Audio Delay Modules
By Steve Strobel, KC7HXB
steve@link-comm.com
http://www.link-comm.com/faq/audio_delay.txt
Last revised 5-23-01

The audio delay modules don't add any functionality, but they can make the
output of the repeater nicer to listen to.  The three main benefits are
elimination of squelch tails, complete muting of DTMF digits and less of a
gap in the audio if the DTMF decoder falses.  In some cases they can also
help you avoid missing the first word someone says if they talk too soon
after keying up.  More complete descriptions of the first three benefits
follow:

1.  When someone that is using the repeater unkeys, the receiver will
usually output squelch noise for a few tens of milliseconds before the
squelch circuit determines that the signal is gone and deasserts the COR
line.  If there is no delay in the system, that squelch noise is
transmitted before the controller (which is watching the COR line) can mute
it.  If an audio delay module is used to delay the audio before the muting
circuit in the controller, the COR line will be deasserted and the
controller can start muting before the squelch tail gets through the audio
delay.  The result is that you won't hear a squelch tail each time someone
that is using the repeater unkeys.  You will still hear a squelch tail at
the end of a conversation when the repeater hang timer expires and the
repeater transmitter drops (an audio delay at the repeater can't do
anything about that).

2.  Most people turn on the DTMF mute feature of their repeater controllers
so that when someone enters DTMF tones to send a command to the repeater
controller, the DTMF tones aren't retransmitted on the repeater output.
Sometimes they also turn on a "cover tone" so the controller will send some
beeps while the DTMF tones are being entered so anyone listening will know
that the repeater isn't just keyed up for no reason.  If the repeater
controller does not have an audio delay, when the first DTMF tone is
entered, 40mS or so of it will be retransmitted before the controller
detects it and starts the mute and cover tone.  If the controller does have
an audio delay, it can start the mute and cover tone before the DTMF tone
gets through the audio delay, so even the beginning of the first digit is
completely muted.

3. When no audio delay is used, the controller is usually programmed to
continue muting for a couple of seconds after each DTMF tone is released
because it is likely that another tone will be entered soon.  That keeps it
from allowing the beginning of every DTMF digit to be repeated;  once the
first digit is detected, all of the digits will be completely muted as long
as you don't pause more than a couple of seconds between them.  A negative
side effect of this several-second long mute is that if someone's voice
"falses" the DTMF decoder (if the decoder thinks their voice sounds like a
DTMF digit and turns on the mute), the controller will mute their next few
words.  If an audio delay is used, the controller can be programmed to quit
muting as soon as the DTMF digit goes away because the beginning of the
next digit (if it ever comes) will be completely muted anyway.  With the
short mute timer, if someone's voice falses the DTMF decoder, their voice
will be muted for only a fraction of a second rather than several seconds.
For more information about DTMF (voice) falsing, see
http://www.link-comm.com/faq/dtmf_voice_falsing.txt.

There are cases where using an audio delay module is not desirable.  The 
most notable of these is a link system with many hops.  Audio delay modules
can be used on the repeater receivers in such a system, but if they are 
used on all of the link receivers, the cumulative delay as audio passes
through the links can cause confusion, similar to international phone calls
that have a lot of delay.  A better way of reducing the length of squelch
tails on such a link system is to use a really fast squelch circuit, such
as the one in the Motorola Micor.  Such a squelch circuit can be added to
other receivers (Link Communications, Inc. sells a small board called the 
RLC-MOT with the same circuit that is used in the Micor).