[sdiy] New to list - and DSP development

[Magnus Danielson]

From: "Kenneth Elhardt" <elhardt at worldnet.att.net>
Subject: Re: [sdiy] New to list - and DSP development
Date: Sun, 12 Dec 2004 14:33:30 -0500
Message-ID: <0a8d01c4e081$77dc0f10$fa9dfea9 at Compaq>

Kenneth,

> Ingo Debus writes:
> >>MIDI is 31.25 kBaud. The external clock (1MHz in most cases) is required
> because this baud rate cannot be generated by dividing down the internal
> clock.<<
> 
> I did notice the divide down problem wouldn't allow for it.  I've never
> understood why the MIDI people didn't use 38.4 kBaud which is a standard
> rate.

To be different? Not invented here (NIH)?

The CPUs in typical synths is an even multiple of 1 MHz, so that way I suspect
they would count on saving an oscillator. I'm not sure I would make the choice
differenly.

> Magnus Danielson writes:
> >>The Mac serial ports have a _very_ nice feature in that you can supply an
> external clock signal to it.<<
> 
> Yeah, I picked up on that from one of those earlier posts.  It's just a bit
> misleading because there is sometimes a dialog box that pops up where the
> user picks the speed from .5 to 2 MHz.  Fools the user into thinking he is
> setting the speed.

Indeed. To do some trivial math (I am sure *you* understands this):

If he/she selects wrong, then that is what is what is happening! What you
really selects is the divisor in the UART. By setting a lower frequency, you
get a higher speed naturally. ;O)

2 MHz / 64   => 31,25 kHz => 31,25 kBd
1 MHz / 32   => 31,25 kHz => 31,25 kBd
500 kHz / 16 => 31,25 kHz => 31,25 kBd

incorrect selection gives:

1 MHz / 64 => 62,5 kHz => 62,5 kBd
2 MHz / 32 => 62,5 kHz => 62,5 kBd
2 MHz / 16 => 125 kHz  => 125 kBd

So, between Macs one could run higher-speed MIDI without much real trouble.
Most other units would also be able to handle this. It's a mystery why this was
not included into the standard, or maybe it's not as much of a mystery :-P

My problem could very well have been that the program was expecting a 500 kHz
or a 2 MHz clock.

Cheers,
Magnus