VocalEasel/mma/text/REALTIME-FILTER

If we had a real FAQ this information would certainly be there. But,
for now we just have a nice little text file.

More than one user has asked about a "real time" or "filter" mode for
MMA. The sort answer is "sorry, this isn't going to happen."

The longer answer follows.

Before you think about "real time" and "filters", let's talk about
what MMA is designed to do. And that is pretty simple: MMA creates
standard MIDI files, also known as SMFs. You can think of a SMF as a
collection of MIDI information (like note-on, note-off and controller
events) packaged with timing information. MIDI itself just has
commands to do simple things like "turn on a note", "turn off a note",
"enable a voice/tone", "turn on vibrato", etc. It doesn't have commands
for musical issues like timing and tempo. Yes, MIDI doesn't understand
anything like "this is a quarter note" or "play at a tempo of 120 bpm".

So, in a SMF each MIDI event is wrapped with a time code. These time
codes are offsets from the start of the file. To play a SMF a program
loads the file, and sends out the MIDI information at intervals
dictated by the time codes. In addition, variables like tempo can be
added to speed up (down) the timing. Remember: timing is not part of
MIDI. Timing is something a sequencer does.

Now, back to MMA. MMA reads a text file, or series of files,
containing information about tempo, volume, patterns, and chords. It
then converts this into a SMF.

When processing the input file(s) MMA doesn't necessarily create the
MIDI data in the same order that it will be played. This is huge
real-time problem.

Not in order? Sure, think about what is going on. MMA reads the input
file and figures out stuff about tempo, patterns, etc. Now, it has to
handle each bar of the song. It does this by looping though each
active track bar by bar. So, our first real-time problem is that the
data for the a bar is not complete until all the tracks for a bar are
processed.

To add complexities to this, MMA also backtracks into previous
bars. Consider the Unify command which joins notes between parts of a
sequence. A note sounding in bar 1 could easily extend into bar
5. And, to figure this MMA needs to adjust previously generated event
data. Consider BeatAdjust which can move the song pointer backwards in
the generated data.

Other commands in MMA can effect future parts of the MIDI. If you
define a Tempo change over several bars MMA immediately creates
entries in a table describing future timing events to incorporate into
the final SMF. And there are many more commands doing nasty things
to complicate life.

Once the entire input file has been read and processed MMA gathers all
the timing and event data it created and processes that into a
SMF. And, really, it can't create the SMF until it knows everything
you wanted to tell it.

So, am I telling you to forget it? Nope. I've certainly given this
issue some thought and do have some ideas which may (or may not) be
implemented in the future. But before I do anything with the following
I need to get some serious ideas from users. So, think/work with me on
this.

INPUT: it is certainly possible to get MMA to read from stdin. But,
it's not as simple as reading a line and generating output. The
biggest problem is the Repeat command. As implemented the
repeat/end/ending code reads all the information in the loop area and
creates copies; then the entire chunk is processed. Acting as an input
filter the reasonable thing would be to disable some commands like
this. Probably a reasonable thing to do. Same goes for Labels and Gotos.

Next, we'd need to rewrite the main parsing loop to handle input on a
line-by-line, rather than file, basis. This is probably less work than
you might imagine.

OUTPUT: To properly do this we'd need to generate output as it's
generated. Certainly doing this as notes or events are generated is not
workable. However, it might be possible to "flush" the buffer
containing the MIDI information after each bar is generated. To do
this one would need to disable a number of commands. Cut, BeatAdjust,
Tempo (future) are just a few which come to mind.

Finally, if we do get the about issues working ... what do we end up
with? Something useful? I'm not convinced that any of the MIDI tools
we normally use can take chunks of MIDI information and do anything
useful with them. Most (all?) players read an entire SMF file, do some
processing and then play the data. Due to the structure of a SMF there
really isn't any other way to handle it.

I think that what people are asking is "can you rewrite MMA from being
a SMF creator into a pattern based MIDI sequencer?" And the answer to
that is "probably not".

So, if you know more about MIDI than I do ... jump in and we'll talk.


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bvdp, June 2009.