Lectrosonics SMDa - Manual (Page 4)

SMDa Series
General Technical Description
Wideband Design
Digital Hybrid transmitters use ±50 kHz wide deviation
for an excellent signal to noise ratio and wide dynamic
range. The DSP controlled input limiter features a wide
range dual envelope design which cleanly limits input
signal peaks over 30 dB above full modulation. Switching
power supplies to provide constant voltages to the trans­
mitter circuits from the beginning (1.5 Volts) to the end
(0.85 Volts) of battery life, and an ultra low noise input
amplifier for quiet operation.
Servo Input
The bias voltage in the input is set by a servo loop that
regulates the DC voltage at the microphone to a user
selectable choice of 2 or 4 Volts. The input can handle
mic bias loads from 1uA to 2000uA while still maintain­
ing full bias voltage regulation. The servo loop also
incorporates a filter that causes it to servo out frequen­
cies below 20 Hz and rolls off the response of the
lavaliere itself to wind noise, thumps and breath pops.
These low frequency excursions are stopped right at
the mic FET so they do not overload early audio stages
in the transmitter.
Digital Hybrid Wireless ® Technology
US Patent 7,225,135
All wireless links suffer from channel noise to some de­
gree, and all wireless microphone systems seek to mini­
mize the impact of that noise on the desired signal. Con­
ventional analog systems use compandors for enhanced
dynamic range, at the cost of subtle artifacts (known as
“pumping” and “breathing”). Wholly digital systems defeat
the noise by sending the audio information in digital form,
at the cost of some combination of power, bandwidth and
resistance to interference.
Lectrosonics Digital Hybrid systems overcome channel
noise in a dramatically new way, digitally encoding the
audio in the transmitter and decoding it in the receiver,
yet still sending the encoded information via an analog
FM wireless link. This proprietary algorithm is not a
digital implementation of an analog compandor but a
technique that can be accomplished only in the digital
domain, even though the inputs and outputs are analog.
Channel noise still impacts received signal quality and
will eventually overwhelm a receiver. Digital Hybrid
simply encodes the signal to use a noisy channel as ef­
ficiently and robustly as possible, yielding audio perfor­
mance that rivals that of wholly digital systems, without
the power and bandwidth problems inherent in digital
transmission.
Because it uses an analog FM link, the Digital Hybrid
system enjoys all the benefits of conventional FM wire­
less systems, such as excellent range, efficient use of
RF spectrum, and resistance to interference. However,
unlike conventional FM systems, it does away with the
analog compandor and its artifacts.
No Pre-Emphasis/De-Emphasis
The Digital Hybrid design results in a signal-to-noise ratio
high enough to preclude the need for conventional pre-
emphasis (HF boost) in the transmitter and de-emphasis
(HF roll off) in the receiver. This eliminates the potential
for extreme distortion on signals with abundant high-
frequency information.
LECTROSONICS, INC. 4