Lectrosonics UDR195 - Manual (Page 5)

Wireless Diversity Receiver
T195 TRANSMITTER
The T195 hand-held transmitter design was the result of
considerable research. The RF and audio performance of the
transmitter was considered first, followed by an analysis of the
typical user’s needs and the practicality of various design
possibilities. The basic circuitry had to accommodate any
frequency in the VHF or UHF spectrums. The mechanical
design had to provide a comfortable “feel,” yet be rugged,
foolproof and easy to operate. The operating features and
functions faced several contradictions in the needs of different
types of users. This led to the development of a very versatile
design wherein the transmitter can be configured for either
“fool proof” operation or to provide user control of the audio
level, metering and indicators. The final T195 design com­
bines the benefits of superior performance, ruggedness , user
convenience, and flexibility.
M195 TRANSMITTER
The M195 is a small belt-pack style transmitter which can be
clipped on the belt, slipped into a pocket, or even taped to the
user's body under clothing. It offers the same high perfor­
mance and wide deviation as the T195 and is compatible with
all of Lectrosonics' 195 series receivers. The M195 comes
with a standard lavalier microphone but practically any mic
can be adapted to work with this transmitter. The transmitter
audio output can easily be monitored and set from the from
panel.
DUAL-BAND COMPANDOR
Compandors have long been a source of audible distortion in
wireless microphone systems. The basic problem is that when
the full bandwidth of the audio signal is processed by a single
compandor, the attack and decay times will always be a
compromise. If the time constants are fast, high frequency
distortion will be low, however, faster time constants in the
compandor create distortion of lower frequencies. If the time
constants are slowed down, low frequency audio distortion
will be low, but high frequency transients will then be dis­
torted. The 195 system introduces a new approach to solving
this basic problem with compandors, called “dual-band
companding.”
There are actually two separate compandors in the 195 system,
one for high frequencies and one for low frequencies. A
crossover network ahead of the compandor separates the
frequency bands at 1kHz with a 6dB per octave slope, fol­
lowed by separate high and low frequency compandors. The
attack and release times in the high frequency compandor are
fast enough to keep high frequency distortion at an extremely
low level. The low frequency compandor uses slower time
constants, reducing low frequency distortion to well below
that of a conventional compandor.
75KHz DEVIATION
75kHz of deviation improves the capture ratio, signal to noise
ratio, and AM rejection of a wireless system dramatically.
NO PRE-EMPHASIS/DE-EMPHASIS
The signal to noise ratio of the 195 system is high enough to
preclude the need for conventional pre-emphasis (HF boost) in
the transmitter and de-emphasis (HF roll off) in the receiver.
Pre-emphasis and de-emphasis in an FM radio system usually
provides about a 10dB improvement in the signal to noise ratio
of the system, but the high frequency boost in the transmitter
must be removed in a purely complementary manner or else
the frequency response of the original audio signal will be
altered.
Pre-emphasis can also cause distortion in the IF filtering stage
in the receiver, since the high frequencies have been boosted,
which increases the level of the harmonics in the FM signal.
As this signal is passed through the IF filters in the receiver,
distortion will be produced, most noticeable at full modula­
tion. De-emphasis cannot be applied until the signal is
converted into audio, so there is no way around this problem
short of eliminating pre-emphasis altogether. Neither of these
problems occur in the 195 system. The dual-band compandor
in the 195 Series system essentially provides a dynamic pre-
emphasis/de-emphasis function with extremely low distortion.
EXCEPTIONAL THERMAL STABILITY
Thermal stability over a wide temperature range is a funda­
mental requirement for any wireless microphone system
intended for professional use. Thermal drift is a major
problem for some wireless systems that are highly recognized
as “high-end” or “high-performance” products. These systems
actually suffer from thermal drift problems that cause as much
as a doubling or tripling of distortion and as much as 5dB
fluctuations in audio output level as the temperature varies.
If temperature shifts cause the oscillators to drift, or values to
change in the detector, serious distortion will result. The
components in the 195 Series systems meet very stringent
tolerances for thermal drift. System distortion in the 195
series remains at very low levels over a very wide temperature
range. This is especially important in applications where the
receiver and/or transmitter must be operated near heat generat­
ing devices, outdoors in direct sunlight, or with the receiver
mounted in an equipment rack.
Rio Rancho, NM – USA 5