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Converter headroom considerations Digital Converters
Old 2nd October 2018
  #1
Gear Head
 

Converter headroom considerations

Hi,

I was looking at the RND Shelford channel and see that it can handle line input and output signals up to 26.5 dBU but that it has a second output which "lowers headroom to +19dBu to accommodate driving the full output range of the channel when running into a device with lower max input levels." (from the manual).

From what I can tell, it seems like other high end analog gear also typically has max output levels in the mid or even high 20dBU's.

I have an RME UFX which can handle up to +19dBU. Thus, relative to +4dBU, there is 15dBU of headroom before hitting 0dBFS. Lynx Aurora converters appear to handle up to 20dBU thus providing 16dBU of headroom. An RME ADI-8 QS offers 20dbU of headroom (max 24dBU) as do the new Apollo X interfaces.

It seems to me that in theory, you want converters with as much headroom as possible, at least if you plan to work in the analog realm (e.g. either by using analog gear on inserts when mixing and/or doing analog summing). However, this is where I get a little confused. If one is operating at +4dBU line level, then is there truly a benefit to having converters with 20dB of additional headroom vs. let's say 16dB? Is this only beneficial if you plan to drive the analog gear harder, or if you have music with wider dynamic range (e.g. large transients)? I'd love to hear from folks who have experience and who understand this better than I do.

Is there a standard that is used by professional mixers (e.g. converters with let's say 20dB of headroom above +4dB)?

Thoughts?
Old 3rd October 2018
  #2
Back in 16-bit days, we used to worry a lot about "wasting bits", so nominal levels and headroom were carefully considered when commissioning a new converter set. Now, with 24-bit conversion the norm, we can easily spare a couple of bits for headroom -- except when we're trying to slam an input for effect. Some converters (e.g. Lavry) are designed with euphonic limiting characteristics, while others don't like being overloaded at all.

If you're equipping a mastering room, then you're likely to have a number of analog processors that you want to run in their "sweet spot". (This could vary from unit to unit, and also depend on the taste of the mastering engineer.) Modern outboard gear tends to be designed with lots of analog headroom, but the same is not true of every vintage piece one might acquire. And there are even some recent designs (e.g. Manley Massive Passive) that have lower-than usual peak level capabilities.

It's easy to get into trouble if you need to interface with semi-pro equipment. I once brought a broadcast-quality converter set made by Benchmark Media into a room based around a Mackie console. I could not drive the ADC input anywhere near full-scale without the Mackie exhibiting audible distress. I had to pop the cover on the Benchmark and move a couple of jumpers before we could mix safely.

Bottom line: It's good to have choices!
Old 3rd October 2018
  #3
Lives for gear
The SMPTE standard is 20 dB headroom above +4 dBu, therefore +24 dBu max. This is why just about every serious piece of gear and LFAC is +24 to +28 dBu.

The reason you find gear that’s lower is pure cost.

That said, headroom is an arbitrary concept. If you want 20 dB headroom in a +20 dBu piece of gear use 0 dBu as your reference.

If you want to have the most dynamic range without limiting, then 20 dB is a good choice for an A/D as it accommodates the crest factor of most musical sources when the average level shows 0 VU.

There’s an argument to use a lower max output level D/A into a console: consoles were designed around tape which maxed out at around +14 dBu. Therefore if you have all your D/A’s at +24 dBu then you’ve just eaten up all the headroom in your mixer.

Lastly, something to ponder: many people will talk about ‘how much headroom’ a piece of gear has. But most gear has the exact same headroom because they are built around the exact same aforementioned standard. There’s no such thing as a +48 dBu output console... The takeaway: many don’t know what headroom is. Wading into headroom discussions here can be fraught!

Last edited by brew; 4th October 2018 at 10:53 PM..
Old 3rd October 2018
  #4
+-15 volt supplies will give you +21 dbu single ended and +27 dbu differential. 18 volt rails might get you an extra db. Pro converters clip at +20 dbu. Analog tape saturates at +13 dbu.
Old 4th October 2018
  #5
Gear Head
 

Guys - thanks so much for taking the time to offer your insights, I appreciate it!
Old 5th October 2018
  #6
Lives for gear
 
cheu78's Avatar
Quote:
Originally Posted by Jim Williams View Post
+-15 volt supplies will give you +21 dbu single ended and +27 dbu differential. 18 volt rails might get you an extra db. Pro converters clip at +20 dbu. Analog tape saturates at +13 dbu.
I do believe it would be also important to say that converter clipping (at any value) sounds like sh*t, tape saturate in a much much different and musical way.. if this is wanted or not is another story, but it's an apple-orange comparison imho.



Cheu
Old 5th October 2018
  #7
Modern "rail to rail" opamps will also get you an extra db or so output for the same power rails. If you use those in a + - 5 volt supply they won't clip before the ADC does.
Old 6th October 2018
  #8
Gear Addict
 
GIACOMO-_'s Avatar
 

Sony DASH digital recorder clip at 19dbu, (like Alesis HD24 XR), with it the most beautiful recordings were made.
Old 7th October 2018
  #9
Lives for gear
 

Sony PCM headroom

Quote:
Originally Posted by GIACOMO-_ View Post
Sony DASH digital recorder clip at 19dbu
PCM-3324: +24 dBm
Attached Thumbnails
Converter headroom considerations-3324.jpg  
Old 7th October 2018
  #10
Gear Addict
 
GIACOMO-_'s Avatar
 

Quote:
Originally Posted by cathode View Post
PCM-3324: +24 dBm
http://www.broadcaststore.com/pdf/mo...8/PCM-3348.pdf
Old 7th October 2018
  #11
Lives for gear
 

3324 specs

Quote:
Originally Posted by GIACOMO-_ View Post
Sony DASH digital recorder clip at 19dbu
2 cue channels.
Old 7th October 2018
  #12
Gear Addict
 
GIACOMO-_'s Avatar
 

Quote:
Originally Posted by cathode View Post
2 cue channels.
You are right! Indeed I was surprised.
+24dbu.

Are there converters that sound like Sony dash machine? Perhaps the old Apogee?
If I remember correctly it was built in collaboration with Apogee.
Old 7th October 2018
  #13
Lives for gear
 

Sony PCM

Quote:
Originally Posted by GIACOMO-_ View Post
Are there converters that sound like Sony dash machine? Perhaps the old Apogee?
If I remember correctly it was built in collaboration with Apogee.
the early Sony digital multi-tracks had SAR or dual slope converters, whatever, they both required analog anti-aliasing and reconstruction filters.
Sony used GIC filters, (a topology, not a brand), made by Soshin.
you can read about the design here:
http://www.ti.com/lit/an/sbaa001/sbaa001.pdf

Bruce Jackson, the founder of Apogee developed his own GIC filter which caught on with all the techie types. The demand was so strong that both Sony and Mitsubishi were installing those filters on new machines before delivery to the customers.
Apogee would show off large glass jars full of extracted Soshin and Murata filters (Mitsubishi 32 track) at AES shows.
Sony DASH machines had their own Sony converters,
modified machines entailed filters, only.
That is the extent of any Apogee/Sony collaboration, to the best of my knowledge.

In my humble opinion, the A2D converter, with original Murata filters in the Mitsubishi 32 track was superior to the Sony DASH.
Murata filters were preferred in the Sony PCM 1610 and 1630 2 track digital setup.

Probably not what you wanted to hear.
Old 7th October 2018
  #14
I measured well over 2500 degrees of phase shift from the Mitsubishi. That's a lot of group delay.
Old 9th October 2018
  #15
Gear Head
 

As I was thinking about some of the early replies to my original post while playing with the various settings on my RME UFX, I realized that for me, and with the gear I have, it probably makes sense to have the AD of the UFX set to +19dBU (15dB headroom, "Lo Gain" setting) so that I have the most headroom the converter offers.

However, I am not so sure if I should always use the same +19dBU ("Hi Gain") setting on the UFX DA. At this setting, the levels going into external hardware are hotter and and thus I won't be able to crank the outputs as much on the external hardware as if the DA is set to say +13dBU ("+4dbU" setting on UFX). I'll need to play around with this some more to compare how things sound and also any potential noise differences.

I guess I wonder, though, do most of you set the AD and DA with the same reference levels and leave those settings alone, or do you frequently change either AD or DA reference levels depending on what you're working on?

Just curious, thanks!
Old 11th October 2018
  #16
Gear Maniac
 

Quote:
Originally Posted by GIACOMO-_ View Post
Sony DASH digital recorder clip at 19dbu, (like Alesis HD24 XR), with it the most beautiful recordings were made.
I agree many great sounding records were made with Sony DASH machines.
Old 11th October 2018
  #17
Lives for gear
Quote:
Originally Posted by Snapchat2314715 View Post
I agree many great sounding records were made with Sony DASH machines.
But is it the DASH, or countless other factors such as the lack of multiband EQ and dynamics, brickwall limiters, an apprentice model of expertise-building...
Old 13th October 2018
  #18
Gear Maniac
 

It's probably just the music. The other factors as you mentioned. Good music sounds good whether it's recorded on a dash machine, PT, or tape.
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