To read part1 click here

With a basic form factor sorted out in the last blog entry, the next step is working out more or less how it's all going to work.

While the signal switching involved could be done using FET's and various CMOS switches, I get far less pushback from the community if I suggest good ol' relays.

With that mind, I decided to reuse the hardware that was designed for the Mic Pre Frontend controller.

However, the switching characteristics now have to change, so it means a small software rewrite. The relays and signal path also need to change.

At a high level the following things have to happen.

  • When the Push-to-talk button is pressed:
    • Mute the speaker (in case of feedback)
    • Switch output path from line-in audio to microphone input
  • When either of the audio source switches are pressed
    • If pushing one that is currently off, change input source
    • if pushing one that is already enabled, mute both.

The interesting thing with reusing the mic pre front panel board is that the LED's are directly connected to the outputs of the board. This means when you want to mute the two sources, you have to do it ONLY WHEN BOTH OUTPUTS are 0. Some additional smarts on the relay board are required.

In the next entry, I'll cover the audio signal chain of the system.



Written by Dafydd Roche — September 16, 2013

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