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Connecting a headset to a handset (audio "null modem"?)

Mihai Beffa
Prodigy 170 points
Other Parts Discussed in Thread: PCM2912A, PCM2912

I need to route the sound to and from a PCM2912A to a telephone built with AS2522. The telephone has a differential mike input and mono audio output, designed to connecto to a typical telephone handset:

The PCM2912 has stereo outputs and a mike input, designed to interface with a typical headset:

Both chips expect input from electret microphones. 

What would be the easiest way to mix the stereo outputs of the PCM2912 to the input of AS2522, and the single ended output of the PCM2912 to the differential input of the AS2522?

  • 0
    TI__Guru**** 189401 points

    Hi, Mihai,

    I apologize, but I don't really follow what you are trying to do. Can you try to explain it in a different way?

    -d2

  • 0
    Prodigy 170 points

    Sure.

    Assume you have to add a telephone chip to a tablet. The tablet does not have audio in, like all laptops do, so I cannot route the sound from the telephone to the tablet. Even if I could, I would have the same problem. I need to connect the output from a handset to the input into a headset, and vice-versa.

    Similar problem: 

    Assume you have two USB dongles implemented with PCM2912. Assume you plug each dongle into one laptop.

    Now you want to route the sound between the two laptops. You cannot simply cross over the cables, at least because the output is stereo and the input is mono, so at a minimum you have to mix the two output channels. An OP-AMP will do it, but I suspect a couple of resistors would do the same. 

    As I mentioned above, the input to the telephone chip is differential and the output from the PCM chip is single ended. Do I need an OP-AMP here, or I can get by with a resistor divider?

  • 0 in reply to Mihai Beffa
    TI__Guru**** 189401 points

    Hi, Mihai,

    The summing circuitry needed depends on the input structure of the telephone chip.

    -d2