[LAU] interfaces with electronic gain controls

[Chris Caudle]

Out of all the interfaces mentioned recently as working well under linux,
all I have looked up seem to still use potentiometers to set gain. That
is fine for recording mono instruments or vocals, but for stereo or
surround acoustic recording I would really like an interface which allows
for very close gain matching between channels, or at the very least very
repeatable gain settings so that I can correct in post production if
needed.
Are there any interfaces which are commonly available and have either
accurate electronic gain setting, or stepped/switched resistors for gain
setting?
The new MOTU devices which are class compliant seem to, starting at around
US$ 1200-1500.
Is that just the price range I have to move to before that feature is
included? I thought Cymatic Audio was going to have a 4 mic amp/2 line
out device for around US$ 500, but it was cancelled before release. That
is closer to the style of device I am looking for.

The MOTU devices without network audio (USB only) seem to have the feature
set and lower price, but has anyone checked whether they can be used
without a proprietary control application? The nice feature of the AVB
interfaces for linux use is that they can be controlled from a web
browser, so no proprietary application needed to set gain and routing from
a linux machine.
The webpage for one of the USB interfaces mentions a template for TouchOSC
which can run on an iPad, but I don't know how iPad software sends OSC
commands. If there is a standard for sending OSC control messages to USB
audio interfaces perhaps it could be adapted to linux. Any of the OSC
users familiar with whether that would be some kind of proprietary
interface, or whether it should be possible to interface one of the linux
OSC applications to the MOTU interface?

[David Kastrup]

Post by Chris Caudle
Out of all the interfaces mentioned recently as working well under linux,
all I have looked up seem to still use potentiometers to set gain.

Its more the preamps that use pots. The interfaces as such tend to have
fixed levels.

Post by Chris Caudle
That is fine for recording mono instruments or vocals, but for stereo
or surround acoustic recording I would really like an interface which
allows for very close gain matching between channels, or at the very
least very repeatable gain settings so that I can correct in post
production if needed.

If the controls are not particularly fiddly, I'd be surprised if they
make for a lot of difference.

Post by Chris Caudle
Are there any interfaces which are commonly available and have either
accurate electronic gain setting, or stepped/switched resistors for gain
setting?

Use a mixer that has some stereo channels? Those tend to have single
gain and (though that's post recording-out anyway) a balance pot with a
center notch.

My Mackie Onyx 1620 (not the one I offered for sale, that's a 1220) has
8 mono inputs (with mic preamp) and 4 stereo inputs (line level).

It's cheaper and more compact than the model with 16 mono inputs (1640).

[Len Ovens]

Post by Chris Caudle
Out of all the interfaces mentioned recently as working well under linux,
all I have looked up seem to still use potentiometers to set gain. That
is fine for recording mono instruments or vocals, but for stereo or
surround acoustic recording I would really like an interface which allows
for very close gain matching between channels, or at the very least very
repeatable gain settings so that I can correct in post production if
needed.

Two things:
look for a switch that controls two channels from one pot. This might be
called dual, link or stereo. I do not know how close this puts the two
channels but even my cheap Art USBDualTubePre has that.

If the device doesn't even have that... set both channels full up as that
is the one place they _should_ match. A matched pad to lower levels could
be added if needed.

However, (the bad part) I suspect anything under about $1k (or higher for
larger channel counts) will have gains that track poorly in any case
because the componants from channel to channel are not matched.

You could try the rack version of the x32 mixer series (XR18), (or the
soundcraft and A&H versions) basically anything that is also meant to be a
live mixer. Some of them will already have stereo channels (at line level)
that should be matched. I should note that the XR12 and XR16 have only 2
channels on the USB, the XR18 has 18 and the x32 rack has 32. They all
seem to have network based control. If they don't have browser control, I
was able to find the OSC map for these devices in their DL files.

The price is still high and you are paying for a lot of lower quality
channels rather than fewer high quality channels.

I am not sure if your choice of USB is for laptop use or "thats what I can
find" reasons. If you can use a PCIe device, I was going to suggest a 4
channel (two stereo stream) audioscience card, but the ASI5620 is already
$1k. Audio Science has had Linux drivers forever. Diggram and Lynx not so
much but some of the cards are less pricy too. I know that there are now
some Digigram modules for Linux but you would have to check for each card
you are interested in.

You are right, though, there is not much out there. I suspect that digital
mixers are the best bet for you though. most of the smaller 4 channel USB
boxes use phyical pots for easy macos (USB2.0) compatability.

--
Len Ovens
www.ovenwerks.net