First of all, if you're plugging this thing into a tape recorder,
what would be wrong with just using a simple capacitor low pass filter
with the 3db down point set around 1300Hz. Or, to be really crude,
is it bad to be recording a square wave? It's not going to come out
square when you play it back, and it's certainly not going to fold
over since you're not using a DAC.

Also, as for the starting and stopping, it might not matter if you
get an audible pop if it's not a signal designed for human listening.
If you're really concerned about it, make a bandpass filter around the
centre frequency, as you'll find that most pops from turning things
on and off tend to create big disturbances in the near-DC range that
cause loud bass thumps.



Andrew


On Thu, Apr 22, 2004 at 08:32:16AM -0500, Mike Hord wrote:
> This is one of those "Rorshach ink-blot tests" for engineers, I think.  ;-)
>
> Software guys will suggest lookup tables, PWM, external DACs, etc.,
> while hardware guys will typically suggest some sort of filter network,
> sometimes based on PWM, sometimes on a triangle wave, or, my
> personal favorite for this application, filtering out the first harmonic of
> a square wave.
>
> That may prove a little tricky, since at such a low frequency your 3rd
> harmonic isn't going to be TOO far away, but you should be able to
> attenuate it below audible range fairly easily.  See FilterLab for details.
>
> The other thing I'll mention is that you may want to leave the signal
> running full time.  Starting and stopping an audio signal can lead to a
> very audible POP at the beginning and end.  TI makes a couple of
> audio amplifiers (unfortunately SMT only), the TPA 122 and 152,
> which have in-built pop reduction.  Hook those up as your output
> stage and connect up to the MUTE lead on the chip, start your PICs
> PWM at 1300 Hz, and away you go!
>
> Mike H.
>
> PS- A more saavy audio/analog guy may know an easier way to
> lose that popping effect, but that's not me!
> >Hi,
> >
> >I need to generate a 1300Hz ish sine wave pulse, from a pic, that I can
> >plug
> >into a tape recorder's line in jack to log.
> >
> >Obviously, it's fairly easy to get a 1300 Hz square wave, but what can I do
> >to simply and easily smooth this out a bit, so it's reasonably close to a
> >sine wave?
> >
> >Any pointers appreciated
> >
> >Thanks
> >
> >Jon
> >
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