Using modulation to send low freq signals through AC-coupled audio line inputs ?

Hello,

In order to avoid serial/USB or Ethernet connections and have a fast and steady datarate for my analog sensors, I was thinking about a way to connect them through the line inputs of my soundcard. As those are AC-coupled, I figured out this would be possible using a modulation/demodulation system: the sensor signal would modulate a high-frequency sine wave to go through the AC coupling. Then using a software demodulation (filter ?) I would get my low-frequency signal again.

The questions:

  1. Which type of modulation would be easier to implement both on the harware (modulation) and software (demodulation) sides : amplitude ? Frequency ? PWM ?
  2. Which one would provibe best results (as for the S/N ratio and cleanliness of the final signal) ?
  3. Do I need an Arduino with "true" (non-PWM) analog outputs such as the Due or the Teensy to produce the analog signals resuting from such modulation ?

Thank you in advance.

Study the cassette-interface used with commodore 64. They used to frequencies while writing and a 555 timer for decoding. It wont be fast..

Hmmm... Remember dial-up Internet and [u]telephone modems[/u]? Those converted digital to audio, and back.

In reality, it's probably easier to use USB or Ethernet, or maybe RS-232.

DVDdoug: In reality, it's probably easier to use USB or Ethernet, or maybe RS-232.

It might be easier but the continuous nature of an audio input sampled by a steady and high-sampling rate clock makes it much more reliable for sensors IMHO.

What about some frequency modulation ? Could I achieve it digitally on the Arduino or should I rather leave everything in the analog domain ? Also, would it be hard to de-modulate in my audio software (Max) ?

By the way, I was thinking about heterodyning too but actually is it something different from amplitude modulation ?

amundsen: By the way, I was thinking about heterodyning too but actually is it something different from amplitude modulation ?

Yes it is frequency shifting not a form of modulation at all.

amundsen: It might be easier but the continuous nature of an audio input sampled by a steady and high-sampling rate clock makes it much more reliable for sensors IMHO.

I don't think you are right. What sort of sensors are these?

Grumpy_Mike:

amundsen: By the way, I was thinking about heterodyning too but actually is it something different from amplitude modulation ?

Yes it is frequency shifting not a form of modulation at all.

I thought it could be considered as a kind of intermodulation (the same way ring modulation is, this one also being based on a multiplication).

Grumpy_Mike:

amundsen: It might be easier but the continuous nature of an audio input sampled by a steady and high-sampling rate clock makes it much more reliable for sensors IMHO.

I don't think you are right. What sort of sensors are these?

These are rather simple sensors (flex, pressure, soft pot) but the question is more about the type of connection between the sensors and the computer than about the sensors themselves. My experience with an interface transmitting sensor data through an audio interface as done by this device has been much better than with any other interface dedicated to sensors and using USB, serial or Ethernet in terms of stability and sampling rate (without making the fans turning crazy as it is when pushing the serial port to high datarates). And as I have an audio soundcard with many free inputs I wonder why not use them.

As I said I don't agree with your analysis.

Anyway, where is the arduino in this question?

Grumpy_Mike: As I said I don't agree with your analysis.

All right, but what do you think about the idea to modulate/hetedrodyne to get through AC coupling ?

Anyway, where is the arduino in this question?

I was thinking about the Arduino to do the modulatng/heterodyning.