Russell McMahon wrote:
> Given PWM of a high enough frequency (20 kHz plus frame rate) you should
> be able to simply gate the audio signal with the PWM OR ground it if the
> supplying impedance is high enough. You will need some sot of electronic
> switching - quite possibly a single FET would do. If speaker is AC
> coupled it will be slightly more complex. Assume for simplicity that
> speaker is DC coupled and driven by a signal never below ground. A FET
> in the speaker ground lead could be gated by the PWM signal. A flyback
> diode would probably be needed across the speaker in series with a diode
> to *attempt* not to modify audio response too much. It would be easier
> if you could control the signal level at lower level and then amplify it
> for the speaker but that wrecks the simplicity.
> 
> See what sense you can make of that and ask more questions.
> 
>        RM

Actually, the frequencies coming out of the speaker rarely exceed
2Khz, so I could even use a lower PWM freq and add a lowpass filter.
But I'm not sure how fast I can push the PWM, since I don't want it
taking too much software time. Anyways I *do* want an amplifier in
there anyway. "Too complicated" would be DACs and stuff, but I don't
mind some transistors in there. Currently I'm using an optocoupler as
a substitute for the speaker for input (I don't trust how the computer
generates output, common ground, common positive, whatever, so an opto
makes sense here), so I can do whatever I want with its output
photodiode/transistor. I've amplified it with a simple BC547
transistor and a pot to (manually) control volume and it works quite
well (although I need a larger transistor, since at higher volumes the
BC547 gets quite warm). Now all I really need to do is replace the pot
with another transistor, something like this:


+12V --------------------+------------------+-------
                  OPTO   |                  |
   +------------+        |                  |   _ /|
   |           _|_     |/C                  +--| | |
 INPUT         /_\ --> |               --------|_| |
   |            |      |\E           |/C          \|
   +--\/\/\/----+        --/\/\/\-+--|
                                  |  |\E
                                |/C    |
     FILTERED PWM INPUT >-------|      |
                                |\E    |
GND ------------------------------+----+------------

(this would of course be an inverted configuration, where a higher PWM
input would mean lower volume, but of course that just means a change
in the software)

But, of course, I either need a filtered mostly clean voltage, or a
very high rate PWM (which I'm not sure I can afford). How clean can
you get with some R-C filters?

-- 
Hector Martin (hector@marcansoft.com)
Public Key: http://www.marcansoft.com/hector.asc

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