynamic range is 130 dB)

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As I reported in the F8 thread, you can overdrive the F8n line input (set to lowest gain) into square waves with it metering -5.5dBFS.   If you get a board feed, you may clip the input without an inline pad; I had a board feed drive it into clipping and ran the limiter just last week).  I frequently run MKH mics at shows with it set to line rather than mic, lowest mic gain setting not low enough. I don't run the limiter, I run gain low enough to avoid it.

Hoping I can do the same as you to avoid clipping if I switch to F8N by using line-in and the input sensitivity turned all the way down.  What is the sensitivity of the MKH mics so that I might compare to better confirm this?   I'd forgotten the -5.5dBFS overload point behavior you found, which doesn't inspire confidence.  Are you able to keep your recordings using MKH > line-in lowest gain > no limiter peaking below -5.5dBFS?
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Offline EmRR

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Re: 32Bit Float recording - The Technical view
« Reply #40 on: October 01, 2019, 04:53:16 PM »
I have not had an F8n problem with MKH into line input, definitely too hot for mic input in many cases with a lot of what I do.  4060 not far behind it. 

Senn. MKH 800 TWIN  40mV/Pa = -28dBV
Senn. MKH 20/30/40    25mV/Pa = -32dBV
Neumann TLM 103 23mV/Pa = –32.5 dBV - they quote a max output voltage +13dBu
DPA 4060 20mV/Pa = -34dBV
Neumann KM184 15 mV/Pa - they quote a max output voltage +10dBu
Neumann KM131  12mV/Pa = -38.4dBV
« Last Edit: October 01, 2019, 05:50:23 PM by EmRR »
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Offline voltronic

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Re: 32Bit Float recording - The Technical view
« Reply #41 on: October 01, 2019, 05:58:52 PM »
I have not had an F8n problem with MKH into line input, definitely too hot for mic input.

That's good to hear that the phantom+line is a useable option for really hot mics.  I've never had a device with that capability, so have never tried it.  I'll have to borrow some hot mics to try it on my F6.

It's worth noting that the Zoom recorders have lower input impedance than the SD MixPres.  The F8n is 2 kΩ mic; 2.6 kΩ line.  F6 is a bit higher at 3 kΩ mic; 5 kΩ line.  MixPre II is 4 kΩ regardless of setting.  That may not be the whole story when it comes to how hot a level it will deal with, but it's part of it.
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Offline Ozpeter

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Re: 32Bit Float recording
« Reply #42 on: October 01, 2019, 08:10:14 PM »
From everything I've read, 32 bit float recording is just another step in this evolution, making it almost stupid easy to make a good recording in the field. The notion of "set it and forget it" is realized to the point that only operator errors like forgetting to hit the record button can get in the way.

Perhaps I'm at risk of picking on something you didn't quite mean the way you worded it, but there's a danger all the way through this discussion that we're saying that a recording which does not suffer from clipping at one end or noise at the other is a good recording.

In recent years, I doubt whether any appraisal of the real merits of a recording by any competent recordist will have included comments about poor level setting.  A good recording in the field is still, and always will be, dictated first by the choice of microphones and their placement relative to the sound source.  Next, these days, comes the choice of preamp, and last in importance is the choice of recording device, and the format used to record in (bits and sample rates etc).  That's not to say it is of no consequence, and of course all items in the signal chain have to be matched at a technical level and at an overall quality level, but it's dangerous to risk any statement that devalues the fundamental skill that separates a competent field recordist from a beginner, that of mic selection and placement.  We have to be careful that we don't make statements about recordings that are the audio equivalent of "that's a great photo, you must have a fantastic camera".

From what I'm reading above, choice and placement of mic is even more important with these devices.  This now becomes the equivalent of level setting - which has been moved from a knob to the mic, and if you've got it wrong your amazing new recorder can't help you.  As always, put a hot mic close to a loud source and you'll still risk clipping - or an unhot mic too far from a low level source, and you'll still risk noise (apart from all the other considerations affecting mic placement).

However, I do agree that we're seeing an important new phase in the development of audio recorders - we started with 14 bits (effectively, as I recall it), then 16, then 24, and now 32 bit float, and then... perhaps we're at the end of the road.

Offline DATBRAD

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Re: 32Bit Float recording
« Reply #43 on: October 02, 2019, 07:46:40 AM »
From everything I've read, 32 bit float recording is just another step in this evolution, making it almost stupid easy to make a good recording in the field. The notion of "set it and forget it" is realized to the point that only operator errors like forgetting to hit the record button can get in the way.

Perhaps I'm at risk of picking on something you didn't quite mean the way you worded it, but there's a danger all the way through this discussion that we're saying that a recording which does not suffer from clipping at one end or noise at the other is a good recording.

In recent years, I doubt whether any appraisal of the real merits of a recording by any competent recordist will have included comments about poor level setting.  A good recording in the field is still, and always will be, dictated first by the choice of microphones and their placement relative to the sound source.  Next, these days, comes the choice of preamp, and last in importance is the choice of recording device, and the format used to record in (bits and sample rates etc).  That's not to say it is of no consequence, and of course all items in the signal chain have to be matched at a technical level and at an overall quality level, but it's dangerous to risk any statement that devalues the fundamental skill that separates a competent field recordist from a beginner, that of mic selection and placement.  We have to be careful that we don't make statements about recordings that are the audio equivalent of "that's a great photo, you must have a fantastic camera".

From what I'm reading above, choice and placement of mic is even more important with these devices.  This now becomes the equivalent of level setting - which has been moved from a knob to the mic, and if you've got it wrong your amazing new recorder can't help you.  As always, put a hot mic close to a loud source and you'll still risk clipping - or an unhot mic too far from a low level source, and you'll still risk noise (apart from all the other considerations affecting mic placement).

However, I do agree that we're seeing an important new phase in the development of audio recorders - we started with 14 bits (effectively, as I recall it), then 16, then 24, and now 32 bit float, and then... perhaps we're at the end of the road.

Sorry, I thought when I said in my semi-rhetorical question at the top of my post "with all other things being equal" that removed those obvious factors from the recording device equation. Of course location, source quality, and mic placement come before the recorder in order of importance. I've actually made the point about skill and experience being more important than gear cost several times over the years on this forum. And while it's true that a naturally gifted or massively practiced golfer can beat a guy with the best clubs made using an old crappy set due to more advanced skill, I'm starting to pull back on that notion somewhat with photography and audio recording with the advancements in technology.
Take photography, when I started out using a 35mm SLR in 1980, it had a manual light meter called "match needle", and was significant because before they were around, light meters were separate hand held devices that photographers had to know how to interpret in order to set f-stop and shutter speed using one. You had to know how things like depth of field were directly related to aperture for portrait work, for example, when you couldn't preview it. And while advancements in camera technology can't improve poor composition and balance, something only skill and training can do, many of the aspects of cameras that also had to be mastered no longer require the same long learning curve. Just like with analog tape saturation, film exposure to maximize the amount of silver left on the negative was very important to understand in the past, but no longer applies in the digital era. It's like knowing how to navigate a boat with paper charts and dead reckoning turned into a quaint ability no longer needed once GPS technology became well established. I guess the point of my remarks was that 32 bit float removes one more skill set from the equation of requirements for a good recording, knowing how to set proper levels to maximize the performance of the recorder.
It means that skills around factors other than location and mic placement are being eliminated, just like portrait mode on modern phone cameras eliminates the need to understand why the subject is in focus and the background is out of focus when it's used, but they do need to know when to use the setting for composition, which absolutely reduces the learning curve just like automatic exposure meters in 35mm did a couple decades earlier. Hope that clarifies my comments from before.
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Offline aaronji

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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"...

I'm referring to the file storage format written to memory, not the analog to digital conversion. 

Gotcha. But further to the conversion itself, I think it is kind of curious that SD doesn't offer 32-bit float on their new pro-level recorders (the Scorpio and 833). Perhaps they were already too far down the development/testing pipeline to add that feature and there will be II versions of those in the near future as well. Or maybe it is not in demand by professional customers. Or some other reason. In any event, I would be interested in hearing why this highly-touted feature isn't on those recorders...

 

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