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Multiplexing audio sources?

Discussion in 'Electronic Design' started by [email protected], Oct 2, 2006.

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  1. Guest

    Hi everyone,

    Good day!

    I have four analog audio sources and one audio amplifier. My objective
    is to amplify an audio input. However, each analog audio source must be
    amplified based on rank. The following example will try to explain

    - There are 4 inputs (A,B,C, an D)
    - Each inputs have the following rank: A = 1st, B = 2nd, C = 3rd, and D
    = 4th.
    - If source C is present and all other sources are off, then source C
    is amplified.
    - If source A and source C are both on, then source A is amplfied. If
    source A goes off and source C stays on, then source C is amplified.
    - If source B and source D are on, then source B is amplified. If
    source A comes on while both source B and source D are still on, the
    amplification switches to source A.

    What would be the best way to tackle this? Is the best way also the
    simplest way?

    Thank you!
  2. On 2 Oct 2006 10:46:41 -0700, in
    Just to clarify
    so what will the gain of C be when A is on, will it be muted or at
    unity gain, while A has a different gain

  3. My interpretation of what you want is called a "priority" system rather than
    a multiplexed system. As I understand it, you have four audio sources
    called A, B, C, and D.

    You only want to hear D if there is no audio present at A, B, or C.

    You only want to hear C if there is no audio present at A or B.

    You only want to hear B if there is no audio present at A.

    You always want to hear A.

    Therefore, Channel A is always connected to a summing port on your

    Channel A also needs a detector of some sort to tell whether or not audio is
    present and output a digital signal if there is audio. Generally you can use
    a comparator with the trigger level set to whatever you say is "audio
    present" and then some sort of rectifier-capacitor with a time constant you
    select to say that there is no more audio present on this channel.

    Channel B needs the same thing, but it also needs a gate in series with the
    audio. Google "CMOS" and "4066" to see a typical analog-digital switch.
    The digital signal from channel A needs to be connected to gate this channel
    off when signal is present at A.

    Channel C needs the same thing, but you need an OR function from both
    channels A & B to gate the signal. It can be as simple as two diodes or you
    can use digital logic.

    Channel D needs no detector, but it needs a gate that is OR by channels A,
    B, and C.

  4. Guest

    I have a need for one of these myself. I would suggest investigating
    "energy detect" circuits used in voice band modems. My recollection of
    how this was done in the analog days was to rectify the AC coupled
    input signal, follow it with a Schmidt trigger, then the averaging
    circuit of your choice. It can be as simple as taking the digital
    signal from the Schmidt trigger and RC filtering it. Then use one more
    Schmidt trigger after this averaged circuit to make the final decision.

    Please post what you come up with. You are really just gating the
    signal, not amplifying them. The amplification, or more accurately
    level equalization, should be done prior to the gating circuit.

    You might also find relevant information on "ducking" circuits, though
    ducking only attenuates one of the signal based on priority.

    You should also consider how to do the switching without creating
    clicks as the muxes are switched. I don't think this is a simple
    project if done well.
  5. PPP

    PPP Guest

    Thanks, Jim!

    I actually saw a sample ducking circuit that used NOR gates. This gave
    me an idea.

    I'm going to try and draw a block diagram of what I had in mind, and
    hopefully, post it soon for some more critiques.

    Muchos Gracias!
  6. PPP

    PPP Guest

    Hey miso,

    Thanks a lot!

    I got plenty of ideas by looking up your suggestion of ducking
    circuits. That's the first time I've heard of it.
  7. PPP

    PPP Guest

    Hi martin,

    If a higher rank signal is present, then all other sources are muted.

  8. Guest

    So, for each channel you need a detector (amplifier, rectifier,
    threshold detector)
    feeding a retriggerable monostable (pulse stretcher to keep quiet
    passages from
    dropping the signal).

    Send all the logic-out from the monostable to a priority encoder
    (CD4532 is a CMOS
    eight-input chip that does this), which performs the
    which-is-highest-priority function
    and has binary number (3 bits) output.

    Then use the 3-bit output to address an analog switch to select between
    inputs (CD4053 is an eight-bit switch that can do this).

    Since ALL inputs might have no signal, it would be worthwhile having a
    resistor to
    ground so as to keep the quiet-noise to a minimum.

    Cool idea; I might use it. But, I'm not sure what priorities I'd give
    that would be
    livable; maybe DVD first, CD second, TV third, FM fourth?
  9. Guest

    I was thinking of using several scanners (radios) with a priority
    system. If you ever ran a scanner, it is a bit annoying if it locks on
    a signal that is kind of interesting, but not your favorite signal. The
    scanners have a priority mode, but what that does is interupt what even
    the scanner is locked on to sample the priority frequency, which might
    not be active. So you would park say 4 scanners on 4 different
    frequencies, but hear them in the order of your priority.
  10. Guest

    Umm... I gibberished that a bit; the CD4051 is the part, and it's an
    eight-way switch
    (an eight-to-one multiplexer analog switch).

    As for the quiet input ('cuz I just HATE the hum that a high impedance
    on the
    input seems to generate), you can just ground the lowest-priority
    channel, and
    strap its detection bit to always ON. That loses one channel (so it's
    a seven-way
    solution). You can also use the group select output on the priority
    encoder; if no
    input is detected, it goes low, and you can use that to turn on a (for
    photoresistor-type optocoupler to ground the output and mute the
  11. PPP

    PPP Guest

    Thanks for the info all! I haven't managed to fully grasp most of the
    other tips. But I was wondering if something like this can work:

    I figured I can just redirect the other signals to ground via a BJT
    transistor if a priority signal is present. Also, I've been playing
    with the PIC microcontroller before, and I think I'll be able to use it
    along with the MAX4562 clickless audio switch.

    Thanks again!
  12. Guest

    I think you need a Schmidt trigger after the rectifier to do the energy

    A uP seems overkill for the application, and it will generate noise.

    Dumb question here, but thinking outside the box, can't you make an
    audio mux using an inverting op amp in the summing configuration? What
    you would do is replace the input resistors with two resistors in
    series, then use an analog gate to pull the unwanted signals to ground
    at the center tap of the resistors. What you are trying to do is set up
    a circuit that doesn't make a click due to charge injection.
  13. PPP

    PPP Guest

    Hi miso,

    You are right about the microprocessor adding noise to the circuit.
    That's why I'm going to have separate ground planes connected at one
    point near the supply. I figured the microprocessor will give me access
    to other functions.

    I don't see anything wrong with a summing amplifier. I was actually
    thinking of using it after the MAX 4562 device.

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