[LAU] USB audio interfaces >= 8 channels

Robin Gareus robin at gareus.org
Tue May 15 18:44:37 UTC 2012

On 05/15/2012 06:10 PM, Fons Adriaensen wrote:
> On Tue, May 15, 2012 at 12:07:36PM +0200, Robin Gareus wrote:
>> It has only balanced outputs but otherwise fits the bill.
> Usually you can use them as unbalanced. In some cases this
> requires a special cable, but that's unlikely.
>> What seems weird with with this box: it has a word-clock out but no
>> word-clock input.
> Not a real one, but it will sync to SPDIF or ADAT.

Excuse my newbie question, but I'm not experienced with SPDIF.

Can one [word-clock] sync two devices using SPDIF or ADAT and still use
the analog I/O on both? -- Is that hardware specific or is there a
standard for that?

I assume the master-device will need to send a signal via SPDIF.

I'll check later if the 1818VSL can do both: output to SPDIF and analog
at the same time. but I don't know if I can do that remotely. What I do
already know is that the clock-source for each input can be configured
individually. but I'm still waiting for Brian to connect the device. my
ssh login already works..

>> What is not clear to me is
>> if the balanced I/Os are [or can be] ground-lifted.
> Better not. Ground lifting can damage your inputs unless
> 1. the inputs are physically isolated by a transformer 
>    (not the case),   
> 2. the common mode voltage (the difference between the two
>    grounds) is small.
> Unless you have transformer isolation, you need a more or less
> common ground. And then, if the inputs are really balanced and
> you use balanced cables, there is no need to lift the ground.

Here it mainly concerns the outputs: In my case there's a ground loop
between my Screen (Asus VE278) and the active speakers when connected
via computer and an USB UA-25 (not UA25-ex which features a ground-lift
switch) -> 1/4inch TRS -> B2031A speakers.

Disconnecting either the screen or the speakers (and using headphones)
works, otherwise there's a hum on the system; luckily no sparks, no
x-runs, either :)

A multimeter shows a constant 1mA current and ~3-4mV AC potential.
(measured between the USB-shielding of the PC and the disconnected
USB-cable of the UA25) when the screen is connected.

Both the screen as well as the speakers (and computer) are connected to
the same power-stripe and share the ground.

Fun-fact: When disconnecting the screen the potential between USB and PC
goes up to ~150mV (IFF the computer is not battery powered otherwise
it's 0) but the current drops to nil.

/me is pondering to cut the ground-wire from the screen.. but I have so
far refrained from doing that.

Anyway at some point the interface may end up on stage somewhere and
ground-lifting may be required for the inputs, too. Although in that
case I'll likely have some ground-lifting DI-boxes around.


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