General Technical Description - Lectrosonics UM700 Operating Instructions Manual

Encrypted digital uhf belt-pack transmitter

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Instruction manual (36 pages)

Quick start manual (2 pages)

Operating instructions and troubleshooting manual (18 pages)

Table Of Contents

Table of Contents

UM700

GENERAL

The 700 series encrypted digital wireless microphones use an all-digital communications link for excellent sound

quality and data security.

In the transmitter, the audio first passes through a DSP-controlled dual envelope analog limiter. The signal is then

digitized and fed to a DSP (digital signal processor). The DSP uses a proprietary audio encoding scheme to lower

the bit rate and provide the high entropy required for secure encryption. The bit stream is then encrypted, appor

tioned into packets, and sent over the air using a proprietary digital modulation scheme.

In the receiver, the digital baseband signal is demodulated to recover the original bit clock and data stream. The

DSP separates out the packet headers and decrypts the audio data. The audio data is then decoded to recover the

original audio signal.

+3.3V Bias

Mic

Supply

Jack

Hi/Lo

Pass

Filter

Encryption

Key Link

Bicolor

Modulation

LEDs

Phase

Locked

DSP-CONTROLLED DUAL ENVELOPE ANALOG LIMITER

In order to make the very best use of the high quality A/D converter, microphone audio is limited in the analog

domain first before being sampled. The DSP controls this process, but because the limiting is done in analog, levels

near the converter's maximum may be used without fear of clipping.

The limiter has a fast attack, but different release characteristics, depending on the nature of the signal that drove

the input into limiting. Brief transients result in a fast decay, to avoid "pumping" effects, while sustained loud signals

result in a slower decay, to keep distortion to a minimum. The result is a transparent-sounding limiter with excellent

low distortion characteristics.

DIGITAL SIGNAL PROCESSING AND MODULATION

The preamplified and limited audio signal is converted to digital using a 24-bit A/D converter and fed to the DSP.

Within the DSP, the audio is encoded to reduce the bit rate and increase entropy in the data stream prior to encryp

tion. The data stream is then encrypted and apportioned into packets, delimited by packet headers. The complete

bit stream is modulated onto the carrier using a modified pi/4 DQPSK (differential quadrature phase shift keying)

method. This modulation method makes efficient use of the RF spectrum and is easy to demodulate reliably.

RF OUTPUT SECTION

Intermodulation (IM) occurs in the final amplifier stages of conventional transmitters when the transmitters are within

a few feet of each other. This can create serious problems in multichannel wireless systems when an IM signal falls

on the carriers, IF frequencies, local oscillator and image frequencies of the systems being operated. To eliminate

this problem in the UM700, the modulated radio signal passes through a circular isolator before entering the an-