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Glossary139
environments such as television studios (news). Impedances: Impedances of headphones range
typically from 2000 ohms to 2 ohms. Common impedances per earphone are 300 ohms, 150
ohms, 50 ohms, 25 ohms. Headphone total impedances depend on the earphone impedance and
whether they are connected in series or parallel. The headphones in standard headsets sold by
RTS™ Systems ranges from 25 ohms to 300 ohms. Military headphones may be very low
impedance, 10 ohms or less. A fuel tank entry system sold by RTS™ Systems has 2000 ohm
headphones. Lower impedance headphones allow a louder sound (up to 110 dB SPL) to be
generated with relatively low voltage in the user station (say 12 volts DC). Microphone Types:
(for headsets) The microphones types may be carbon, carbon emulate, dynamic, electret. The
carbon types produce high output levels but have higher distortion, the carbon emulate types, put
out high levels with low to moderate distortion but require special electronics and a way to power
the electronics. The electrets usually have electronics built on the microphone, but there is no
voltage gain from this electronics, just impedance matching (from megohms to kilohms).
Electrets have about 10 dB more level than dynamics, but are very prone to “popping”. To
prevent popping, windscreens need to be installed or placed over the microphone element, and the
following circuit should have a circa 500 hertz high pass roll off. Dynamic and electret
microphones usually have low distortion and good frequency response (100 to 8,000 hertz). Some
dynamic microphones made with low technology may have poor frequency response. Some
typical microphones impedances are as follows: carbon: small button 600 ohms, large button, 50
ohms; dynamic: 2 ohm (military), 150-200 ohm (RTS™ Systems recommends), 600-1000 (lower
cost push-to-talk and others). Microphone impedances can also be higher such as 50 kilohms, but
these usually are not on headsets. Most RTS™ Systems User Stations microphone inputs allow
for an impedance range of 50 to 1000 ohms for dynamic microphones, 1000 to 2000 ohms for
electrets, 50 to 200 ohms for carbon or carbon emulate.
Headroom
The difference between the instantaneous level of a signal and the peak signal possible in a given
system. Headroom is often expressed in decibels. System headroom in the TW system is about
eight to ten dB. Headroom for the microphone input is an apparent 40 dB because of the 30 dB
limiter compression ratio. Because of the design of the TW and 800 series systems, and the
consistency of levels, the peak to average speech ratio is close to 10 dB.
Hertz
The unit of frequency, cycles per second. One thousand hertz equals one kilohertz equals one
thousand cycles per second.
Home Run
Running the user station system connection cables to a central point (as opposed to Daisy
Chaining).
Hot
1. A wire actively carrying power or signals. 2. Equipment that is turned on, for example a “hot”
microphone.
Hum
Hum is an interfering addition to audio. Its frequency is within that of human hearing and it is at
the frequency of the power line or its harmonics. For example: a pickup of the fundamental will
result in a 50, 60, or 400 hertz tone in the audio. If the hum is due to excess ripple in a full wave
rectified supply the frequency will be 100, 120, 800 hertz. If the power line waveform is distorted
(which it often is), other harmonics will be heard. Hum is induced electrostatically via unshielded
wires in high impedance circuits, or electromagnetically via unshielded dynamic microphones,
transformers, tape recorder heads, or ground loops.
Hypercardioid
A microphone pick-up pattern. This pattern has its maximum rejection at 100° off axis. This
pattern has good rejection of far field sound and room reverberation. Good in house speaker
systems.
I
I
Symbol used to designate current.
IBEW
International Brotherhood of Electrical Workers
IFB
The IFB * System is a special intercom system used for television shows with highly flexible
formats or where important program changes are likely, for example, newscasts or special events
telecasts. The IFB system connects control room personnel such as the director, producer, audio
mixer, and technical director directly with the performers or “talent”. The performer wears a