The myth on MIDI Thru propagation delay.

The reason for writing this article was a recent forum discussion (see the link below).
So, let us turn from words down to business. First, let us take a look on the input/output schematic recommended by the MIDI specifications.

MIDI Standart Hardware

As you can see, the only element that can introduce a noticeable delay is the optocoupler.
The specifications recommend using Sharp PC-900. We, however, use something simpler and widely available in sufficient quantities, namely EL3H7C. In our experiment we connect six MIDI Thru cascades in series.
According to the data-sheet, the CTR (Current Transfer Ratio) ranges from 200% to 400%. Taking the average results in a pull-up resistor value of 390 Ohm. To demonstrate the effect of incorrectly calculated values, one of the resistors is set to a much higher value (4,7kOm).
To make the experiment closer to the reality we add protection diodes, and connect every two cascades via a regular 5m MIDI cable.


Schematic

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2098x1276


The rest is pretty simple. We connect a logic analyzer to the test points, power the circuit up, and forward a pulse with duration of 32μs to the input.

Observing the waveforms, note a leading edge pulse delay,

Delay Front One
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1259x603



along with the falling edge delay

Delay Decay One
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1259x603


of about 5μs, as it is mentioned in the datasheet.
The presence of cables does not change anything.


Delay Decay All
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1259x603


When using a "wrong" resistor, the leading edge got slightly steeper, but the falling edge delay is stretched out and is about twice longer now. This is an inacceptable distortion of waveforms, and using such cascade at the specified speed rate is inappropriate.


Delay Front All
View (download)
1259x603


The resulting total delay of six stages is 34μs.

I do not have any exact information concerning the ability of human’s ear to distinguish sounds. However, according to my own experience the threshold of sensitivity is above several tens of milliseconds. This is at least 3 orders of magnitude higher than the delay obtained in our experiment.

From the above experiment we deduce two conclusions:
1. The "Pros" sometimes (really do not want to use the word "often") do not know what they are talking about.
2. Outtricking the laws of physics failed again. This, in turn, implies that if your MIDI Thru adds a noticeable delay into the sound then:
a. Your MIDI Thru does not meet the specifications, and the manufacturer is simply deceiving you (e.g. slips ECHO instead THRU),
b. Or you just do not know how to tune/set up your instruments.

Needless to say that all written here refers to the case of using identical samplers/ton generators (or at least the ones that have similar playback delays).






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Copyright © 2015 by Dmitry Dubrovenko.