You are here

Advice on Higher End AD/DA Converter/Interface

All about the tools and techniques involved in capturing sound, in the studio or on location.

Advice on Higher End AD/DA Converter/Interface

Postby DDDDDoug » Sun Apr 23, 2017 10:20 pm

My RME Fireface 800 has served me well but I’ve upgraded the rest of my home studio over the years and I think it’s time to upgrade my AD/DA conversion. I don’t need built-in mic pres anymore so I’d like either a line level interface or a standalone converter. I understand that a standalone converter will also require something like an RME Madiface depending on the digital format. I need 16 channels and don’t care if they’re all in one box or two 8-channel boxes. I prefer converters to be more on the neutral and transparent side as long as that doesn’t translate into thin or harsh. Also, my Fireface is silent (no fan) and I’d like the replacement to be as well. I don’t mind buying used gear.

One issue I have with the Fireface and most other semi-pro interfaces is that the high gain settings are only +19 dBu. Hugh Robjohns gave a great explanation of this in a few other threads. I have to run my external equipment very low when feeding to it and then deal with low signal level when feeding from it. I can make it work but it’s a pain, therefore I will only consider boxes that can handle +24 dBu.

Conversion quality from the Fireface is fairly good to my ears but the consensus seems to be that you can do quite a bit better nowadays so I’d like to move up to something that will keep me happy for some time.

My current equipment:
DDA S Series 16 channel analog mixer
Vintech 473 mic pres
Several AKG 414 B-ULS, Peluso P-84 stereo set, Advanced Audio CM47 FET, and some other mics
Charter Oak PEQ-1 and SCL-1, Drawmer 1968 ME
Audio Accessories Mini Shorti patchbay
All Mogami cabling
Mac Mini with Thunderbolt, Firewire, and USB connections. 2.3 GHz quad-core Intel Core i7.
Cubase 7.5
Professionally treated room
Dynaudio BM5a and BM14s monitoring

I record things ranging from singer-songwriter to alt-country to harder rock and I do some hybrid mixing. I heard a huge difference when I upgraded to the Vintech, Mogami, and AKG and again when I added the DDA. Even simple cassette and vinyl transfers sound much better going through the DDA rather than directly into the Fireface.

I’m considering the following boxes and am wondering how they compare soundwise. Are they all at roughly the same level but with different flavours or are some clearly better? I’m not looking for “this is great for the money” comments, I’m looking for, “if these were all the same price, I’d put these ones in the top tier, these ones in the second, etc.”

Apogee Symphony I/O mki
Apogee Symphony I/O mkii
IZ Technology ADAii with Classic 96 cards
IZ Technology ADAii with Ultra Nyquist cards
Lynx Aurora 16 VT (older model, not the new “n”)
Metric Halo Lio-8
Mytek 8x192 (not sure if it can go to +24 dBu)
Prism Sound ADA-8XR
RME ADI-QS
Any others that I’ve missed?

Thanks in advance for any help you can provide!

Doug
DDDDDoug
New here
Posts: 8
Joined: Mon Jul 29, 2002 12:00 am

Re: Advice on Higher End AD/DA Converter/Interface

Postby resistorman » Mon Apr 24, 2017 3:24 am

I wish I had first hand experience with all these items! I wish I could walk into a store where all these items were hooked up in a way that I could compare them accurately in real time!
User avatar
resistorman
Frequent Poster
Posts: 591
Joined: Sun Nov 22, 2015 1:00 am
Location: Asheville NC
future26.com

Re: Advice on Higher End AD/DA Converter/Interface

Postby molecular » Mon Apr 24, 2017 9:05 am

Likewise.

But I do run my current setup on an RME madi face as you describe (in my case connected to a second hand SSL alphalink) and it is flawless.

I think MH Lio is still FireWire only? Although I had a ULN2 before this setup and it was also rock solid, never had a single issue and the software is great. Built like a tank too. I would go back to MH if I was doing more portable/mobile stuff.
User avatar
molecular
Frequent Poster
Posts: 1303
Joined: Sat Dec 13, 2003 1:00 am
Location: The turn off where the main road goes over the river. If you're at the post box you've gone too far.
Anto mo Ninja, Watashi mo Ninja http://www.hectormacinnes.com

Re: Advice on Higher End AD/DA Converter/Interface

Postby Hugh Robjohns » Mon Apr 24, 2017 9:41 am

This is a case of 'yer pays yer money and takes yer choice'. There are plenty of converter and interface options and, by and large you get what you pay for... You've listed many good quality alternatives and we've reviewed a lot of them. If money is no object then it will come down to a personal preference of features, facilities and sound character -- some having more 'character' than others! Personally, I'd go for the Prism if someone else is bank-rolling it! ;-)

However, I fear there may be some underlying misunderstandings generating part of your desire for change:

DDDDDoug wrote:One issue I have with the Fireface and most other semi-pro interfaces is that the high gain settings are only +19 dBu. ...I have to run my external equipment very low when feeding to it and then deal with low signal level when feeding from it. I can make it work but it’s a pain, therefore I will only consider boxes that can handle +24 dBu.

You say '...have to run external equipment very low'? This is utter madness. :-( If you are concerned that the analogue levels are too low in comparison to your digital levels, you need to use a conversion alignment with a LOWER clipping level, not higher. Try switching to the +4dBu mode on the RME's analogue input and output settings instead.

The plain fact is that there are only a very few situations where a clipping point of +19dBu is a problem (and they relate to live recordings with no rehearsal and lots of unpredictability!), and it really shouldn't be bothering you to the extent you suggest.

A clipping level of +19dBu should provide more than enough headroom for any sensibly calibrated and operated analogue/digital signal chain within the controlled environment of a project studio. Thousands of broadcasters, studios, mastering houses, and project studios all across Europe seem to cope perfectly well with it... as do I. ;-)

The reality is that RME's adherence to the EBU standard is perfectly logical and practical given their Germanic origins. In fact they actually provide 1dB more headroom than the EBU alignment calls for (making precise alignment less critical).

It's also worth noting that -- and this is the REALLY important bit -- the EBU alignment provides 18dB of headroom (and RME are giving you 19dB) whereas the SMPTE standard calls for 20dB of headroom.

Are you really trying to tell us that 1 or 2dB more headroom margin would make a stunning difference to your system's sound? If you are I fear there really is something terribly awry either in your system's setup or its operation -- most likely your analogue reference levels are wildly incorrect or you don't understand the concept of headroom.

Have a read of this: http://www.soundonsound.com/techniques/ ... w-software

H
User avatar
Hugh Robjohns
Moderator
Posts: 21952
Joined: Fri Jul 25, 2003 12:00 am
Location: Worcestershire, UK
Technical Editor, Sound On Sound

Re: Advice on Higher End AD/DA Converter/Interface

Postby Mixedup » Mon Apr 24, 2017 9:57 am

What Hugh said! If the FF800 is so bad, I'll take it off your hands to add more I/O to mine ;) :headbang:

But if you were to want to upgrade, how many channels do you require?

Note that the FF800 has 16 channels (at 44.1/48 kHz; 8 at 88.2/96 kHz; 4 at 192kHz) of ADAT and 2 of AES or S/PDIF digital audio, so you could consider standalone converters and keep using your FF800 and its software.
User avatar
Mixedup
Jedi Poster
Posts: 4217
Joined: Wed Sep 03, 2003 12:00 am
Location: Laputa

Re: Advice on Higher End AD/DA Converter/Interface

Postby mjfe2 » Mon Apr 24, 2017 10:02 am

How about this? http://www.ebay.co.uk/itm/152407290893? ... EBIDX%3AIT

That said, apart from a slightly better SNR I don't think you can go wrong with the Fireface's converters, even nowadays. I'm impressed at how many of classical label BIS's newer releases are still recorded on their old Octamic D's, which came out around the same time as the FF800.
User avatar
mjfe2
Frequent Poster
Posts: 609
Joined: Sun Oct 11, 2009 12:00 am
Location: London, UK

Re: Advice on Higher End AD/DA Converter/Interface

Postby DDDDDoug » Wed Apr 26, 2017 2:42 am

Hugh, first, let me say that it’s amazing to have knowledgeable people such as you willing to answer questions from amateurs like me. I’ve been reading the magazine for about 15 years and have a lot of respect for it. Thank-you for your reply. :thumbup:

Hugh Robjohns wrote:This is a case of 'yer pays yer money and takes yer choice'. There are plenty of converter and interface options and, by and large you get what you pay for... You've listed many good quality alternatives and we've reviewed a lot of them. If money is no object then it will come down to a personal preference of features, facilities and sound character -- some having more 'character' than others! Personally, I'd go for the Prism if someone else is bank-rolling it! ;-)

I’m paying for it myself so I can’t really afford the Prism either unless I find a great used deal. But I’ll know to jump on it if one does! :) The others are more likely to come up used at a price I can afford and I was curious how they’d compare to something at the next price level.

Hugh Robjohns wrote:You say '...have to run external equipment very low'? This is utter madness. :-( If you are concerned that the analogue levels are too low in comparison to your digital levels, you need to use a conversion alignment with a LOWER clipping level, not higher. Try switching to the +4dBu mode on the RME's analogue input and output settings instead.
H

I think I didn’t explain myself clearly enough in the original post regarding the +19 dBu and +24 dBu issue, so let me try again. According to the manual, the DDA mixer’s “output capability” is +26 dBV (which I believe is the same as +28 dBu – correct me if I’m wrong). Since the FF800 maxes out at +19dBu, I have to run the mixer level low when recording in order to maintain adequate headroom in the digital domain and avoid clipping.

Here’s an example that I did today to illustrate it:
I recorded a track off an LP so I’d have a repeatable source.
Turntable => Phono preamp => DDA channel returns (unbalanced) => FF800
With Lo Gain In and High Gain Out settings on the FF800, I set the levels on the mixer to leave “sensible” headroom and got the following levels:
Cubase meters average around -20 dBFS
Cubase meters peak = -13.7 dBFS
DDA VU meters ranged from -10 to -15 dBu

My concern here is that the DDA VU meters are near the bottom of their range and I suspect that the mixer may be operating a little below its optimum range. Are the meters unusable at this level? Is the sound terrible? No and definitely no. But if I can address an apparent mismatch when upgrading converters, then why wouldn’t I?

I then recorded the LP again, changing the FF800 settings to +4dBu as you suggested but leaving everything else exactly where it was. I got the following levels:
Cubase meters average around -14 dBFS
Cubase meters peak = -7.9 dBFS
DDA VU meters ranged from -10 to -15 dBu

This makes things worse since I’m now closer to clipping in the digital side but the analogue mixer levels are no higher than before. Please let me know if there’s something that I’m still misunderstanding…

Thx,

Doug
DDDDDoug
New here
Posts: 8
Joined: Mon Jul 29, 2002 12:00 am

Re: Advice on Higher End AD/DA Converter/Interface

Postby DDDDDoug » Wed Apr 26, 2017 2:54 am

mjfe2 wrote:How about this? http://www.ebay.co.uk/itm/152407290893? ... EBIDX%3AIT

That said, apart from a slightly better SNR I don't think you can go wrong with the Fireface's converters, even nowadays. I'm impressed at how many of classical label BIS's newer releases are still recorded on their old Octamic D's, which came out around the same time as the FF800.

Thanks, I'll add that to my list. That's a great comment about the Fireface, too - it's good to know that they still hold up well today.
DDDDDoug
New here
Posts: 8
Joined: Mon Jul 29, 2002 12:00 am

Re: Advice on Higher End AD/DA Converter/Interface

Postby DDDDDoug » Wed Apr 26, 2017 3:02 am

Mixedup wrote:But if you were to want to upgrade, how many channels do you require?
16, as stated in the initial post. ;)

Mixedup wrote:Note that the FF800 has 16 channels (at 44.1/48 kHz; 8 at 88.2/96 kHz; 4 at 192kHz) of ADAT and 2 of AES or S/PDIF digital audio, so you could consider standalone converters and keep using your FF800 and its software.
Thx.
DDDDDoug
New here
Posts: 8
Joined: Mon Jul 29, 2002 12:00 am

Re: Advice on Higher End AD/DA Converter/Interface

Postby Hugh Robjohns » Wed Apr 26, 2017 9:00 am

DDDDDoug wrote:I’m paying for it myself so I can’t really afford the Prism either unless I find a great used deal.

;-) I know that problem, too -- but you did list it! :D

According to the manual, the DDA mixer’s “output capability” is +26 dBV (which I believe is the same as +28 dBu – correct me if I’m wrong).

Yes, you are correct. It is specified with a generous headroom margin befitting its live sound intentions... but that doesn't mean you have to use all of it; It just means it isn't going to restrict headroom in the overall system.

By default it is designed to operate with a reference level of +4dBu (= 0VU), but the versions fitted with PPMs were aligned for a reference level of 0dBu (=PPM4) -- this being the appropriate calibration alignment for use with EBU converters.

Since the FF800 maxes out at +19dBu, I have to run the mixer level low when recording in order to maintain adequate headroom in the digital domain and avoid clipping.

Well... yes, technically you should be operating with a reference level of 0dBu, so 4dB lower than the stock design. And in that case you'll have 19dB of digital headroom instead of 20dB, which isn't something I'd lose any sleep over. You'd also have 4dB less signal-noise ratio in the console, but again, not something I'd worry about in a project studio situation.

Here’s an example that I did today to illustrate it:
Cubase meters average around -20 dBFS
Cubase meters peak = -13.7 dBFS
DDA VU meters ranged from -10 to -15 dBu

In general the Cubase levels look about right which suggests the levels through the desk are about right. The desk VU meters sound a little low, though... but you really can't judge levels very well with programme material on VU meters.

Steady state test tones are far more revealing. The meter sensitivity in the DDA is also easily adjustable (the process is described in the manual).

My concern here is that the DDA VU meters are near the bottom of their range and I suspect that the mixer may be operating a little below its optimum range.

Yes, I can understand your concern, but the raw analogue signal levels as indicated by your cubase meters sound pretty much spot on.

Are the meters unusable at this level? Is the sound terrible? No and definitely no.

..which also suggests the desk meter calibration might be in error, rather than the actual signal levels through the desk.

But if I can address an apparent mismatch when upgrading converters, then why wouldn’t I?

Switching to a converter that requires +24dBu for 0dBFS instead of +19dBFS will require you to push the analogue side 5dB harder for the same digital-side levels.. so I get your thinking... but I'd investigate the meter alignment in the DDA first because from your description that sounds the most likely problem to me.

Quick and easy calibration test: With the RME set to the +hi/Lo gain modes so it expects +19dBu for 0dBFS, use the desk's line-up tone or another source of clean sine-wave, and adjust the desk's main stereo output level to read an RMS average level of -15dBFS on each channel in cubase.

At that point, the desk's master VU meters should read 0VU, which is the original factory calibration for a desk with VU meters. I fear they might have been tweaked lower than that...

It's quite possible that a previous user adjusted the meters deliberately to drive the desk harder for effect, or because they didn't want headroom in the digital side!

So check the desk meter alignment... What I'd then do -- if you are happy to stick with the RME converters -- is adjust the desk output level so that the Cubase meters read an RMS average of -19dBFS, and then tweak the desk's meter drivers to raise the VU meters (which should now be showing -4VU) back to 0VU. That would be the desk's factory PPM alignment, and while you wouldn't be driving it any harder the meter readings wouldn't worry you so much. ;-)

H
User avatar
Hugh Robjohns
Moderator
Posts: 21952
Joined: Fri Jul 25, 2003 12:00 am
Location: Worcestershire, UK
Technical Editor, Sound On Sound

Re: Advice on Higher End AD/DA Converter/Interface

Postby mjfe2 » Wed Apr 26, 2017 9:12 am

Hugh Robjohns wrote:
DDDDDoug wrote:
Since the FF800 maxes out at +19dBu, I have to run the mixer level low when recording in order to maintain adequate headroom in the digital domain and avoid clipping.

Well... yes, technically you should be operating with a reference level of 0dBu, so 4dB lower than the stock design. And in that case you'll have 19dB of digital headroom instead of 20dB, which isn't something I'd lose any sleep over. You'd also have 4dB less signal-noise ratio in the console, but again, not something I'd worry about in a project studio situation.

As a side note, the FF800 offers more headroom on the front line inputs, which you try A/B testing if you're worried about SNR. Certainly worth a try before splashing out on new converters!
User avatar
mjfe2
Frequent Poster
Posts: 609
Joined: Sun Oct 11, 2009 12:00 am
Location: London, UK

Re: Advice on Higher End AD/DA Converter/Interface

Postby James Perrett » Wed Apr 26, 2017 10:55 am

Hugh mentions the most likely problem here - your desk meters are going to react more slowly than the peak meters in your software. It is also worth re-iterating his advice to use the desk oscillator to line up levels as the meters will be most accurate with continuous sine waves.

Even LED bargraph meters on some desks will be artificially damped in order to mimic the behaviour of analogue meters - presumably engineers at the time preferred this behaviour.
User avatar
James Perrett
Moderator
Posts: 7606
Joined: Mon Sep 10, 2001 12:00 am
Location: The wilds of Hampshire
JRP Music - Audio Mastering and Restoration. JRP Music Facebook Page

Re: Advice on Higher End AD/DA Converter/Interface

Postby Mixedup » Thu Apr 27, 2017 12:52 pm

DDDDDoug wrote:
Mixedup wrote:But if you were to want to upgrade, how many channels do you require?
16, as stated in the initial post. ;)

Sorry - completely skimmed past that :headbang:

So... note that many audio interfaces with ADAT ports can be configured as standalone converters, meaning you might not need to sacrifice your Fireface in order to use their preamps/converters. That makes the number of possible candidates rather longer. That said, I'd personally still hang on to the Fireface as it doesn't sound like it's doing anything wrong in your setup :)
User avatar
Mixedup
Jedi Poster
Posts: 4217
Joined: Wed Sep 03, 2003 12:00 am
Location: Laputa

Re: Advice on Higher End AD/DA Converter/Interface

Postby DDDDDoug » Mon May 01, 2017 5:45 am

Hugh Robjohns wrote:Quick and easy calibration test: With the RME set to the +hi/Lo gain modes so it expects +19dBu for 0dBFS, use the desk's line-up tone or another source of clean sine-wave, and adjust the desk's main stereo output level to read an RMS average level of -15dBFS on each channel in cubase.

At that point, the desk's master VU meters should read 0VU, which is the original factory calibration for a desk with VU meters. I fear they might have been tweaked lower than that...

It's quite possible that a previous user adjusted the meters deliberately to drive the desk harder for effect, or because they didn't want headroom in the digital side!

So check the desk meter alignment... What I'd then do -- if you are happy to stick with the RME converters -- is adjust the desk output level so that the Cubase meters read an RMS average of -19dBFS, and then tweak the desk's meter drivers to raise the VU meters (which should now be showing -4VU) back to 0VU. That would be the desk's factory PPM alignment, and while you wouldn't be driving it any harder the meter readings wouldn't worry you so much. ;-)
H

I ran the test you suggested using the DDA’s 1 kHz tone generator and got these results:

-15.0 dBFS in Cubase => +0.9 VU (left) and +1.0 VU (right)
-19.0 dBFS in Cubase => -3.9 VU (left) and -3.4 VU (right)

So the mixer’s meters seem to be calibrated pretty close to where they should be, with an offset of up to 1 dB. Maybe I’ve just been too cautious with my levels in Cubase and need to interpret it’s meters a little differently than I have been… :oops: I may try that for a little while and then tweak the meters if it still doesn’t feel right (or if I feel braver about opening the mixer up :? ).

FYI, since the offset wasn’t constant, I ran a few additional tests. Here are all the results:

Cubase (dBFS) / Left DDA (VU) / Right DDA (VU)
-13.2_______________+3.0___________+3.0
-14.0_______________+2.0___________+2.0
-15.0_______________+0.9___________+1.0
-17.0_______________-1.5___________-1.2
-19.0_______________-3.9___________-3.4
-24.0_______________-9.8___________-9.5
-26.0_______________-12.5__________-12.0
-28.0_______________-15____________-15

Thanks for all the advice, Hugh!
DDDDDoug
New here
Posts: 8
Joined: Mon Jul 29, 2002 12:00 am

Re: Advice on Higher End AD/DA Converter/Interface

Postby Hugh Robjohns » Mon May 01, 2017 9:48 am

Hmmm... From what you had been describing I really was expecting the desk meter alignment to be well off... but it seems they are more or less where they should be.

Metering is a nightmare because there are so many different 'standard' metering systems and they all give such radically different results with different signals.

Rule 1: NEVER compare meter readings with anything other than steady sine-wave tones!

Rule 2: In almost all cases, meters are only a visual aid to help keep signals in the right ball-park.

They are only there to confirm what your ears are telling you -- and it is what your ears tell you that is the only really important thing. So if what comes from your desk sounds good to you -- as in, it isn't distorted and it isn't noisy -- then forget the desk meters and just get on with it!

That's a large part of the reason for having headroom too, of course -- a working space to accommodate fast transients that analogue (and a lot of digital) meters just can't show. It's there specifically so that you don't have to worry too much about the meters.

:-D

H
User avatar
Hugh Robjohns
Moderator
Posts: 21952
Joined: Fri Jul 25, 2003 12:00 am
Location: Worcestershire, UK
Technical Editor, Sound On Sound

Re: Advice on Higher End AD/DA Converter/Interface

Postby Loueasy » Sun Dec 02, 2018 5:22 pm

It is very confusing to me. I have a new Rme Fireface UFX II and it is really my understanding that in low/high gain settings have 15db of headroom not 19.

When I try and study and find information on calibrating different components within a hybrid setup, I get so much different information it is hard to get my head around it. I have spent a lot of time and research on this topic and still have quit a bit of confusion. It may be that I am over complicating things.

It still goes back to the 15db of headroom. I would like to add a Grace Designs M108 via ADAT and possibly a Dangerous Music Convert 8 via ADAT. Considering the grace is +24 and alignments can be set to either -14 or -20 dbfs and the Convert can be aligned to with -14, -16 or -18 it gets confusing to me how all this would be calibrated with the Fireface. There will be times that I would use the pre and line in’s in the Fireface along with the Grace for inputs

Anyway, I remain confused.
Loueasy
Posts: 1
Joined: Tue Dec 13, 2016 5:38 am

Re: Advice on Higher End AD/DA Converter/Interface

Postby James Perrett » Sun Dec 02, 2018 9:29 pm

Loueasy wrote:Anyway, I remain confused.

It might help if you tell us what you are planning on feeding into the various analogue inputs. If it is something like a multi-track tape machine then there is a good argument for having all the input levels set to the same reference but if you are using a collection of microphones or keyboards then it is less important to have everything set to the same reference as it is best to match the gain to the source level in this situation. Just make sure that the highest output of each source is less than the maximum input level of the input you are feeding.
User avatar
James Perrett
Moderator
Posts: 7606
Joined: Mon Sep 10, 2001 12:00 am
Location: The wilds of Hampshire
JRP Music - Audio Mastering and Restoration. JRP Music Facebook Page

Re: Advice on Higher End AD/DA Converter/Interface

Postby Hugh Robjohns » Mon Dec 03, 2018 1:19 pm

Loueasy wrote:It is very confusing to me.

You're not alone in that! And part of the problem is understanding what is meant by the term 'headroom'...

Headroom is the space between the defined maximum working level of the system and the level to which normal programme peaks are allow to reach -- something which you or the manufacturer decides.

headroom.png


In the case of analogue equipment, the nominal 'alignment' or 'reference' level -- which is usually where the average level of normal signals sits -- is well defined. In European broadcast markets it is 0dBu while in professional recording studios it is +4dBu (in most cases). This alignment level is also shown on as a reference mark on metering systems (eg. PPM4, 0VU, Test, etc).

In an analogue system, normal programme peaks are allowed to rise 8dB (in the UK) or 9dB (in Europe) above that reference or alignment level -- so typically, +8dBu for UK broadcast, or +13dBu for a recording studio (and on meters it would be PPM6 or off the red end of the scale on a VU! :D ).

So normal programme peaks would be expected to fall somewhere in the yellow region of that virtual meter above. The average level would be expected to sit around the transition between green and yellow, but obviously the quieter bits will fall down into the green area. Very loud uncontrolled transients might reach up into the read -- and the headroom is there to accommodate them without overload distortion.

However, the 'maximum working level' in analogue equipment is a bit vague as the amount of distortion increases rapidly as the level increases, long before it actually clips. In the days of analogue tape machines the nominal peak level was generally chosen to keep distortion at or below 3% THD.

In practice, though, most analogue audio electronics will cope with much higher signal levels, albeit with rising distortion before the system finally clips. High-end analogue equipment like mixing consoles and outboard gear will usually cope with +24dBu before clipping (and some may go even higher), so +24dBu is generally taken as the maximum practical pro-audio level.

So with analogue systems, the manufacturers specify a nominal alignment level, and the user is expected to allow signal peaks no higher than 8 or 9dB above that... knowing that larger transient peaks can still be accommodated but possibly with some distortion. Some users will want to avoid that increasing distortion, while others might actively make use of it for musical colour...

In digital equipment, the situation is very different. As the signal level rises the amount of distortion remains extremely low right up to the point where the converter runs out of quantising levels, and then it clips horribly!

So the maximum allowable level is clearly defined here (0dBFS)... but sadly there is no built-in 'alignment level' like there is in the analogue world -- that's (unhelpfully) left entirely to the user's own preferences.

Having said that, the SMPTE (in America) recommends an equivalent alignment level of -20dBFS, while the EBU (Europe) requires -18dBFS... and mastering engineers working on pop music might choose -15, or -12dBFS, or something else...

So that's 'headroom'. Where this becomes important (and confusing) is in the analogue-digital calibration -- how we translate between the analogue and digital domains.

In general, it makes sense to choose a calibration that provides similar headroom margins for both the analogue and digital domains, so that neither one runs out of space prematurely and thus becomes the limiting factor when moving signals between the two domains.

However, that's not always practical, depending on the equipment you're working with. For example, semi-pro devices may well not have the same peak-level capacity as high-end professional equipment, and some European equipment designed to the EBU spec will not be fully compatible with American equipment designed to the SMPTE spec.

I have a new Rme Fireface UFX II and it is really my understanding that in low/high gain settings have 15db of headroom not 19.

RME products usually offer several selectable calibration options, and on the UFX2 they are labelled as 'LoGain' and '+4dBu' on the line inputs, with the corresponding (if slightly confusing) 'HiGain' and '+4dBu' options on the line outputs. A third output alignment is labelled '-10dBV'.

Ignoring the -10dBV option for the moment, both of the calibrations (lo/hi and +4) assume a nominal alignment or reference level on the analogue side of +4dBu. The difference between them is that the lo/hi option is configured to reach maximum digital level (0dBFS) at +19dBu, while the +4 options does so at +13dBu.

So, relative to the analogue alignment level of +4dBu, the lo/hi mode has 15dB of digital headroom (19-4 dB), while the +4 mode has 9dB of digital headroom (13-4 dB). The manual is correct in that!

Just for completeness, the -10dBV mode provides 12dB of digital headroom, but its analogue calibration is designed to match the semi-pro standard alignment level of -10dBV rather than +4dBu.

It might make more sense, and help to put all this in context, to look at a graph:

Digital relative Levels Chart.png


So here I've shown the professional analogue world on the left, with the 'alignment level' at +4dBu on the border between the green/yellow sections of a virtual meter. I've shown the the nominal working range (normal signal peaks) as a 9dB window in yellow (to match RME's specifications), and the remaining headroom in red above that.

As you can see, the SMPTE calibration exactly matches the nominal professional analogue world.

For comparison, I've also shown the EBU calibration which uses an alignment level of 0dBu and a peak level of +18dBu. It obviously has 6dB less headroom than the SMPTE standard, and that's because it is expecting 'controlled' broadcast-ready audio where someone has already mixed and managed the sound levels, so there should be no uncontrolled transients, and less need for a large headroom margin.

RME's lo/hi gain calibration sets a maximum peak level slightly (1dB) above the EBU requirement but it can't go as high as the SMPTE standard. In practice, this is rarely a problem as only the most enormous uncontrolled transients would normally be affected... but it does mean that you may need to be a little cautious when setting levels in high-end analogue equipment.

The RME +4 option reduces the analogue peak level even further, and is really intended for use with highly-compressed and commercially mastered audio where there is no need for a large headroom margin at all.

I would like to add a Grace Designs M108 via ADAT and possibly a Dangerous Music Convert 8 via ADAT. Considering the grace is +24 and alignments can be set to either -14 or -20 dbfs and the Convert can be aligned to with -14, -16 or -18 it gets confusing to me how all this would be calibrated with the Fireface.

So you're connecting some outboard preamps/converters via a digital interface, which means the RME's own internal converter calibrations are irrelevant. They only matter if you're passing analogue signals into or out of the RME directly.

So if you were taking the analogue line output of the Grace into the RME you'd need to think about the RME's calibration... but you're not as the conversion is all happening inside the Grace or Dangerous boxes.

However, if you might be connecting an analogue line source to either the Grace/Dangerous, or the RME's analogue inputs, AND you want the meters on those devices to give broadly the same kind of level indications, then you should choose similar calibrations.

In Hi/Lo mode the RME has a nominal 15dB range between alignment level and the maximum (0dBFS) level. So the closest option to that in the Grace is the -14 meter mode, and I'd go with the same setting in the Dangerous box.

Hope that helps!

H
User avatar
Hugh Robjohns
Moderator
Posts: 21952
Joined: Fri Jul 25, 2003 12:00 am
Location: Worcestershire, UK
Technical Editor, Sound On Sound

Re: Advice on Higher End AD/DA Converter/Interface

Postby Martin Walker » Mon Dec 03, 2018 2:35 pm

Is that Volume 1 Issue 1 of the monthly Hugh Robjohns magazine?

If so, please put me down for a subscription :clap:


Martin
User avatar
Martin Walker
Moderator
Posts: 12824
Joined: Wed Jan 13, 2010 9:44 am
Location: Cornwall, UK


Who is online

Users browsing this forum: No registered users