How about real waveforms with PWM (hardware or software) output?

You can easily store a sampled (or synthesized) waveform in a table. You
then index through the table at various rates to generate different notes.
This is the basic idea behind "wavetable" sound cards.

The trick is that you maintain a 16 bit value for your current position in
the table, but only use the top 8 bits to index.

The lower 8 bits are a running 'error value'.

Here is the idea in sorta-C

unsigned char sound_table[256] = {
    ... a bunch of numbers for the desired waveform ...
};

union {
    unsigned short as_short;
    struct {
        unsigned char ls_bits;
        unsigned char  ms_bits;
    } as_bytes;
} index;

/*
 the following variable determines the frequency of the note to be
generated. You think of it as a fixed point binary number with 8 bits on
each side of the binary point

 assume you are sampling at 8,000 samples per second

  if note_value == 0x200    (ie: 2.00 in fixed point)
  then it will take 128 samples or 128/8000 of a second
  to generate one cycle of output. Thus the frequency will be 62.50 Hz.

  because the time taken to cycle through the table is proportional to
note_value
  so will the frequency be.
*/

unsigned short note_value;

// assume you are sampling at 8,000 samples per second
//     if note
//
unsigned short note_value;

index.as_short = 0;

for (;;)    // loop forever
{
     index.as_short += note_value;
     output(sound_table[index.as_bytes.ms_bits]);
}

You can use a similar technique to perform envelope generation, but then you
will need to multiply the envelope value by the waveform value to compute
the output.

At 8000 samples per second and a 4MHz clock you will have 125 instructions
per sample. This should be enough to do quite a bit.

btw: I did this sort of thing over 20 years ago using a Data General
minicomputer and it sounded great.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

----- Original Message -----
From: Brian Jones <bejones@HURSLEY.IBM.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Saturday, August 19, 2000 11:39 AM
Subject: Music


> I run a 'warm iron' evening at my ham radio club every December.
> The idea is I design and kit something simple for the members to
> build - must take less than a couple of hours and cost less than 10
> dollars. Last years 20 sequenced LED Xmas tree driven by a
> 12C509 was a great hit.
>
> I'd like to add music this year but can't find a cheap source of
> Holtek tune chips so was wondering what I'd need to make the
> 16F84 (or whatever) sound decent. Obviously a starting point is a
> simple square wave with the roughness filtered. But how do I make
> a richer tone - what harmonics do I need? Is it a simple job of
> adding harmonic square waves suitably attentuated and feeding all
> the outputs to the filter/speaker?
>
> Any ideas ??
> Brian E Jones
> Centre for Java Technology
> IBM HURSLEY
>
> --
> http://www.piclist.com hint: The list server can filter out subtopics
> (like ads or off topics) for you. See http://www.piclist.com/#topics

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