On Mon 2012-Jun-11 06:47, Mike Rivers writes:
For our example, we're doing a radio commercial. We want
the music bed up full until the announcercomes in to tell
us about the great deal you can get if you call before
midnight tomorrow.

That's the problem with the analogy, though. You're already
saturating the channel before you add the "announcement".
So, you have to attenuate that signal if you want anything
to come across "above" it.

This illustrates the difference between "mixing" in the
sense of audio production and "summing" which just piles one thing
on to another.

Indeed, now if you listen to some ham radio repeater systems where they do summing often the repeater's identification or other announcements overpower the people actually
*communicating* using the system.

Contrast that with many repeater systems used by law
enforcement wehre they sidestep the mixing vs summing issue
altogether, if dispatcher or a unit needs to use the
frequency and keys the mic in the middle of that automatic
identification the automatic id is immediately silenced so
that the users can actually communicate.

Iow summing doesn't work reliably, so you either have to mix the signals to make sure what you want communicated is
foreground, or mute the program.

Them's the two reliable choices ya got.

Think, instead, of how you would add that announcement over
a soft, slow classical piece. Chances are, you wouldn't
further attenuate it as it would fall into the noise.
Rather, you would mix in your announcement with that signal
unchanged. Probably even attenuating your announcement so
that it isn't overpowering in the context of the other signal!

This is one I wasn't really paying attention to in my recent reply to Don, but the isntruments used in that soft slow
classical piece are also going to have some bearing on what
to do with that announcement or alert. Where that
announcement might cut through that soft slow classical
piece which is made up of female vocal and nylon string
guitar it might not do as well cutting through the string
quartet, even though relative levels are the same. This one could get a bit deep for a man still trying to ingest enough coffee to realize what/if he is <grin>.

I hope this isn't how they do it when there's a fire in the concert
hall. <g>

Usually if they have such a system it's going to totally
preempt other materials. Again repeater systems for public
service come to mind, paging with bg music in large public
spaces, etc. YOur choices are ducking or some other way of
automagically mixing the two which ensures that your
announcement is foreground, or totally muting the program
for the duration of the announcement. Those are the only
methods which will reliably get the announcement or alert to the ears of those who are intended to act upon it. In the
case of two way relay links, called repeaters for those not
familiar with the jargon, the automatic station id of the
link is secondary, the users actually communicating on it
are primary, so that id needs to duck under them, or be
preempted altogether.

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