[sdiy] (OT) Doepfer MIDI-CV oddity

[Oren Leavitt]

On 1/19/2011 4:45 AM, Colin f wrote:
>
>> I did quite a bit of research looking for MIDI input circuits
>> based on the 6N138/9 and found a large range of resistor
>> values used.
>
> I've been there too...
> Take a look at this image:
> http://www.colinfraser.com/images/6n139-fairchild-vs-vishay.jpg
>
> It's a scope trace of the output from a 6N139 in a MIDI input stage.
> Top trace is a Fairchild part, bottom is a Vishay.
> These two are just swapped over in the same circuit.
> All other values are the same.
> So clearly, all optos are not.
> Fairchild worked perfectly, Vishay was hopelessly unreliable, missing more
> bytes than it managed to receive intact.
> I've always stuck to Fairchild since.
>
> Cheers,
> Colin f

There could be logic-threshold differences in the various pieces of 
equipment.
If the input conditioning circuitry has a lower logic threshold, say 
1.2V, I would think it would be indifferent to the short pulses in the 
bottom trace.

- Oren