General technical description, General, Dual band compandor – Lectrosonics UM300b User Manual
Page 4: Um300b block diagram
GENERAL TECHNICAL DESCRIPTION
The UM300B transmitters are comprised of a number of functional sub-systems as shown in the block diagram
below.
GENERAL
The 300 system uses 50kHz wide deviation for an extremely high signal to noise ratio. The transmitter circuits are all
regulated to allow full output power from the beginning (9 Volts) to the end (6.5 Volts) of battery life. The input ampli
fier uses a Motorola 33178 op amp for ultra low noise operation. It is gain controlled with a wide range input com
pressor which cleanly limits input signal peaks over 30dB above full modulation.
DUAL BAND COMPANDOR
Traditionally, compandors have been a source of distortion in wireless microphone systems. The basic problem with
conventional systems is that the attack and decay times are always a compromise. If the time constants are fast,
high frequency transients will not be distorted, but this will cause low frequency distortion. If the time constants are
slower, low frequency audio distortion will be low, but high frequency transients will then be distorted. The 300
system introduces an entirely new approach to solving this basic problem, called “dual-band companding.”
There are actually two separate compandors in the 300 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 transient distortion at a low level, and the low frequency compandor uses slower time con
stants, reducing low frequency distortion to well below that of a conventional compandor.
TRANSMITTER
Vref
BASS
TREBLE
LP FILTER
HP FILTER
LIMIT
LED
COMPANDOR
Vreg
Vreg
+5VDC
+3.6VDC
SHUNT
LIMITER
INPUT
AMP
AUDIO
LEVEL
LP
FILTER
PEAK AUDIO
INDICATOR &
LIMITER
DRIVER
PILOT
TONE
OSC
5
4
3
2
1
MIC
JACK
+5V BIAS
SUPPLY
LF
ROLL-OFF
COMPANDED AUDIO
TO XMTR
COMPANDED
AUDIO
VOLTAGE
CONTROLLED
OSCILLATOR
FREQ
SWITCHES
LOW PASS
FILTER
BUFFER
PWR
DELAY
+9VDC
PWR
LED
SYNTHESIZER
uP
E PROM
2
SET
LED
UM300B Block Diagram
4