Re: [AH] MIDI wiring mess

From Brian Willoughby
Sent Thu, Dec 27th 2018, 05:11

On Dec 26, 2018, at 6:54 PM, Michael E Caloroso =
<xxx.xxxxxxxxxxx@xxxxx.xxx> wrote:
>> 3) Although 99% of MIDI devices have MIDI Thru wired properly with 0 =
latency from MIDI
>> In, I seem to recall hearing about a few MIDI devices that have CPU =
processing on the
>> MIDI stream for MIDI Thru, and that would introduce latency. That =
would be in violation of
>> the MIDI standards, so I hope you don=E2=80=99t have any devices like =
that.
>=20
> The telltale is a device with a port labeled "MIDI OUT/THRU" or only
> IN/OUT ports but no THRU.  Buried in the menu system may be a
> configuration to change the OUT port to a THRU.  Under this condition
> the "THRU" signal isn't the circuit spelled out by the MMA and is
> actually coming from the CPU on the device, which WILL have some delay
> to it.

Theoretically, a MIDI device could use a digital multiplexer to switch =
the MIDI jack between CPU output and a textbook MIDI Thru circuit. I=E2=80=
=99m not aware of any synths actually doing this, but it would be =
possible to have a switchable port that doesn=E2=80=99t introduce =
latency when set to Thru.


> I'm not a fan of chaining THRU ports as one too many can distort MIDI
> signals to the point where they are not recognized at the receiving
> device or can appear as another erroneous MIDI message.  I'm not
> talking clock messages, I'm talking performance messages like note
> on/off and CC messages.

Perhaps I=E2=80=99m missing your point, but there is nothing =
electrically different between MIDI clock messages and MIDI Note On/Off =
and CC messages. They=E2=80=99re all 8-bit serial words with 1 start bit =
and 1 stop bit. If distortion were to occur, it could affect any bit, =
and there=E2=80=99s no reason that MIDI clock messages would be immune =
to such bit errors.

However, there=E2=80=99s no evidence that any synth manufacturer has =
problematic MIDI Thru circuitry that would actually cause bit errors, no =
matter how far down the chain they are placed. There are actually a pair =
of inverter gates ahead of every MIDI Thru circuit, and if those are =
Schottky inverters, such as common 74LS04/14/06 hex gates, then =
they=E2=80=99ll actually clean up any slew rate problems on the input =
such that the output is just as good as the original. If someone has a =
reproducible test case to the contrary, I=E2=80=99d be very interested =
to read the details.

Brian Willoughby