Instruction Manual

HMa, HMa-941, HMa/EO1, HMa/E01-B2, HMa/E02, HMa/E06, HMA/E07-941, HMa/X
LECTROSONICS, INC.
4
The HMa transmitter uses a DSP controlled input limiter
featuring a wide range dual envelope design which
cleanly limits input signal peaks over 30 dB above full
modulation. Switching power supplies to provide con-
stant voltages to the transmitter circuits from the begin-
ning (3 Volts) to the end (1.7 Volts) of battery life, and an
ultra low noise input amplifier for quiet operation.
Digital Hybrid Wireless
®
Technology
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 Wireless
®
systems over-
come 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 informa-
tion 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
Wireless
®
simply encodes the signal to use a noisy
channel as efficiently and robustly as possible, yield-
ing audio performance that rivals that of wholly digital
systems, without the power and bandwidth problems
inherent in digital transmission.
Because it uses an analog FM link, Digital Hybrid
Wireless
®
enjoys all the benefits of conventional FM
wireless 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 Wireless
®
design results in a signal-to-
noise ratio high enough to preclude the need for con-
ventional pre-emphasis (HF boost) in the transmitter and
de-emphasis (HF roll-off) in the receiver. This eliminates
the potential for distortion on signals with abundant
high-frequency information.
General Technical Description
Low Frequency Roll-Off
The low frequency roll-off can be set for a 3 dB down
point at 35, 50, 70, 100, 120 and 150 Hz to control
subsonic and very low frequency audio content in
the audio. The actual roll-off frequency will vary slightly
depending upon the low frequency response of the
microphone.
Excessive low frequency content can drive the trans-
mitter into limiting, or in the case of high level sound
systems, even cause damage to loudspeaker systems.
The roll-off is normally adjusted by ear while listening
as the system is operating.
Input Limiter
A DSP-controlled analog audio limiter is employed
before the analog-to-digital (A-D) converter. The limiter
has a range of more than 30 dB for excellent overload
protection. A dual release envelope makes the limiter
acoustically transparent while maintaining low distor-
tion. It can be thought of as two limiters in series, a
fast attack and release limiter followed by a slow attack
and release limiter. The limiter recovers quickly from
brief transients, with no audible side effects, and also
recovers slowly from sustained high levels, to keep
audio distortion low and while preserving short term
dynamics.
Signal Encoding and Pilot Tone
In addition to controlling the limiter, the DSP also
encodes the digitized audio from the A-D converter
and adds an ultrasonic pilot tone to control the re-
ceiver’s squelch. A pilot tone squelch system provides
a reliable method of keeping a receiver output muted
(squelched) even in the presence of significant inter-
ference. When the system is operating in the hybrid
mode, a different pilot tone frequency is generated
for each carrier frequency in 100 kHz increments to
prevent inadvertent squelch problems and simplify
multi-channel coordination.
Microprocessor and DSP
A microprocessor monitors user command inputs from
the control panel buttons and numerous other internal
signals. It works intimately with the DSP to ensure the
audio is encoded according to the selected Compatibil-
ity Mode and that the correct pilot tone is added to the
encoded signal.