-
Thread: Lighting Desks creating
RF interference in wireless mic receivers
-
-
- From: Curt Taipale (Taipale Media Systems, Inc.)
- Subject: digital interference
-
-
- Hi Y'all!
-
- I've run into a problem that's new to me, and would appreciate
all comments.
- On a recent installation, one of our techs discovered
that the wireless mic
- systems (Audio Technica 1100 series) were picking up
a surprising amount of RF
- when the transmitters were powered down. The wireless
system rack was located
- at the house mix position. After a little investigation,
they found that
- moving around the lighting control cable coming out of
the digital lighting
- desk caused the RF lights on wireless receivers to dance.
-
- Our ultimate solution was to relocate the wireless receiver
rack to the stage.
- If I remember right, the lighting desk was a Leprechon
(sp?) product. This is
- not the first time our installers have found this problem.
Does anyone in the
- group have any solutions other than moving the wireless
receivers? Are there
- lighting desks that are better shielded that wouldn't
likely create such a
- problem, or is this similar in nature to the interference
one might expect
- when putting wireless receivers in a rack with digital
effects processors?
-
- Many thanks!
-
- Curt
- curt@churchsoundcheck.com
-
- ------------------------------------------------------------------------------
-
- From: Thundercraft (Las Vegas, NV)
- Subject: Re: digital interference
-
- Our AT-1031 also can have the meters light up when the
transmitter is
- turned off. Besides the effects processor (which we has
discussed), the
- motors from the cassette decks (next to the receiver)
are also picked up
- in the meters, although no detectable noise in the system.
-
- Besides coming under the RF banner, it also comes under
the heading
- "Electro-Magnetic Radiation" which is another
topic unto itself.
-
- As discussed earlier, you could try relocating the antena's.
-
- Rob
-
- ------------------------------------------------------------------------------
-
- From: Steve Barbar
- Subject: digital interference
-
- Curt:
-
- It's definitely not a good idea to place wireless receivers
near digital
- stuff. Any digital processor generates RF. If the receiver
has very good
- front end rejection (4th order filtering), this problem
is minimized. It is
- also not a good idea to have inexpensive wireless receivers
near each other,
- or terminate the antennas to a single dipole. The local
osc. in each
- receiver generates RF that is not always filtered well
in inexpensive
- designs. Hence, you can get intermod distortion from
the receivers that is
- not easily overcome when the transmitter is electrically
distant from a
- receive antenna.
-
- Best,
- Steve
-
- ------------------------------------------------------------------------------
-
- From: Jim Brown (Chicago, IL)
- Subject: Re: digital interference
-
- <<Are there lighting desks that are better shielded
that wouldn't
- likely create such a problem, or is this similar in nature
to the
- interference one might expect when putting wireless receivers
in
- a rack with digital effects processors?>>
-
- This is yet another head of the same EMI snake, Curt.
One
- thing to look for is FCC Class B per Part 15 on all of
the
- elements of the interfering equipment, which means that
it
- meets the most stringent FCC standards for
- non-interference. Unfortunately, most lighting equipment
- vendors have taken the position that because their
- equipment is not used in residential settings, it is
not
- subject to Class B (which is legally true).
-
- We take the position that because lighting equipment
will
- be used around wireless mics, we require (via our
- specification language for the construction of the
- building) that all computer equipment and digital equipment
- meet Class B. Thus it takes the force of contract, but
not
- the force of FCC regulation.
-
- Another good indication of compliance with
- anti-interference standards is the CE sticker, which
is
- required by the European Union. This standard is roughly
- comparable to but somewhat more strict than the FCC Class
B
- standard.
-
- There is another way that interference can get out to
- bother us. It is via the many interface connectors to
- external accessories, like video monitors, mice, control
- interfaces, and the like. Here, you can help yourself
by
- using properly shielded interface cables and accessories.
- Shielded printer, video, and serial cables can cost
- anywhere from half again more to twice as much as standard
- cables, and are available in better computer stores.
This
- is one place where spending more money for cables IS
a good
- idea.
-
- Jim Brown
- Audio Systems Group, Inc.
- Chicago
-
- ------------------------------------------------------------------------------
-
- From: Barry Birdwell (Nashville, TN)
- Subject: Re: digital interference
-
- Curt, since you have dropped the carrier to your receiver
by powering
- down the transmitter, your receiver has scanned for another
carrier
- and found some spurious oscillation and locked on to
that. As others
- have pointed out this could be anything from a cassette
player to a
- digital delay to a radio station.
-
- I would recommend using the mic mute switch during the
service and
- once it is all over turn off the receiver first.
-
- Barry
- Birdwell Acoustics
-
- ------------------------------------------------------------------------------
-
- From: Curt Taipale (Taipale Media Systems, Inc.)
- Subject: Re: digital interference
-
- <<I would recommend using the mic mute switch during
the service and
- once it
- is all over turn off the receiver first.>>
-
-
- Barry & Y'all!,
-
- I've taught and practiced that rule for many years. It
clearly is the safest
- way to operate. In this case, I didn't personally get
to experience what our
- installers discovered, but apparently even when the transmitters
were powered
- up there were interference problems. That's what caused
them to go looking
- for the source of the problem in the first place. It
took them - and me - by
- surprise.
-
- Blessings!
-
- Curt
- curt@churchsoundcheck.com
-
- ------------------------------------------------------------------------------
-
- From: Bob Lewis (Garrisonville, VA)
- Subject: Re: digital interference
-
- The FM receivers in wireless mike systems are "captured"
by the strongest
- signal on the frequency. When the wireless mike is transmitting
an RF signal
- it will generally be the strongest signal and block interferrence.
When the
- mike transmitter is off, the receiver is more susceptable
to any interference
- on frequency. Mute the mixer channel any time the wireless
mike is off.
-
- ------------------------------------------------------------------------------
-
- From: Glen Farrell (Los Angeles, CA)
- Subject: Re: digital interference
-
- Curt,
-
- Does it use the DMX (usually on a mic type cable) control
signals? One
- of the things that you may be running into is poor shield
termination on
- the part of the dimmer and lighting console.
-
- This _will_ turn the cable into a fairly efficient antenna
if it is any
- useful length at all. This is especially true if the
digital device(in
- this case the lighting console) has a "pin 1 problem"
(they exist
- outside of audio as well). This radiated signal may be
overloading the
- front end of the receiver (kind of like when the kid
down the street put
- a 1000W power amp on his CB radio and transmitted to
everyone's TV).
- Since digital signals are rich in harmonics, it probably
has enough
- field strength to be received. During normal operation
the wireless
- mics will easily overcome the relatively small signal
level radiated
- from the cable shield.
-
- What _can_ happen is that the digital noise (which is
of a higher
- frequency) doesn't get shunted to chassis effectively
and thus it will
- radiate. In this case, an improperly terminated shield
_can_
- exacerbate the situation. What ends up happening is that
at the higher
- radio frequencies, a short or open at the other end can
look to the
- noise signal like a short or open and the shield will
tend to radiate.
- The fact that moving the cable made a difference would
seem to point in
- that direction.
-
- One thing that you might try is, if possible bring the
shield on the
- lighting cable(if present) to chassis GND on the board
as close to the
- point of entry as possible.
-
- Comments that others have had about requiring Class B
FCC certification
- and CE marking are also significant, but you may want
to attack it as a
- "pin 1 problem"
-
- Hope this helps,
-
- Glen Farrell
- JBL Professional
-
- ------------------------------------------------------------------------------
-
- From: Dave Meyer (Los Angeles, CA)
- Subject: Re: digital interference
-
- Barry,
-
- I like your suggestion about using the mic mute before
turning off the
- wireless xmitter. An added benefit is that the whole
congregation dosen't
- hear the miked individual turning off his mike (usually
evidenced by a loud,
- low-frequency POP). Your woofs will thank you, too.
-
- Dave
-
- ------------------------------------------------------------------------------
-
- From: Stephen Krone (Westlake, OH)
- Subject: Re: digital interference
-
- You say that moving the cable caused the lights to dance.
Perhaps it is a
- common mode rf signal from the microprocessor circuitry
escaping down the
- outside of the cable shield. This is a very typical problem
with digital
- equipment. The cable just becomes an antenna for rf leakage
if the
- entrance/exit is not treated properly. Often this just
means attaching the
- shield to the case where the cable exits the box, so
that the shield of the
- cable becomes an extension of the case. (Same as Neil
Muncy's "pin 1
- problem".)
-
- Wrapping the cable through a ferrite core several times
often helps. I had
- a computer mouse that interfered with FM radios whenever
it was moved and
- the ferrite fixed that problem. A lot of computer cables
now have a core
- attached to the outside to minimize such problems.
-
- Stephen Krone
-
- ------------------------------------------------------------------------------
-
- From: Dan Bureman (Ames, IA)
- Subject: Re: digital interference
-
- <<I like your suggestion about using the mic mute
before turning off the