directly, as with an electric guitar. You need
to adjust the sensitivity of the transmitter appropriately; otherwise, it can distort right away
and there’s absolutely nothing you can do to fix
it down the line.
To adjust input trim, have the presenters/per-formers speak or sing in a normal tone. If the
level starts to bounce a little bit while the person
is talking or singing, that’s good. If it sits, or if
it just goes up and hits the top of the audio level indicator, it’s too loud. If they’re talking and
it doesn’t light up, you’ll have to add so much
gain later that you’ll raise the noise floor and
hiss. The same principle applies to instrument
mics. Have the performers play they way they
will during the performance and set the transmitter’s input levels accordingly.
Dealing With Hum
Another very annoying problem with audio is
hum. Usually it’s a 60Hz (in the U.S.) hum from
AC power. It comes when the ground used for
the audio is coupled to different things in different places. This happens, for instance, when
the venue is mixing sound behind the stage and
run a couple mic lines to you at the back of the
house, and you’re also plugged into a circuit
that’s not grounded in the same way the house
audio is. Suddenly, the 60Hz hum crops up and
invades everything. If you hear it when you plug
in, or even touch, the long audio lines into your
mixer, and it goes away as soon as you remove
them, you’ve pinpointed the problem.
The first thing to try is a ground lift adapter.
You should always have at least one of these
in your kit, because they can help you solve so
many different problems. That said, it’s a temporary fix, and not one you should rely on in a
If a ground lift adapter doesn’t solve the problem, try to work on the audio cables themselves.
There are multiple ways of doing this. Simple
accessories such as an XLR adapter can be put
inline on the XLR cables to try and find which
pin is carrying the offending voltage. Alternatively, there are more substantial transformer
devices that completely isolate the in and out,
while still passing audio. These devices can usually resolve the most stubborn hum problems.
Lastly, make sure power cables aren’t taped
down along with the audio cables. That’s also a
great way to pick up extra noise. They should be
separated by at least a few inches if not a foot.
If you’re still working with old 700mHz wireless microphones on frequencies that have been
allocated for other uses—including public service—you should replace these systems as soon
If you’ve moved on to systems that use approved frequency ranges, you’re less likely to
encounter interference, but it can still happen
under certain circumstances. For example, if
you’re filming a lecturer who has his lav belt pack
just a few inches from his cellphone, you’re going
to get RF interference from his phone. He needs
to leave it on the podium, or on a table. The farther the phone is from the microphone, the less
it will affect your signal.
The same goes for your own crew. Don’t put
your phone on the audio mixer, even if you’re
using it to play music into your system. The RF
noise can get into any part of the signal path.
Overmodulated Mixer Input
Another problem that arises frequently when
you’re mixing audio for live events is overmodulation (or overmod) on the mixer input. Several
factors can cause overmodulation, some easy
to avoid, others a bit more subtle. To begin, if
you’re feeding line-level input, don’t plug it into
the mic jack. It will distort. Some mixers have a
universal gain that you can dial down to avoid
clipping. If yours doesn’t, you’ll need an external PAD to knock the line level XLR down to mic
level. These should be in your kit and always
stay with the audio mixer.
On the other hand, if you feed a mic level
source into the line-level jack, you’ll find yourself cranking everything all the way up and
wondering, “Why the heck am I not getting any
sound out of this?” The more you crank things
up, the more hiss you’ll get in your signal path.
Next we’ll look at equalization (EQ) adjustments (Figure 3, next page). Let’s say you want
someone to have a strong, booming presence in
the room. You’ll need to increase the gain of the
low frequencies, noting that the input level you
had previously set is now considerably higher.
The EQ does affect the input level, even though
it technically shouldn’t since it’s further down
in the audio path. But it does, and you have to
go back to the input level and readjust it after
setting your EQ.