> > I had a look at it, but it is implementing a SIN wave and there is noth=
ing in
> > that code dealing specifically with a SWEEP across a start and end freq=
uency
> > within a given time window.
>=20
>=20
Subject added ...

> I did an image generator for SSTV (Slow Scan TV) a long time ago.
> It used DDS techniques to generate a sine wave at the output. It used a
> pic16c54 (!) to do that, so any more modern processor should be able to d=
o
> it faster and this produce a better quality sound.
>=20
> Though I don't have the code at hand here, here's the idea:
>=20
> - It uses a fixed frequency interrupt. Each interrupt, a fixed number is =
added
> to what is called a 'phase accumulator'. This phase accumulator points to=
 a
> table of, say, 16 or 32 values of a sine table (2 pi corresponds to 16 or=
 32
> values).
>=20
> - But wait - this technique doesn't give enough frequency resolution (tho=
ugh
> the 16 levels - or 32) approximate the sine enough, the frequency step is=
 too
> coarse. This is where a second accumulator comes in. A delta is summed to
> this accumulator which effectively corresponds to a value from 0 (0x00) t=
o
> 0.996093 (255). Each time there's a carry, you increment the first
> accumulator an extra 1. This effectively gives you a decimal part to the =
fixed
> number from the previous point, which gives you extremely good frequency
> resolution. So effective you are dividing your clock by
> (fixed) to (fixed+1) in steps of 0.004!
>=20
> - And if you don't want a D/A converter (you only need 4 or 5 bits for a =
16 or
> 32 bit table), you can use the trick with Walsh coefficients (using coeff=
icients
> and a special lookup table).
> There's an Application Note from Motorola about using Walsh transform to
> generate sine waves.

This one? (now NXP) ... http://cache.freescale.com/files/microcontrollers/d=
oc/app_note/AN1771.pdf
Or this one (also now NXP) http://www.nxp.com/files/microcontrollers/doc/ap=
p_note/AN1222.pdf (this appears to use the process described above without =
talking about Walsh coefficients).


I presume the OP has seen this microchip AN ...
http://ww1.microchip.com/downloads/en/AppNotes/00001523A.pdf




--=20
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