Audio Ducking Amp

[rwingraham]

Has anyone had experience with building a ducking amp?

Or know where I can get some schematics or a kit?

The problem:

I have four prioritized audio inputs and one audio output.

The audio inputs are prioritized as follows: 1 is the highest priority, 2 is next highest, etc., 4 is the lowest.

What you want is the highest priority *active* audio input to be gated to the output, squelching the lower-priority inputs until it is done. Then control returns to the next highest audio input, if any.

If no audio inputs are active, then there is no output signal on the audio output.

It has to be automatic, so the circuit has to auto-detect voltage on each audio input and arbitrate access to the output based on the competing inputs' relative priorities.


Applications of this circuit are found in aircraft, ham radio rigs, etc.

In an aircraft, for example, the highest priority channel is the air traffic controller audio input and the lowest is the pilot's CD player... (My application is for ham radio, so not so mission-critical as aircraft operations.)


What's weird is that I can't find anything that does this that isn't expensive/commercial-grade.

I'm trying to build my own on a modest budget, if possible.

Any help would be most appreciated.

Thanks,
Bob

[MrAl]

Hi there,

The circuit can be divided into two basic parts: the priority detector and the audio switcher.

i would have to ask you if you would like to use a microcontroller to do the selection part of the task.
The micocontroller would detect the audio on all four channels and make the necessary decisions to
switch the audio switcher to the right state so the right audio channel gets through.

If you do not want to use a microcontroller, then one idea for the priority detector is to use four
comparators each with their own pot for setting the squelch. Each comparator feeds the input
to a logic ic like the 74LS148, which is a 1 of 8 priority encoder. The output of the 74LS148 then
feeds a CD4051B which does all the switching of the audio from 4 channels in to 1 channel out.
Note you only use 4 of the 8 channels, and only 4 of the 8 possible priorities, but you could
always use the others too if you find that you need more than 4 inputs.
Total device count:
(1) LM339
(1) 74LS148 (or CMOS equivalent)
(1) CD4051B
and of course at least 4 pots and some resistors, a few caps.

The microcontroller version would feed the CD4051B and all the squelch levels could be
programmed internally, or set with push button switches by the user, depending on how
much programming you want to do/write.

[rwingraham]

Hi MrAl,

This is cool - thanks for the reply!

I have no problem using a microcontroller - Is there an advantage to using the microcontroller version vs. the disrcrete component version? (e.g., cost, complexity, reliability?)

Thanks,
Bob

[dacflyer]

i have some units that i think are what your looking for.
we used them on jukeboxes, when the jukebox was not playing, then background musik ( radio ) would play thru..when the jukebox was played, the modual would auto switch from the radio to the jukebox, when the audio was done, then it would switch back over to the radio..i suppose you could have 2-3 of them connected in order. i think i have 2-3 units if interested.
or you might be able to find something local

[MrAl]

Hi again,

Assuming you still want to build it yourself, one of the advantages of using a microcontroller
is that you can do everything with two chips: sample the four audio channels, based on priority
decide what channel should get through, then output the signal to the CD4052 or similar to
actually get the audio feed through.
Another advantage is you can avoid using four pots to set the desired 'detect' level by either
hard coding the levels right into the chip or providing a couple push button switches to allow
setting UP/DOWN of the detect level for any given channel.
Another advantage is if everything is set inside the chip (or with the push buttons) probably
no resistors would be required, and no delay caps / timing network either.
Hopefully the audio level for all four channels is high enough to allow detection by the uC
though, or else some amplification might be needed, but i dont think this will be necessary.

[rwingraham]

Thanks MrAl - I definitely want to build one, and since I'm a software engineer by trade, going the microprocessor route is appealing.

[rwingraham]

dacflyer - how much do you want for your jukebox units?

Thanks,
Bob

[dacflyer]

well they was origionally over 100.00 each when i got them, but i have had them sitting around for so long. i'd be happy with 25.00 each.
i think i have 3, 2 for sure. just have to dig them out..lol

[MrAl]

Hi again,


dac:
Wouldnt it be a lot cheaper to build a four channel device? Just a thought.

[dacflyer]

i am not building anything..lol these moduals are just what i happen to have on hand..lol..they were origionally designed for jukeboxes.

[MrAl]

i am not building anything..lol these moduals are just what i happen to have on hand..lol..they were origionally designed for jukeboxes.

LOL back @ya I was talking about the OP building one, not you

[dacflyer]

oh, ok,, i thought you was talking to me (no problem) cause it said...

"dac:
Wouldnt it be a lot cheaper to build a four channel device? Just a thought."

[Robert Reed]

OK, after following this post for a while, I just have to ask - what the h*ll is a "Ducking" amp and how did it ever aquire this name? To my thinking it appears as a "Voting" amp commonly used in radio communications networks.

[dacflyer]

lol. i never heard it called that either.. actually do not know what it is properly called..
but one channel always has priority over the other.

according to the ones i have, they are called "audio activated line level switch"
made by commercial sound & electronics inc.

[MrAl]

Hello,


I have read statements with the phrase "ducking" in them with respect to audio.
An example is in the supermarket, where the background music plays continuously
and now and then an announcer from the store comes on and says something like:
"Hello shoppers, we have a sale on dinosaur eggs from Egypt today in isle five,
buy two and get a free ostrich egg".

The announcement cancels the music for those few seconds it takes to talk
and then comes back on. There is usually a small delay before the music comes
back, like 1 second after the announcer stops talking.

I dont know how or where the term originated. A guess would be that since
one of the definitions of "ducking" is "to lower", i would guess that it is in
reference to the lowering of the music channel to make the announcers
voice more prominent.