[sdiy] MIDI phantom power...over 5 pin MIDI connector ?

[rsdio at audiobanshee.com]

On Sep 11, 2015, at 12:22 PM, mskala at ansuz.sooke.bc.ca wrote:
> On Fri, 11 Sep 2015, rsdio at audiobanshee.com wrote:
>> The point of termination is to allow an alternate path for the signal to
>> flow so that it doesn't bounce back from the receiver to the
>> transmitter.
> 
> RF termination is supposed to prevent reflections, but it does that by
> absorbing power, not by giving it an "alternate path."  That seems to be a
> DC concern; so I think it's clear that the audio concept of "termination"
> is something different from the RF concept that only happens to have the
> same name.  Like I said, it wouldn't be the first time.

It's more likely that termination circuits for RF and audio are basically the same (apart from frequency), but that I'm using the wrong terminology.

It seems like the same thing. The "alternative path" is what absorbs the signal. If the primary path were able to absorb the signal, then termination would not be needed.

We're probably getting off topic, although I am enjoying the discussion in terms of the MIDI current loop, but perhaps it would be interesting to look at the basic types of RF termination to see if they really differ from audio termination. My assumption is that RF terminating resistors are always tied to a separate circuit node that is driven to a different voltage than the signal, whether that's ground, a power rail, or an inverted version of the signal that is separately driven (differential).

If you look at the Wikipedia definition of "Electrical termination," which focuses on RF, all examples of termination include either ground or rails. According to Wikipedia, the 220 ohm series resistors could potentially create reflections because they represent a change in impedance for the signal. I'm assuming that MIDI cables have incredible low resistance compared to the 220 ohm resistors.

Brian