audio question - multiplexing audio channels 2

[monkeysolder]

I have 8 audio channels that I would like to multiplex together (4 headsets from cell phones, 2 channels per headset (speaker/mic)). Only one cellphone will be operating at a time, so it will be more like a switch.

Would it be electrically sound if I simply connected all the outputs(earphone) together, all the inputs(mic) together, and all the grounds together? I'm thinking something like a poor man's multiplexer. This way, I don't have to build in any additional circuitry and save myself a multiplexing chip.

Only one of the cell phones will be operating at a time, so only one will output to the headphone, correct? Can anyone see any impedance or other issues with this scheme?

I suppose the right way to do this would be to use an analog multiplexer, but that has been introducing noise on the audio.

Thanks for the help and sorry if the question is not very clear,
MS

 

[monkeysolder]

To clarify, I meant I wanted to splice all of the individual lines together to one single headset output.

 

[IRstuff]

No, it would not be electrically sound. The inactive outputs would be driven by the active output. Depending on the circuit design, you may or may not have a low impedance path that will suck up all your output current.

TTFN

 

[BrianR]

You don't need a multiplexer but an 8 channel mixer.

 

[monkeysolder]

Thanks, I will nix this idea and look around for a mixer.

 

[itsmoked]

Better would be hook each phones output to an opamp and tie their outputs together. Then you can work in volume control/matching.

 

[monkeysolder]

So would this IC do the trick?

PDF datasheet:

http://www.analog.com/UploadedFiles/Data_Sheets/79221617ADG526A_7A_b.pdf

Multiplexer page:

http://www.analog.com/en/prod/0,2877,768%5F836%5FADG527A,00.html

How should I minimize the noise pickup from ground and the rest of the board? Thanks,

 

[itsmoked]

No and no!

Those are multiplexers.. They give you one selected channel at a time. This would mean you need to actually select each phone one at a time with strange buttons, rather than which ever phone is on just works.

 

[VisiGoth]

Would everyone want those wires tangling each other up?

What about a TDMA signal MUX for existing USB earphone/mic combinations? someone must have invented this already, at least for 8, which I think is the number of endpoints available. But maybe USB on the go does something different.

 

[monkeysolder]

Wow, so they make an IC that will detect which phone is talking and automatically switch to that line with no microcontroller involved (excuse my ignorance)? Sign me up!

Does anybody know of a part number or what search terms I should be using? I need a surface mount part.

Thanks

 

[VisiGoth]

The analog mixer would not care who is talking. You would summ them at low levels using resistors, then send them to each phone, possibly with a buffer for each one.

You did not say if you wanted this gang of 4 to be able tohear each other. It seemed like you wanted only one phone in operation, but you wanted to use their microphones and speakers. Or did you just want to use the each of the phones speakers, or did you have an individual speaker for each person using a phone?

The digitial mixer would probably use TDMA or the iso mode on USB. Each phone would have to be part of the circuit on the RF USB. The advantage is that you might not have to have any wires. If 3 of the 4 phones are turned off, then USB would simply ignore them.

 

[monkeysolder]

No USB will be involved... just audio signals. I'm thinking about using a 4 op-amp IC (like itsmoked said), tying the outputs together after a non-inverting stage, and tying all of the returns together.

crude diagram:

===|>---
|
===|>---
|----------- COMBINED OUTPUT
===|>---
|
===|>---

-------------------- COMBINED RETURNS



I don't need the phones to hear each other. I just need to reduce the number of wires output audio is transmitted over, since only one phone will be in operation at a time.
It should be safe to tie all of the mic input lines together, since that will act as a splitter to four phones being driven by one mic.


Thanks for all the help!

 

[IRstuff]

Depends on the op amps. You SHOULD run the outputs into a summer with a final op amp. If the other outputs are inactive, they present a zero volt output that must be driven by the one active output.

Hypothetically, you should run your single mic into individual buffers that drive each mic input separately. The input impedance of a mic input may not be that high and ganging 4 together may reduce the input load sufficiently to degrade the performance of the mic.

TTFN

 

[fsmyth]

Provided that you have a high enough gain on the amp you
are feeding, you could simply add a resistor in series with
each source, and tie all together at the amp input.
10k works well with line-level amp inputs.
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