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Disabling M-Audio Transit USB Analog Input DC Bias

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Anonymous
November 17, 2004 2:15:03 AM

Archived from groups: rec.audio.pro (More info?)

Looking for a better audio recording interface for my Gateway (Motion
Computing) Tablet PC, I ran across the M-Audio Transit USB. It
immediately worked for playing sound, but I've been through a major
hassle with recording. It turns out the Transit is designed with 2.24
VDC of bias on each analog audio input, to power cheap PC microphones.
When connected to other audio devices, this bias voltage fights the
source you are trying to record. Depending on the source impedance, it
might even draw enough current from the Transit's tiny USB-driven
power supply to interfere with the USB signal and cause full-volume
hissing in place of music. With any audio source, I found I could have
only one end of the USB cable connected to grounded equipment. If the
audio source was grounded, the computer could not be - not even by the
supposedly isolated AC-powered battery charger.

I spent over a month eMailing M-Audio about these issues, and received
no help, only the typical runaround about drivers, SpeedStep, and the
kind of IRQ conflicts that went away with the ISA bus. I finally
started calling their phone support with the single DC bias issue, and
in only a day got them to confirm that the 2.24 V bias was by design.
In another hour I had cut the traces and solved almost all of the
problems with recording through the Transit. Suddenly there was no
more hissing, no more problem with using the battery charger while
recording, and the extreme loading of the source signal went away.

I don't guarantee that I've found the best way to accomplish removing
the DC bias, but here's how I did it. (For a photo of the area I'm
referring to, see <http://www.psychoros.org/Transit-Input.jpg&gt;.)

The output end of R4 is a 4.4V supply. R33, R31, and C36, and R34,
R32, and C37, form a pair of voltage dividers, creating two
independent filtered sources of the 2.2V bias voltage. R21 and R22
connect the bias voltages to the actual analog audio input lines, and
in the process connect C36/37 between the audio inputs and ground.
(The equivalent sized C32 and C33 connect the audio inputs to the ADC
circuits, so the loading is significant.)

You can probably see my ugly cuts in the traces to the right of R21
and R22, separating the DC bias supplies from the audio inputs, in the
photo. (Guess I need a new X-Acto blade...) Those two tiny cuts solved
the problems with my Transit and the Tablet PC, stopped the excessive
loading of the audio source, and dramatically improved the recorded
sound. With this issue fixed, I'm very pleased with the sound quality
of both recording and playback through the Transit. I can't say the
same for M-Audio's product documentation or support.

Loren
Anonymous
November 17, 2004 5:38:49 AM

Archived from groups: rec.audio.pro (More info?)

Loren Amelang <loren@pacific.net> writes:

> Looking for a better audio recording interface for my Gateway (Motion
> Computing) Tablet PC, I ran across the M-Audio Transit USB. It
> immediately worked for playing sound, but I've been through a major
> hassle with recording. It turns out the Transit is designed with 2.24
> VDC of bias on each analog audio input, to power cheap PC microphones.
> When connected to other audio devices, this bias voltage fights the
> source you are trying to record. Depending on the source impedance, it
> might even draw enough current from the Transit's tiny USB-driven
> power supply to interfere with the USB signal and cause full-volume
> hissing in place of music. With any audio source, I found I could have
> only one end of the USB cable connected to grounded equipment. If the
> audio source was grounded, the computer could not be - not even by the
> supposedly isolated AC-powered battery charger.
>
> I spent over a month eMailing M-Audio about these issues, and received
> no help, only the typical runaround about drivers, SpeedStep, and the
> kind of IRQ conflicts that went away with the ISA bus. I finally
> started calling their phone support with the single DC bias issue, and
> in only a day got them to confirm that the 2.24 V bias was by design.
> In another hour I had cut the traces and solved almost all of the
> problems with recording through the Transit. Suddenly there was no
> more hissing, no more problem with using the battery charger while
> recording, and the extreme loading of the source signal went away.
>
> I don't guarantee that I've found the best way to accomplish removing
> the DC bias, but here's how I did it. (For a photo of the area I'm
> referring to, see <http://www.psychoros.org/Transit-Input.jpg&gt;.)
>
> The output end of R4 is a 4.4V supply. R33, R31, and C36, and R34,
> R32, and C37, form a pair of voltage dividers, creating two
> independent filtered sources of the 2.2V bias voltage. R21 and R22
> connect the bias voltages to the actual analog audio input lines, and
> in the process connect C36/37 between the audio inputs and ground.
> (The equivalent sized C32 and C33 connect the audio inputs to the ADC
> circuits, so the loading is significant.)
>
> You can probably see my ugly cuts in the traces to the right of R21
> and R22, separating the DC bias supplies from the audio inputs, in the
> photo. (Guess I need a new X-Acto blade...) Those two tiny cuts solved
> the problems with my Transit and the Tablet PC, stopped the excessive
> loading of the audio source, and dramatically improved the recorded
> sound. With this issue fixed, I'm very pleased with the sound quality
> of both recording and playback through the Transit. I can't say the
> same for M-Audio's product documentation or support.
>
> Loren

Dear Loren,

Hey, while you've got that Transit apart, can you show us the rest of the
insides??? I'd like to know what ADC chips and mic preamp (opamp?) chips it
uses. I'm looking for a cheap USB input device for Laptop recording.

It says it has mic inputs, but I don't know how it senses the difference
between mic and line.

Also, are you able to control the input level via software? That is, can you
control the input level *before* ADC is done.

Thanks,
Richard
Anonymous
November 17, 2004 2:43:48 PM

Archived from groups: rec.audio.pro (More info?)

On 17 Nov 2004 02:38:49 -0500, Mannr@uwaterloo.ca wrote:

>Loren Amelang <loren@pacific.net> writes:
>
>> Looking for a better audio recording interface for my Gateway (Motion
>> Computing) Tablet PC, I ran across the M-Audio Transit USB.
....
>Dear Loren,
>
>Hey, while you've got that Transit apart, can you show us the rest of the
>insides??? I'd like to know what ADC chips and mic preamp (opamp?) chips it
>uses. I'm looking for a cheap USB input device for Laptop recording.

Decent photos and some info on playback mods are on the web at
<http://headwize.com/ubb/showpage.php?fnum=3&tid=4974&gt;

There are four chips - AKM AK4584VQ (audio), TI TAS1020A (8052 w/USB
audio streaming), NS LMV722M (amp), and uC 24LC64 (serial EEPROM).

>It says it has mic inputs, but I don't know how it senses the difference
>between mic and line.

The software includes a control panel with tray icon that selects
input gain.

>Also, are you able to control the input level via software? That is, can you
>control the input level *before* ADC is done.

In the +0/14/20/26 db steps of the control panel, yes. Beyond that you
get Windows Volume Controls. Some audio software level controls don't
seem to connect with the Transit...

>Thanks,
> Richard
Anonymous
November 17, 2004 4:27:39 PM

Archived from groups: rec.audio.pro (More info?)

Loren Amelang <loren@pacific.net> wrote:

>Looking for a better audio recording interface for my Gateway (Motion
>Computing) Tablet PC, I ran across the M-Audio Transit USB. It
>immediately worked for playing sound, ....

It's a fine low-cost S/PDIF input device for Mac and PC laptops too.
We've tested it an found it's bit-for-bit accurate.

[Disclaimer: Core Sound is an M-Audio dealer.]

--
Len Moskowitz PDAudio, Binaural Mics, Cables, DPA, M-Audio
Core Sound http://www.stealthmicrophones.com
Teaneck, New Jersey USA http://www.core-sound.com
moskowit@core-sound.com Tel: 201-801-0812, FAX: 201-801-0912
Anonymous
November 17, 2004 4:27:40 PM

Archived from groups: rec.audio.pro (More info?)

On 17 Nov 2004 13:27:39 -0500, moskowit@panix.com (Len Moskowitz)
wrote:

>
>Loren Amelang <loren@pacific.net> wrote:
>
>>Looking for a better audio recording interface for my Gateway (Motion
>>Computing) Tablet PC, I ran across the M-Audio Transit USB. It
>>immediately worked for playing sound, ....
>
>It's a fine low-cost S/PDIF input device for Mac and PC laptops too.
>We've tested it an found it's bit-for-bit accurate.
>
>[Disclaimer: Core Sound is an M-Audio dealer.]

I've seen some loopback tests that show it is bit-for-bit during
simultaneous play and record on the Toslink side.

Just don't think you are going to use it for analog monitoring while
recording the optical in - Connecting the Toslink-to-mini adapter to
the dual-purpose mini jack input appears to mechanically switch off
the analog output.

Not that it is such a great loss - there is no direct monitor ability,
you must use playthrough in software. The latency is pretty long, more
of a distant memory than an echo.

Still, for the price, the compromises are tolerable, and the sound is
great.

Loren
Anonymous
November 17, 2004 6:27:23 PM

Archived from groups: rec.audio.pro (More info?)

Loren Amelang <loren@pacific.net> writes:

> On 17 Nov 2004 13:27:39 -0500, moskowit@panix.com (Len Moskowitz)
> wrote:
>
> >
> >Loren Amelang <loren@pacific.net> wrote:
> >
> >>Looking for a better audio recording interface for my Gateway (Motion
> >>Computing) Tablet PC, I ran across the M-Audio Transit USB. It
> >>immediately worked for playing sound, ....
> >
> >It's a fine low-cost S/PDIF input device for Mac and PC laptops too.
> >We've tested it an found it's bit-for-bit accurate.
> >
> >[Disclaimer: Core Sound is an M-Audio dealer.]
>
> I've seen some loopback tests that show it is bit-for-bit during
> simultaneous play and record on the Toslink side.
>
> Just don't think you are going to use it for analog monitoring while
> recording the optical in - Connecting the Toslink-to-mini adapter to
> the dual-purpose mini jack input appears to mechanically switch off
> the analog output.
>
> Not that it is such a great loss - there is no direct monitor ability,
> you must use playthrough in software. The latency is pretty long, more
> of a distant memory than an echo.
>
> Still, for the price, the compromises are tolerable, and the sound is
> great.
>
> Loren

More questions.

Does this operate in "standalone" ADC mode? That is, if you put analog in,
can you get digital out without connecting to a computer? Or could you hack
it to do that? This could be a really cheap input device, powered from a 6 or
9V battery.

Anyway, I think I'll pick one up for my (Windows) laptop.

Richard
Anonymous
November 18, 2004 11:59:42 AM

Archived from groups: rec.audio.pro (More info?)

Loren Amelang <loren@pacific.net> wrote:

>Just don't think you are going to use it for analog monitoring while
>recording the optical in - Connecting the Toslink-to-mini adapter to
>the dual-purpose mini jack input appears to mechanically switch off
>the analog output.

You can use the laptop's internal sound card for monitoring. We've done
that here and it works just fine as long as you can stand the latency
delays.

--
Len Moskowitz PDAudio, Binaural Mics, Cables, DPA, M-Audio
Core Sound http://www.stealthmicrophones.com
Teaneck, New Jersey USA http://www.core-sound.com
moskowit@core-sound.com Tel: 201-801-0812, FAX: 201-801-0912
Anonymous
November 18, 2004 8:56:59 PM

Archived from groups: rec.audio.pro (More info?)

On 17 Nov 2004 15:27:23 -0500, Mannr@uwaterloo.ca wrote:

>> >Loren Amelang <loren@pacific.net> wrote:
>> >
>> >>Looking for a better audio recording interface for my Gateway (Motion
>> >>Computing) Tablet PC, I ran across the M-Audio Transit USB.
....

>More questions.
>
>Does this operate in "standalone" ADC mode? That is, if you put analog in,
>can you get digital out without connecting to a computer? Or could you hack
>it to do that? This could be a really cheap input device, powered from a 6 or
>9V battery.

I've seen that question asked, and the unchallenged answer (take it
with the confidence of anything you see on the net) was no. I don't
think anything is going to happen without some USB setup of the
internal microcontroller. I don't even know any way to monitor an
input through an output without the signal going into the computer,
through software, and back out.

The ART DI/O is a great standalone ADC/DAC with similar performance
for not much more money, but it uses AC input from its wall wart, and
I decided after peeking inside that converting it to battery power
would be excruciating. And it is S/PDIF on RCA rather than Toslink.

Loren
Anonymous
November 18, 2004 9:34:54 PM

Archived from groups: rec.audio.pro (More info?)

On 18 Nov 2004 08:59:42 -0500, moskowit@panix.com (Len Moskowitz)
wrote:
>Loren Amelang <loren@pacific.net> wrote:
>
>>Just don't think you are going to use it for analog monitoring while
>>recording the optical in - Connecting the Toslink-to-mini adapter to
>>the dual-purpose mini jack input appears to mechanically switch off
>>the analog output.
>
>You can use the laptop's internal sound card for monitoring. We've done
>that here and it works just fine as long as you can stand the latency
>delays.

I do. Luckily the tablet has an unusually good internal DAC - but only
a mono audio input. Hence the Transit.

Unfortunately, the volume controls in some audio apps, and the
keyboard button volume controls on both my notebooks, seem to get
really confused by having both internal and USB audio connected.

In case someone hasn't noticed, you can get to the Windows Volume
Controls for the built-in sound device even while the Transit is
connected and its controls are the default. Go to Volume Control ->
Options -> Properties -> Mixer device: and select the other device.

Now... Does anyone know how to make that selection from a command
line or batch file? Like you can do "sndvol32.exe /r" to get the
Recording Volume Control without having to go through the menus in the
Play control?

Loren
!