Line level audio and muxing

Question

I am developing an OSD project which also manipulates audio signals.

The aim is to add a 'mux' system, which allows a microcontroller to select between the input audio and an internally generated audio signal as the output. This allows it to add a voice to the signal or transmit data in the right audio channel.

Does anyone have any solutions for this? I would like to avoid something like a 4066 which takes up valuable space. Also, since line level is alternating current, it needs to somehow work well with this.

Answer 1

You need something like the 4066. Luckily, there are smaller versions with only one switch inside, like a tiny 6-pin SPDT, with 1/3 the circuitry of a 4053.

• NLAS4599: Single SPDT Analog Switch Single Supply
• NLASB3157: Single SPDT Analog Switch
• FSA4157 Low Voltage 1 Ohm SPDT Analog Switch

(I was just looking at these parts for this question.)

You can find examples of how to bias and interface with these kinds of parts here:

• Bypassing and Switching with the CD4053 CMOS Analog MUX
• Audio Switch Notes (also shows a JFET switch)
• "The Juggler 4 is a CMOS based effects order switcher"

Answer 2

I have come up with my own solution to this.

As I'm not worried about distortion I simply use a passive adder circuit which adds 3V to a ±2V signal. This can be switched using conventional, ground referenced circuits. Then, when I want to output it, I pass it through a capacitor + emitter follower amplifier.