Demod PC Generated ELF

[Robert Davis]

I would like to use commercial software to generate complex sub-audio
waveforms. Obviously, these cannot be output directly from the
soundcard. So I am planning to have them amplitude modulate an audio
sinewave and then apply the composite signal to a demodulator external
to the PC to recover the sub-audio waveform.

Can anyone please provide advice regarding the best design practice
for this application? What type of existing circuitry might I base
this on?

Thank you,

Robert Davis

 

[Nico Coesel]

I would like to use commercial software to generate complex sub-audio
waveforms. Obviously, these cannot be output directly from the
soundcard. So I am planning to have them amplitude modulate an audio
sinewave and then apply the composite signal to a demodulator external
to the PC to recover the sub-audio waveform.

Can anyone please provide advice regarding the best design practice
for this application? What type of existing circuitry might I base
this on?

I have a simpler solution: remove the DC blocking capacitors from the
soundcard and create an output stage that uses the reference (usually
half the supply voltage) of the soundchip.

 

[Robert Davis]

If you only need one output, you could use both channels of the sound
card and feed a mixer to make the demodulated sub-audio. This could
be more linear than just rectifying one channel to make the
demodulated signal. I would base the demodulator of the HC4053 analog
switch.

Are you referring to a balanced demodulator as described here?

http://www.spelektroniikka.fi/kuvat/mc1496appl.pdf

I gather both an external mixer and demodulator are required. Is this
available in one IC?

Could you please describe in a little more detail how the technique
you kindly described would work with the software/soundcard approach
in my OP?

I did a web search but found mostly RF apps.

If I was taking the signal from the sound card, the first thing I
would do would to high pass filter it to remove any low frequency
noise that the PC may be putting on it. Then you know that the output
would be all the demodulated result and not feed through of the noise.

I was planning to use 1KHz as the carrier for the ELF. According to
your suggestion, what would be the recommended cutoff frequency for
the high pass filter?

Robert Davis

 

[Phil Allison]

"Robert Davis"

I would like to use commercial software to generate complex sub-audio
waveforms.

** You hope to do this or you ARE doing this now?

Obviously, these cannot be output directly from the
soundcard. So I am planning to have them amplitude modulate an audio
sinewave and then apply the composite signal to a demodulator external
to the PC to recover the sub-audio waveform.

** You hope to do this or you ARE doing this now?

Can anyone please provide advice regarding the best design practice
for this application? What type of existing circuitry might I base
this on?

** A half wave, precision rectifier is all you need to demod AM.

http://en.wikipedia.org/wiki/Precision_rectifier

See: " Improved Circuit ".



...... Phil

 

[Jasen Betts]

I would like to use commercial software to generate complex sub-audio
waveforms. Obviously, these cannot be output directly from the
soundcard.

not without bypassing the output capacitors, a fairly easy task.

So I am planning to have them amplitude modulate an audio
sinewave and then apply the composite signal to a demodulator external
to the PC to recover the sub-audio waveform.

seems like doing it the hard way.