Lectrosonics T195 - Manual (Page 3)

Hand-held Transmitter
GENERAL TECHNICAL DESCRIPTION
The T195 / UT195 transmitter is comprised of a number of functional sub-systems as shown in the
block diagram below.
The T195 and UT195 utilize 75kHz deviation for compatibility with the 195 series receivers. The transmitter circuits
are all regulated to allow full output power from the beginning (9 Volts) to the end (7 Volts) of battery life. The oscilla­
tor crystal is shock mounted to provide ruggedness. The input amplifier uses a Motorola 33079 op amp for ultra low
noise operation and is gain controlled with a wide range input compressor which cleanly limits signal peaks up to
30dB above maximum modulation.
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 slower, low frequency
audio distortion will be low, but high frequency transients will then be distorted. The 195 system introduces an
entirely new approach to solving this basic problem, called “dual-band companding.”
There are actually two separate compandors in the 195 system, one for high frequencies and one for low frequen­
cies. A crossover network separates the frequency bands at 1kHz with a 6dB per octave slope, followed 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 con­
stants, reducing low frequency distortion to well below that of a conventional compandor.
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. As this signal is passed through the
IF filters in the receiver, distortion can be produced, most noticeable at full modulation. 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.
MIC
ELEMENT PREAMP INPUT
Vref
BASS
TREBLE
LP
FILTER
HP
FILTER
COMPANDOR
Vreg
Vreg
+5VDC
+3.6VDC
XTAL OSC
X3 X2 X2
XTAL OSC
X4 X2 X2 X2
UHF TRANSMITTER
VHF TRANSMITTER
PWR MUTE
+9VDC
AMP
AUDIO
LEVEL
LIMIT
LED
SET
LED
LP
FILTER
PEAK AUDIO
INDICATOR &
LIMITER
DRIVER
PILOT
TONE
OSC
COMPANDED AUDIO
TO XMTR
SWITCH
DISABLE
LED
DISABLE
PWR
LED
COMPANDED
AUDIO
COMPANDED
AUDIO
Rio Rancho, NM – USA 3