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Author Topic: New Sound Devices MixPre II-series coming?  (Read 8395 times)

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Offline justink

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Re: New Sound Devices MixPre II-series coming?
« Reply #75 on: September 04, 2019, 05:21:38 PM »
dBu and dBV measurements
-----------------------------------

Can someone explain this like I’m 5?

It can get a bit confusing when some measurements are quoted in dBu & some in dBV, but that's life! However, it's simple to convert between them.

dB in any measurement is always a ratio between two values, and so one of these needs to be your reference point:

dBV is referenced to 1.0 Volts
dBu is referenced to 0.775 Volts

When you have 2 signals, a & b,    the difference in dB  = 20*log(a/b)

So to convert a measurement in dBV to dBu, just add 2.21dB.

Paul quoted the MixPre's max mic input level  as 12dBV.
Just add 2.21dB to get to Jerryfreak's "official"  spec of 14dBu (more or less - don't lose sleep over fractions of a dB!)

This works out to be 3.88V.   (BTW, original & MkII are identical spec here)

Thanks. How does this relate to our use?

Appreciate it.
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Offline Gutbucket

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Re: New Sound Devices MixPre II-series coming?
« Reply #76 on: September 04, 2019, 05:33:39 PM »
Of course dithering will eliminate the "stairsteps" but still dithering is noise, so one way or another you signal to noise is lower.

The point of contention regards what occurs within the well established dynamic range limitations of any digital storage format.  It is not about the change in dynamic range storage capability (No one is contending dynamic range storage capacity is not dramatically increased with 32 bit floating point representation), nor is it about the vastly more restrictive real-world dynamic range limitations imposed by the recording environment, the limitations of the microphones, the analog stages through which the signal passes prior to digitization, and the digitization process itself, but rather the "resolution" with which a signal is represented within the well established dynamic range limits of a particular file recording format.  ..and indeed if "resolution" actually has any agreed upon meaning at all in that context.

This sums it up:
Quote from: DSatz
[snip]..equate bit depth/word length with the noise floor and nothing else.
[and]
The other thing is that there's some strange hand-waving going on in this thread about low-level signals needing to have the same s/n ratio as high-level signals. That makes no sense at all, as a few people have pointed out--but the hand-waving continues.

Quote from: borjam
Well, when recording it's useful to have a lot of headroom so that unexpected transients won't ruin your day. We use to record on 24 bit formats because of that. With more resolution available to the low level signals you have more freedom.

An "unexpected transient which might otherwise ruin your day" is high-level signal by definition, one which would have otherwise resulted in clipping or otherwise unacceptable levels of harmonic distortion.  A low-level transient does not require an extreme dynamic range to capture it completely, like any other it only requires enough dynamic range to contain the peak level of the transient.  We need enough headroom to capture high-level transients, but that's essentially a different discussion which concerns what happens at the "other end" of the dynamic range spectrum.

Quote
If you happen to record at a very low level and you raise the volume in postproduction that extra resolution should help.
^
What extra resolution?  This is the recurring claim which no one has been able to back up.  Please explain or point us to a technical explanation for this.. other than a gut-feeling "that's how it seems like it should work", which seems to be the only basis for the claim thus far.

No animosity intended by the way, I'm only trying to get to the bottom of what seems to me to be a strawman argument supporting this rather bold claim.
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Offline mjwin

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Re: New Sound Devices MixPre II-series coming?
« Reply #77 on: September 04, 2019, 06:40:17 PM »
Re. explanation of clipping levels for the Mixpre mic inputs:
 
Thanks. How does this relate to our use?

You might need to know whether your recorder input will clip at a loud show.

But first, you need to know the sensitivity spec of your mic.  Sensitivities are always quoted at a level of 94dB SPL, (equivalent to air pressure of 1Pascal)  Most SDCs have sensitivities  between -30 & -40dBV.
Say you were using  Schoeps MK4, with a sensitivity of   -36.5dBV.
You plug it into a Mixpre with a max input level of +12dBV
-36.5 - 12 =  -48.5dB difference between the mic output at 94dBSPL and the output when it clips the mic pre.
So the input clips at 48.5+94 = 142.5dB SPL which is louder than a close-miked drumkit, and also much higher than the rated spec of the microphone (so the mic signal will be very distorted at this point)!

Although I've veered off topic a bit with these calculations, what they show is that the Mixpres (new & old)  have a impressively wide input range. +12dBV at one end, and the noise floor of -130dBV at the other = 142dB dynamic range. And that's back to pretty much where this thread is at.

Offline DATBRAD

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Re: New Sound Devices MixPre II-series coming?
« Reply #78 on: September 04, 2019, 08:17:09 PM »
So, I've reversed my position on 32 bit in the field, after reading some 9 year old Pro Tools materials. I now see in practice 32 bit can allow you to set super conservative levels that are adjusted up in post just like 24 bit, but unlike 24 bit, you can fly closer too the Sun, so to speak. Imagine setting ideal levels at the show, based on average signal without concern for unexpected peaks. The preamps won't overload if analog limited engage, and you can't go "over" on the recording, just the display. I'd much rather err on the side of being a little too hot and having to dial it back in post, instead of boosting a weak level recording. Could be all in my head, but it just seems right.
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Offline Amir

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Re: New Sound Devices MixPre II-series coming?
« Reply #79 on: September 05, 2019, 12:33:58 AM »
badly needed? a usb-c to AC power adaptor can be had for $20....
I'd rather stick to the manufacturers' recommended AC adapter products so as not to risk damaging my purchased electronic goods. SD's own adapter definitely provides a lot more peace of mind.

Offline AB52

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Re: New Sound Devices MixPre II-series coming?
« Reply #80 on: September 05, 2019, 01:53:33 AM »
I record thunder sometimes - it seems like 32 bit would be idea.  A lot of times - my levels cannot get it right and I end up with distortion (i.e. the storm is at such a low level versus a great thunder that would have been worth recording correctly).

Offline voltronic

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Re: New Sound Devices MixPre II-series coming?
« Reply #81 on: September 05, 2019, 05:54:11 AM »

Quote
If you happen to record at a very low level and you raise the volume in postproduction that extra resolution should help.
^
What extra resolution?  This is the recurring claim which no one has been able to back up.  Please explain or point us to a technical explanation for this.. other than a gut-feeling "that's how it seems like it should work", which seems to be the only basis for the claim thus far.

No animosity intended by the way, I'm only trying to get to the bottom of what seems to me to be a strawman argument supporting this rather bold claim.

Barring a more detailed technical explanation, I suppose the only way to really know is that someone who gets their hands on one of these units is going to have to run parallel 24-bit fixed and 32-bit float recordings at very low level, normalize, A/B them, null test, etc.  I don't see simultaneous 24-fixed and 32-float recording as an option in the MixPre II manual, but it is an option on the (still not released) Zoom F6.
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Offline jerryfreak

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Re: New Sound Devices MixPre II-series coming?
« Reply #82 on: September 05, 2019, 06:18:01 AM »
re: dBU vs dBV question from yesterday, this has always been a handy page for me

http://www.sengpielaudio.com/calculator-db-volt.htm
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Offline mjwin

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Re: New Sound Devices MixPre II-series coming?
« Reply #83 on: September 05, 2019, 06:38:46 AM »
Quote
If you happen to record at a very low level and you raise the volume in postproduction that extra resolution should help.
^
What extra resolution?  This is the recurring claim which no one has been able to back up.  Please explain or point us to a technical explanation for this.. other than a gut-feeling "that's how it seems like it should work", which seems to be the only basis for the claim thus far.

I think part of the issue here is the failure to grasp how the omnipresent noise impinges on the desired signal. 

There seems to be a common misconception  that the noise floor in a system is something "down there" which the signal needs to climb above if it's to be heard and, once that's happened, it is magically noise-free with the potential for infinite resolution (limited only by the convertors). This is simply not the case. The noise, whether we like it or not,  floats on our signal, effectively adding a random value to each sample and inherently limiting its resolution.

Here's a simple comparison between two digital systems. It assumes that the convertors and electronics are  "perfect" & is very much a first-order approximation. It doesn't take into account the dithering & resolution enhancing effect of noise, nor all manner of psycho-acoustic phenomena, etc. It's just a relative comparison. (Disclaimer!)

Say we're recording an orchestral concert and set our mic gain so the recorder clips at 120dB SPL.
We're in a *very* quiet hall, everyone holds their breath, and the background noise is only 24dB SPL. (if only...)
Then the music begins with a single quiet note, which peaks at 54dB SPL.

That note has a dynamic range of 30dB.   As the reverb dies away, those of us present hear the sound smoothly decay into the background ambiant noise.

24 bit recording
The 24 bit convertor has a dynamic range of 144dB.  We've set 0dB (clipping) to occur at a sound pressure level of 120dB SPL, so the background noise  at 24dB SPL is 96dB down from clipping, & occupies the bottom 8 bits of our digitized signal .
Our Convertor samples the note, capturing the peak at the 13 bit level . The "gut feeling" is that, the peak of the captured waveform will smoothly follow the instrumental sound in small increments of 1/8192 (13 bit resolution) of the peak value.   

But we've forgotten about the noise!  The bottom 8 bits of the convertor are jumping about randomly, with the effect that they add a random value to each  sampled point.  We can't be sure exactly where the wanted signal is, as the bottom 8 bits are meaningless. The peak of the musical note is only 30dB above the noise, so we have only a 30dB dynamic range. (Resolution approx 5 bits). But it won't sound "granular" or look like a staircase  on the screen. If we zoom out our display we'll just see noise superimposed on our waveform; which is exactly what we hear when sitting in the hall.

32 bit recording
So what if we're also recording  with a (hypothetical) 32 bit convertor?  The sound levels are obviously the same, and again we set our clipping level to be 120dB SPL.

Now our convertor has a dynamic range of 192dB. The extended low level resolution means that the noise now occupies the bottom 16 bits of our digitized signal, with the first musical note peaking at the 21 bit level.  Wow! we think, "the resolution  is now 1 part in 2.097 million!"

But it isn't. Again, we've forgotten  the noise! Now the bottom 16 bits are meaningless in terms of resolution as they're simply noise. The signal peaks at 54dB SPL, the noise is at 24dB SPL, the dynamic range is 30dB. 

Nothing we can do in the digital domain will increase the resolution of our wanted signal.  In order to achieve this, we have to reduce the noise level at the recorder inputs. All we can do is to place the mic closer to the instruments, switch off the a/c, record at night,etc.

It's a pity that the conversation has gone down this particular rabbit hole, as the upgraded MixPres potentially have a lot more to offer.   As a few people have already mentioned, the dual convertor topology should allow us to record low level signals using maximum gain from the low-noise preamps, whilst also allowing capture of high level transient peaks up to the clipping level of the input. Effectively that allows us to utilize the full dynamic range of the analog front end, and not worry about clipping. Ever.  And that can only be a "good thing".



Offline mjwin

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Re: New Sound Devices MixPre II-series coming?
« Reply #84 on: September 05, 2019, 07:04:17 AM »
I record thunder sometimes - it seems like 32 bit would be idea.  A lot of times - my levels cannot get it right and I end up with distortion (i.e. the storm is at such a low level versus a great thunder that would have been worth recording correctly).

Paul used the thunderstorm example a few pages back, and I then commented on how I do mental calculations to ensure that I capture the full dynamic range without compromising resolution.

But this is exactly the situation where the freedom from potential clipping would be of practical benefit.  You simply don't need to worry about the levels. Just setting up your mics out of the rain, not being in a vulnerable spot, etc. is plenty to think about in the heat of the moment.

There are parallels with photography here. I remember using color transparency film and needing to be very careful to meter for the highlights so that they didn't crush to white. The dynamic range of transparency film (& early digital) was quite low & if you weren't careful you'd end up with everything washed out, or no shadow detail at all. Modern cameras can achieve fantastic results with just point & shoot - allowing you to concentrate on the composition, timing & other artistic elements.  Just like sound recording.

Offline justme

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Re: New Sound Devices MixPre II-series coming?
« Reply #85 on: September 05, 2019, 08:04:03 AM »
There's 24-bit and 32-bit fixed point value.
Those not the same as 32-bit floating point value which is what SD and Zoom is using in their recorders although Sound Devices uses 3 parallel preamp/ADC stages with low/mid/high gain which can be combined into a 2 stage output if needed. Exactly how Zoom have implemented their solution we will need to wait for.

While 32 bit fixed point offers a better resolution than 24 bit fixed, both will suffer from a reduction of resolution bits the lower the signal gets.
A 32-bit floating point value basically contains of a 24-bit value (mantissa) which always retains its precision by floating the decimal and 8 bit exponent which defines the signal level of it.

It appears as if SD is reaching the need dynamic range with their use of three parallel stage input feeding the 32-bit FP signal processor.

Offline Amir

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Re: New Sound Devices MixPre II-series coming?
« Reply #86 on: September 06, 2019, 12:14:48 PM »
Hi all,

Would it be possible for you to send the following to SoundDevices' Tech Support team on my behalf? Since I don't have the MixPre-6 II at the moment, I can't send it without a serial number via their support contact form. My request is related to implementing an accessibility feature for the visually impaired via a future firmware update.
............
Dear SoundDevices tech support representative,

Hope this finds you well.

My issue is that the MixPre-6 II doesn't provide audio messages, via headphones, to assist the visually impaired with the operation of the device. As such, making changes to its menus and settings is quite tricky and challenging for those who have little or no vision.
When a USB keyboard or a controler is connected to the MixPre-6 II, it would be quite useful to hear audio prompts, via headphones, upon pressing keys and keyboard shortcuts. Otherwise -- and as a visually impaired person, I can't comfortably utilize such a capable and robust device. In short, I want audio and speech prompts, possibly via a future firmware update, which -- if enabled -- would allow me to alter most or all of the device's settings via my USB keyboard or -- if applicable -- via a controler, independently.

Hope SoundDevices can implement this suggestion which would be a tremendous boon to the visually impaired.

All the best
............
Thanks beforehand for your assistance.

Offline justme

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Re: New Sound Devices MixPre II-series coming?
« Reply #87 on: September 06, 2019, 12:34:34 PM »
I sent your excellent request to the SD support.
And also included a link to your post.

I’ll post their reply.

Hi all,

Would it be possible for you to send the following to SoundDevices' Tech Support team on my behalf? Since I don't have the MixPre-6 II at the moment, I can't send it without a serial number via their support contact form. My request is related to implementing an accessibility feature for the visually impaired via a future firmware 
............
Thanks beforehand for your assistance.

Offline Amir

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Re: New Sound Devices MixPre II-series coming?
« Reply #88 on: September 06, 2019, 12:41:41 PM »
I sent your excellent request to the SD support.
And also included a link to your post.

I’ll post their reply.
Thanks for your prompt and considerate action!
Looking forward to their reply.

Offline justme

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Re: New Sound Devices MixPre II-series coming?
« Reply #89 on: September 06, 2019, 02:40:37 PM »
SD support just got back to me.
Here's their initial response. If I get any follow ups I'll post them to.

Thank you for contacting Sound Devices. I will certainly pass this feedback along to our engineering team for evaluation, though I cannot promise if or when these features would be added.

Thanks,
Daniel
Sound Devices, LLC


Thanks for your prompt and considerate action!
Looking forward to their reply.

 

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