Re: EEG trigger - technical question

[Bob Masta <audio@xxxxxxxxxxx>]

Do you really need true TTL?  Most inputs that state they 
are TTL compatible just need something in the 0-5V range.  
If that's true in your case, then probably all you really 
need is to get rid of the AC coupling capacitor on the 
output of the sound card.  With some sound card chipsets 
that will give excellent "TTL" levels (0.1 to 4.0 V).  See
"Simple Sound Card DC Output Modification" at 
<http://www.daqarta.com/dw_ggll.htm>.  

Note that the 5.1 channel unit discussed there not only has 
good TTL levels, it also has multiple output channels.  So 
you may be able to use it for your whole output solution, 
sound as well as trigger pulse.  That's important because 
in general there is no easy way to synchronize separate 
sound cards to use the same clock signal.

That particular unit is very cheap (under US$20), so you 
may want to get a couple to experiment with.  There is no 
particular model number, and in fact the unit you get may 
not be identical.  But as long as it uses the same chipset 
you should have no problems.

Not also that to get the full signal swing your digital 
signal must go from *negative* full-scale to positive full-
scale (-32768 to +32767), since negative full-scale will 
become (close to) 0 VDC.  (Binary 0 becomes about 2V.)

If you really need "ideal" TTL outputs, and you don't mind 
building circuits, look at "DC Pulse Output Circuits" at 
<http://www.daqarta.com/de_gg0o.htm>.  But that wouldn't be 
my first choice.

Best regards,

Bob Masta

==============================

On 19 May 2016 at 17:26, Bastian Epp wrote:

> Hi everyone!
> 
> I have a rather specific and technical question - before starting to 
> develop something from scratch:
> 
> Is there a commercially available solution to translate a digital audio 
> signal (preferrably from an ADAT channel, otherwise SPDIF) into parallel 
> TTL (ie "printer port")? I am looking for something (let's call it "THE 
> BOX") that does not require too much overhead in terms of devices.
> 
> Some background information:
> We want to send our EEG trigger as a digital number on the sound card in 
> one channel, along with two channels containing the stimulus. So "THE 
> BOX" needs to decode the digital signal/number and encodes it onto 0/5V 
> on a d-sub connector - without jitter!
> 
> We do have a box that does it with SPDIF, but I wonder if that is 
> possible with ADAT directly? Looking into audio forums, it seems doable, 
> I just couldn't find someone that actually did it. And as I said - 
> before digging into digital electronics...it looks like a fun project 
> though.
> 
> Any pointers welcome and (of course) I would be willing to share a 
> concise summary if there is interest.
> 
> Thanks in advance and a wonderful day to everyone from sunny Copenhagen!
> 
> BAstian

Bob Masta
 
            D A Q A R T A
Data AcQuisition And Real-Time Analysis
           www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
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