[sdiy] New to list - and DSP development

[Magnus Danielson]

From: Rainer Buchty <rainer at buchty.net>
Subject: Re: [sdiy] New to list - and DSP development
Date: Sun, 12 Dec 2004 21:48:38 +0100 (CET)
Message-ID: <Pine.LNX.4.56.0412122137050.24483 at laptop.buchty.net>

> >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.
> 
> Actually, that might well be the reason.
> 
> In PC/XT the UART frequency (1.8432MHz) was derived from 14.7456MHz
> which IIRC is 4 times the NTSC color subcarrier.

No, you had 14,31818 MHz = 4 * 3,579545 MHz. You also had the CPU running at
14,31818 MHz / 3 = 4,7727267 MHz (if you guys where wondering wherever that
came from).

1,8432 MHz = 96 x 19,2 kHz

The ratio between these are now 14,31818 MHz / 1,8432 MHz which is about
7,7681. This is why the serial interfaces had their own crystal oscillator at
1,8432 MHz.

When IBM created the IBM PC they did these choices. I guess they had the
intention that graphical cards would easilly be able to be simplified by
already operate synchronous with the NTSC colour-burst. I have only seen one
graphical card able to output TV-signal, and then only in greyscale. Actually,
that card was sitting in the spare-part built PC I run my first UNIX ever on,
the SCO Xenix. It had a wooping 8 MHz 286 motherboard, a 20 MB HDD and a 360 kB
FDD. I used the familly 26" TV for that one, using a SCART contact to get in
there. It worked, but I never lifted with XENIX but kept dreaming about hacking
a real OS like UNIX. Today I am very happy hacking away on Linux I might add.

> As you say, MIDI timing is based on an even multiple of 1MHz. If you
> want to convert any vanilla serial card to MIDI, either apply
> 16MHz to the 14.7456MHz input or 2MHz to the 1.8432MHz input. Then use
> OS settings for 28k8 to get 31k25.

Except for the numbers not being quite correct, yes.

You could do that by replacing the 1,8432 MHz crystal with a 2 MHz crystal and
the choose 28,8 kBd.

> >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
> 
> Interestingly, a number of internal buses of older machines run off
> "double MIDI".

Standards are good, everyone should have their own, but in reality everyone
run multiple standards for themselfs.

Cheers,
Magnus - pausing in the repairs of my new Synthi-A