572 Chapter 16
Quantity and Frequency Coordination. Determine
how many wireless microphones, wireless monitor
systems, intercoms, etc. are required or in use for your
job. With the information you gathered from step one,
you can begin the system design. You now have the
available TV channels and the number of wireless
systems you want to use.
With this know-how you can start the frequency
coordination of your system inside the vacant TV chan-
nels. This is supported by software that is available
from various companies. The key here is to prevent
intermodulation products (unwanted frequencies gener-
ated by harmonic distortion) from interfering with the
wanted frequencies of your wireless systems.
A check in the venue is also necessary. If you have
the chance, scout the location with a spectrum analyzer,
Fig. 16-154. With this tool, you can verify that the
information from the internet is correct. Alternately, you
can scroll through the tunable frequencies of your wire-
less receivers to scan the RF activity in the venue. Many
receivers also have an auto scan function to find open
frequencies. This cross-check is necessary to find out
whether other wireless devices are in use that you do
not have on your list, which could interfere with your
signal during operation.
Tune Your Components. Set your individual transmit-
ters and corresponding receivers to their coordinated
frequencies. Switch on all components and perform a
final test of compatibility. Physically space the transmit-
ters a couple feet apart and at least 10 feet from the
receiving antenna. Listen for any interference. Compati-
bility between components of a system is achieved if the
following requirements are met: each link in a multi-
channel wireless system functions equally well with all
other links active and no single link—or any combina-
tion of multiple links—causes interference.
16.11.11 Future Considerations: Digital Wireless
TransmissionDigital is a buzz word that many presume solves all the
technical issues we face today. More and more digital
equipment, such as mixing consoles, audio signal
processors, and the like, are used for several applica-
tions, as a digital audio signal chain offers many advan-
tages. A digital signal on a wire (i.e., fiber optic cable)
is easier to handle than on a copper wire because 48, 64,
or more audio channels can be transported on one thin
fiber optic cable. If an audio signal is already in the
digital domain, it makes sense to keep it in this domain
as long as possible.
As for digital wireless transmission, a digital wire-
less system is beneficial when the sound, occupied RF
spectrum, and battery lifetime is as good or even better
than an analog system. On top of this, latency (time
delay between input and output) is always a very impor-
tant topic to keep in mind.16.11.11.1 Starting with Sound and the Related Data
RateThe best sound can be expected if there is no audio data
compression used in the wireless system. This will lead
to a very high data rate.- Minimum for 20 kHz audio and approximately 110 dB
dynamic range: 18 Bit × 48 kHz = 0.864 Mbit/s. - Necessary overhead (framing, channel coding) leads
to even higher data rate (factor approximately 1.5
[1.296 Mbit/s]).
When transmitting this high amount of data, it is no
longer possible to use a simple and robust digital modu-
lation scheme like FSK (frequency shift keying), ASK
(amplitude shift keying), or PSK (phase shift keying),
because these concepts will be not able to fulfill the
spectrum mask, d200 kHz of occupied RF spectrum,
defined by the FCC. Even if this constraint didn’t exist,
greater occupied RF spectrum could inhibit large multi-
channel systems.
To improve this, it is necessary to use a more
complex modulation scheme with narrow filtering, Fig.
16-155.
The amplitude and the phase of the transmitted
signal must be very precise when using this approach.
Behind every point of the constellation diagram, a
digital word is deposited, which the receiver has to pick
up and transfer back into an audio signal.
This requires a very linear RF amplifier. This is a
current-hungry device. The unwanted effect is reducedFigure 16-154. Plot of the RF spectrum in Athens outside
the Olympic Stadium (450–960 MHz).
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