RE: Syncing the EDP to the Repeater

["Kevin Mulvihill" <kmulvihill@mediaone.net>]

Kim, all I can say is that this is really useful, specific information...
and that I hope Electrix sends you a check for doing their beta testing for
them... I haven't done any MIDI syncing yet, but this should've been caught
before release.

Kevin

> sorry for copying the mail below whole, but it was from a while ago so I
> figured the context was needed again. I was concerned about this problem
> quite a bit, and one of my reasons to get a repeater was just to figure
> this one out. (how's that for dedication? :)
>
> Anyway, it is true that when you try to sync an Echoplex to the
> Repeater's
> midi clock out there will be problems. The Echoplex loop time ends up
> slightly longer than the Repeater, and then shifts against the Repeater's
> loop as they play together. I thought this was really weird, since I had
> never seen anything like it with the EDP.
>
> I did some analysis of the Repeater's midi clock output, and sure enough
> there is a subtle bug in the Repeater clock out.
>
> If you aren't familiar with how midi clock works, basically it uses the
> ancient 24 pulses per quarter note method that dates back to the days of
> analog synths. in midi though, the "pulses" are actually a particular
> System Realtime byte. The clock generator sends 24 of these per
> beat at the
> given tempo. The clock pulses are normally spaced evenly in time so the
> receiver can use whichever pulse it needs to time most any sort
> of rhythm.
> so the time interval between these clock pulses should be equal to 1/24th
> of a beat.
>
> I made a little max patch that times the interval between midi
> clock pulses
> in milliseconds, and saves the data it collects into a text file.
> With the
> Repeater clock, I see the 23rd pulse of every beat is delayed by
> an entire
> interval. The 24th pulse then comes immediately after that, with no
> interval at all. so the 23rd pulse is practically right on top of
> it. So if
> the tempo is 125 BPM, the usual interval between pulses is 20ms. Between
> the 22nd and 23rd pulses the interval is 40ms, and between the 23rd and
> 24th pulse is less than 1ms. (Max says the delay is 0ms between these,
> since it resolves to 1ms.) That means these last two pulses in
> the beat are
> as close together as two midi commands can be.
>
> The echoplex apparently decides this situation of two clock
> pulses so close
> together and completely different from all the other pulses is nonsense,
> and doesn't count one of them. It counts the first pulse in the next beat
> as the 24th pulse. So it ends up thinking the beat is 1/24th of a beat
> longer than the Repeater thinks it is. This is why a 2 bar loop at 120BPM
> shows as 4.1 seconds long on the EDP instead of 4.0 seconds.
>
> I tried the midi clock outputs from some other devices into my
> max patch to
> get some comparison data and they did not have any problem like this. All
> the pulses were evenly spaced as they should be.
>
> My guess is, this delayed pulse also why drum machines and sequencers
> synchronized to Repeater's midi clock out sound a little wobbly, even
> though they don't always get off so much like the echoplex does.
> Some notes
> probably get shifted around a little while it tries to sort the
> weird clock
> pulses out.
>
> hopefully electrix fixes this one quick, since it is really a drag. The
> only work around I can see here is either use something else for a midi
> clock to both Repeater and EDP, or slave the Repeater to the EDP
> midi clock
> out.
>
> hope this helps,
> kim