R J Bettis
Some time ago (AMPLINEX issues 006 and 007) a couple of
articles by Scott Mackie described the construction and use of a MIDI interface
for the BBC running AMPLE.
This article describes a modified version of that interface,
which I have adapted to work with the Music 2000. It also works with my master keyboard
(a Cheetah) which uses the MIDI 'running status' system in its data output.
1) History
Shortly after the interface articles appeared I acquired a
Music 2000 interface, and since then my MIDI system has grown, while the Music 500(0)
has lost out as a sound source. This is mainly because of the increased
expression available from a touch-sensitive keyboard (and the sound quality of
relatively cheap MIDI units).
Some of the Music 5000 facilities, such as a wide range of
readily programmable sounds have been tempting me back every now and then, and eventually
I decided to try and incorporate the Music 5000 as a MIDI expander (as opposed
to using the Music 2000 as its controller).
I remembered Scott's articles, dug out the discs, and
examined the software. There were three programs, a main AMPLE control program,
a machine code program to run the interface, and a BASIC source program for the
machine code.
I then made the modifications needed to run the program with
my system - these were almost all in the machine code.
2) Technical details
The original hardware and software used the (rather drastic)
NMI interrupt, whereas the Music 2000 uses the normal interrupt system. Hence
the program had to be altered to (a) use this interrupt, (b) set itself up in the
interrupt chain and (c) avoid acting upon or corrupting other interrupts using
the same system.
The result was that I replaced the whole of the original
interrupt handler with a new section of code. This then made the program too
long for its original location (page &900) so I had to expand downwards to
start at &8D0. This area is used by the operation system for sound
envelopes 2-4, so seemed fair game in an AMPLE environment.
The second problem I discovered was that my keyboard uses
'running status'. A typical MIDI message consists of a command byte followed by
(usually two) data bytes; however, the specification allows for just the two data
bytes to be sent if the command hasn't changed.
For example, a 'note on' message might be three bytes:
<note on> <note number> <velocity>
but with running status a three note chord could be sent as:
<note on> <note 1> <velocity 1>
<note 2) <velocity 2>
<note 3> <velocity 3>
<note 2) <velocity 2>
<note 3> <velocity 3>
saving a couple of bytes in transmission.
This means that the receiver now has to keep a note of the
last command so that this can be employed.
The BASIC program for the new code is in file U.MXbasic, and
the code this produces is called U.MXcode.
3) In AMPLE
The AMPLE program ('U.MXample') is largely the same as Scott
Mackie's original (on the principle of 'if it works, don't mess with it'). It
has a quite comprehensive 'help' facility written into it.
Calls to the code have been altered to reflect the new
locations, and two new commands have been added:
'go' gives a list of all
available instruments and ask for a choice
to be made before using MIDI
to be made before using MIDI
'ins' sets up to use an instrument 'inst', so that new sounds can
be
tried easily (alter 'inst', MAKE and run 'ins')
tried easily (alter 'inst', MAKE and run 'ins')
Related files on this disc:
Published in AMPLINEX 019, September
1990
