Lectrosonics SMDa User Manual download pdf (Page 5)

Super-Minature Belt Pack Transmitters

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 sub-

sonic and very low frequency audio content in the au-

dio. The actual roll-off frequency will vary slightly depend-

ing upon the low frequency response of the microphone.

Excessive low frequency content can drive the transmit-

ter into limiting, or in the case of high level sound sys-

tems, can 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 recov-

ers slowly from sustained high levels, to keep audio dis-

tortion low and while preserving short term dynamics.

Signal Encoding and Pilot Tone

In addition to controlling the limiter, the DSP also en-

codes the digitized audio from the A-D converter and

adds an ultrasonic pilot tone to control the receiver’s

squelch. A pilot tone squelch system provides a reliable

method of keeping a receiver output muted (squelched)

even in the presence of signiﬁcant interference. When

the system is operating in the hybrid mode, a differ-

ent pilot tone frequency is generated for each carrier

frequency to prevent inadvertent squelch problems and

simplify multi-channel coordination.

Microprocessor, PLL and VCO Circuits

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.

Compatibility Modes

SMDa transmitters were designed to operate with

Lectrosonics Digital Hybrid receivers and will yield the

best performance when doing so. However, due to the

ﬂexibility of digital signal processing, the transmitters

can also operate in various compatibility modes for use

with IFB.

Control Panel

The control panel includes four membrane switches and

an LCD screen to adjust the operational settings. Multi-

color LEDs are used to indicate audio signal levels for

accurate gain adjustment and for battery status.

Wide-Band Deviation

±50 kHz deviation improves the signal to noise ratio and

audio dynamic range of a wireless system dramatically.

Wide deviation combined with a high powered transmit-

ters makes a signiﬁcant improvement in signal to noise

ratio and operating range.

Battery Options and Operating Time

Switching power supplies convert regulated battery

voltages to operate various circuit stages with maxi-

mum efﬁciency. With the variety of alkaline, lithium and

rechargeable NiMH batteries available today in the AA

format, there are many choices to maximize operating

time or minimize cost as needed for any application.

Frequency Blocks

Lectrosonics established a “block” numbering system

years ago to organize the range of frequencies avail-

able from the low 500 MHz band to the upper 700

MHz band. Each block includes 256 frequencies in 100

kHz increments. The block number is part of a simple

formula to derive the frequency. The block number is

multiplied by 25.6 to produce the lowest frequency in

the block. For example, block 27 x 25.6 = 691.200 MHz.

Circulator/Isolator

The RF output circuit includes a one way circulator/isolator

using a magnetically polarized ferrite. This device greatly

reduces the RF intermodulation produced when multiple

transmitters are used in close proximity to one another

(several feet apart). The isolator also provides additional

RF output stage protection.

Rio Rancho, NM

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Lectrosonics SMDa User Manual Page 5

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