Multiple frequencies on one output audio feed

[dallas75248]

When listening to activity at a large airport, e.g., SLC, how do I hear a specific frequency such as ground control or approach control?  Are they all piggybacked onto one outgoing audio signal that we hear?  If so, do you programatically delay them to serially step them into the outgoing audio stream or do they simply step on each other (loudest wins as in simplex radio)?

[Biff]

Some feeds are set up with different audio sources (freqs) on the left and right channels.  To listen to one or the other exclusively, simply set your audio balance fully left or right.

Most feeds are set up with multiple freqs on a single audio channel.  These are generally fed by radio scanners.  The scanner is programmed to listen to one or more frequencies.  It will step through each freq, listening for a transmission.  Depending on the scanner model, this could be at the rate of anywhere from 10 to 100 steps per second.  When it 'hears' a transmission, it stops on that freq until the transmission ends.  Some scanners are programmed to continue listening on that freq for a short time after the transmission ends, expecting that there will be a reply. 

So in effect, it's "first come first served" when it comes to what you hear on the feed.  If the scanner is stopped on one freq listening to a transmission and someone keys up on another freq, you won't hear the second conversation.