differences in relative level betwixt various AVB interfaces

Moderator: James Steele

Forum rules
Discussion related to installation, configuration and use of MOTU hardware such as MIDI interfaces, audio interfaces, etc. for Mac OSX
Post Reply
EMRR
Posts: 1079
Joined: Sat Jan 07, 2006 11:17 am
Primary DAW OS: MacOS
Contact:

differences in relative level betwixt various AVB interfaces

Post by EMRR »

I think I've addressed this in some other AVB related posts, but I have some new shiny pictures to share. Boring technical words ahead, beware.

The short version is that a 16A and a Monitor 8 do not have equivalent loopback (DA patched to AD) levels, so you cannot reliably move an analog mix from one device to another without skewing the mix.

I've said elsewhere, MOTU could and should add an AVB interface utility to trim levels exactly, as they only give you 1dB steps, and those 1dB steps do not correlate from one interface to another, so precisely matched levels are impossible. As well, that utility should allow for unity gain in a loopback situation.

16A:
Spec says +20dBu max output, +24dBu max input.
In reality measurements for a particular channel showed FS output will clip the input at +2 input setting, but not at +1, and the decimal point gain difference that can not be trimmed exactly was +0.26dB. Those are RMS readings.

Monitor 8:
Spec says +22dBu max output, +24dBu max input. On paper, already a 2dB difference. One would assume a 2dB output trim difference to match a 16A.
In reality measurements show that line outs 1-8, the Main out, and the Aux out all have differing relative output levels, with 1-8 being within about 0.03dB of each other, Main being 0.1dB lower, and Aux being yet another 0.1dB lower. Those are RMS readings.

On to the graphics, which are DP's peak reading meters, which appear to have a max resolution of 0.05dB, so things could be slightly better or worse than metered. Right side shows internal tone level of DP. Left side shows returning levels at input, which would include the combination of analog output and input gain errors. 16A output channels (1-16) are trimmed '-4dB', Monitor 8 output channels (17-24) are trimmed '-9dB'. On paper there should be a 2dB difference between the two interfaces. The closest the two can be trimmed to each other is roughly 0.4dB. The Main and Aux channels were not included in these loops and are not shown.

In contrast, the older 2408mkIII and 24i/o delivered much tighter unity gain performance, with lower measured differences. They sure don't sound as good as the AVB stuff though, you gain something, you lose something.

We see that a full analog mix will be affected by any change in interface sequence, or even output/input assignment within a single interface. It would be most peoples' assumption that this would not be a place to look for gain errors, and we also have to acknowledge the principal assumption would be that gain errors would be found primarily in any analog outboard equipment. Even a stereo mix sent to analog mix/mastering hardware can be altered upon recall if the hardware assignments change, with possible stereo skewing or overall level shifts needing to be accounted for.

Click this link for full size
Image

Click this link for full size
Image
Last edited by EMRR on Mon Jan 16, 2017 11:33 am, edited 1 time in total.
Doug Williams
Electromagnetic Radiation Recorders
The Martha Bassett Show broadcast mixer
Tape Op issue 73

DP 11.31
Studio M1 Max OS12.7.3
MOTU 16A and Monitor 8
M1 Pro MBP for remotes and editing
avsessions
Posts: 3
Joined: Sun Jan 15, 2017 5:16 am
Primary DAW OS: MacOS

Re: differences in relative level betwixt various AVB interf

Post by avsessions »

I'm interested in this topic because the issue I'm having is that i cant calibrate my Audio Express interface to my standard -16dbfs=0VU studio cal. When I send out a 1k tone out of it I'm -1.5dbVU down and cant trim up any more.
EMRR
Posts: 1079
Joined: Sat Jan 07, 2006 11:17 am
Primary DAW OS: MacOS
Contact:

Re: differences in relative level betwixt various AVB interf

Post by EMRR »

I haven't encountered an Audio Express unit to say for sure, but it looks on paper like they trim gain in a similar manner to the 16A and Monitor 8. Neither of those devices are capable of unity loopbacks, which throws a kink into anyone using them for insert purposes at very least. At a glance neither the website nor the manual has a tech page to reveal max input/output specs, which is, well, lame. That may or may not be a clue as to why you can't get closer than 1dB, which you at least can with the AVB line.
Doug Williams
Electromagnetic Radiation Recorders
The Martha Bassett Show broadcast mixer
Tape Op issue 73

DP 11.31
Studio M1 Max OS12.7.3
MOTU 16A and Monitor 8
M1 Pro MBP for remotes and editing
Post Reply