Lectrosonics UM500 User Manual download pdf (Page 4)

UM500

GENERAL

GENERAL TECHNICAL DESCRIPTION

The 500 system uses 50kHz wide deviation for an extremely high signal to noise ratio. The switching power supplies

provide constant voltages to the transmitter circuits from the beginning (9.3 Volts) to the end (5.5 Volts) of battery life.

The input amplifier uses an ultra low noise op amp for quiet operation. It is gain controlled with a wide range dual

envelope input compressor which cleanly limits input signal peaks over 30dB above full modulation.

DIGITAL HYBRID TECHNOLOGY

All wireless links suffer from channel noise to some degree, and all wireless microphone systems seek to minimize

the impact of that noise on the desired signal. Conventional 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.

The Lectrosonics Digital Hybrid system overcomes 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

which can be accomplished only in the digital domain, even though the inputs and outputs are analog signals. (As of

this writing, the patent is still pending, so we cannot reveal detailed information about the algorithm at this time.)

Channel noise still has an impact on received signal quality and will eventually overwhelm the receiver. The Digital

Hybrid simply encodes the signal to use a noisy channel as efficiently and robustly as possible, yielding audio

performance that rivals that of wholly digital systems, without the power and bandwidth problems inherent in digital

transmission. As always, these advantages come at a cost. The Digital Hybrid system requires fairly intensive digital

processing in both the transmitter and the receiver. These processors cost money, take up space and consume

power. The Digital Hybrid system also requires that the underlying RF link be of excellent quality, with better fre-

quency response and distortion characteristics than that required by conventional systems.

Because it uses an analog FM link, the Digital Hybrid enjoys all the benefits of conventional FM wireless systems,

such as excellent range, efficient use of RF spectrum, and long battery life. However, unlike conventional FM sys-

tems, the Digital Hybrid has done away with the analog compandor and its artifacts.

NO PRE-EMPHASIS/DE-EMPHASIS

The signal to noise ratio of the 500 system is high enough to preclude the need for conventional pre-emphasis (HF

boost) in the transmitter and de-emphasis (HF roll off) in the receiver. Pre-emphasis and de-emphasis in an FM

radio system usually provides about a 10dB improvement in the signal to noise ratio of the system, but the high

frequency boost in the transmitter must be removed in a purely complementary manner or else the frequency

response of the original audio signal will be altered.

Pre-emphasis can also cause distortion in the receiver. As this signal is passed through the IF filters in the receiver,

distortion can be produced, most noticeably at full modulation. De-emphasis cannot be applied until the signal is

converted into audio, so there is no way around this problem short of eliminating pre-emphasis altogether. Neither of

these problems occur in the 500 system

PILOT TONE SQUELCH

The 500 system utilizes one of 256 different ultrasonic tones between 25 and 32 kHz, that modulate the carrier to

operate the receiver squelch. The pilot tone frequency is chosen according to which of the 256 channels has been

selected by the frequency switch setting. The basic benefit of the pilot tone squelch system is that the receiver will

remain muted until it receives the pilot tone from the matching transmitter, even if a strong RF signal is present on

the carrier frequency of the system. The UM500 extends this concept even further by insuring that all transmitters in

a system have different pilot tone frequencies so that even spurious RF from the wrong transmitters can’t open the

receiver squelch.

WIDE-BAND DEVIATION

±50kHz deviation improves the capture ratio, signal to noise ratio and AM rejection of a wireless system dramatically,

compared to the more commonly used ±15kHz deviation.

LECTROSONICS, INC.

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Lectrosonics UM500 User Manual Page 4

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