[sdiy] midi clock

[Magnus Danielson]

From: patchell <patchell at silcom.com>
Subject: Re: [sdiy] midi clock
Date: Sun, 01 Dec 2002 16:14:34 -0800

Hmm... this makes me think of another of those great projects in the drawer...
a propper House Clock. You know, which has a good internal reference, may lock
onto external signals (both frequency and/or timecodes) and that can spit out
different formats. All of the production timecodes also have the specific
aspect of different frame-rate (of the supposed TV production) which helps to
confuse things.

In audio/video world I know of these timecode formats:
SMPTE Time Code (Also put into standard by ITU-R) - LTC & VITC
MIDI Time Code - MTC (really just a MIDI "backend" to the SMPTE machinery)
AES/EBU, SPDIF (IEC spec) - Frequency locking (AES 11, EBU etc) and TC subcode
SDI with Timecode auxiliary data - Frequency locking and TC subcode
(suppressing all knowledge of MPEG at this point)

Timecode-less formats:
DIN-Sync (BPM related)
Gate/Trigg ticks (BPM related)
10 MHz, 5 MHz, 1 MHz, 100 kHz, 1 PPS (frequency transfer)
"Word clock" - sample rate

Then also from various worlds:
IRIG A, B, C etc... - Military subcodes also used in military stuff and NIST
		      radio transmissions
DCF77
GPS
GLONASS
GALILEO (OK, several years to launch)
NTP (which you run on Internet)
+ a bunch of others if I care to dig up in them books

Well, all of the first category is really nothing but different forms of the
same tiger, they use the same time "core", as found in SMPTE/ITU-R specs.
The LTC, VITC and MTC should all be quite easy to accomplish. The AES/EBU and
SPDIF variants should be just slightly worse. The SDI stuff becomes easier if
you use up existing building blocks (saves a lot of trouble when dealing with
it's 270 Mbps signal, but I've looked down that pipe before...).

Generating DIN-Sync and Gate/Trigg signals should both be very easy, but they
involve actuall setting a frequency synthesis locked to the reference. It also
raises the issue if not the MIDI signal should have Song Pointers, and all of
a sudden we've oppend up a can of worms.

Well, this is not much of a op-amp, resistor, capacitor and pots kind of work
I guess. Much of it (but not all) falls into the hands of logic.

Another issue is, OK... it's simple to make a tune do exactly 123 BPM,
throughout, but is this really what we want and need?
Also, will our outputs allways want to sit in the same relation to each other
all the time?

Both goes by the answer of NO if we think of it... and there we saw the project
exploging in out hands... (actually, I naturally can think of a solution of
a few).

However, is there any interest in discussing the issues around all this?
Do people think timing issues are of interest?
Do people need a helping hand in designing stuff like these?

Cheers,
Magnus - who just came "out" as being a timing maniac