No Pre-Emphasis/De-Emphasis; Pilot Tone Squelch; Input Limiter; Wide-Band Deviation - Lectrosonics UM400 Operating Instructions Manual

Frequency-Agile UHF Belt-Pack Transmitter

NO PRE-EMPHASIS/DE-EMPHASIS

The signal to noise ratio of the 400 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 10 dB 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 400 system

PILOT TONE SQUELCH

The 400 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 UM400 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.

INPUT LIMITER

The 400 series transmitters employ a digitally-controlled analog audio limiter just before the analog-to-digital con-
verter. 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 distortion. It can be thought of as two limiters in series,
connected as a fast attack and release limiter followed by a slow attack and release limiter. The limiter recovers
quickly from brief transients, so that its action is hidden from the listener, but recovers slowly from sustained high
levels, to both keep audio distortion low and preserve short term dynamic changes.
The audio level LEDs indicate limiter activity. The first red LED indicates that the limiter is active and that the trans-
mitter is fully modulated (audio level is between +0 and +10 dB). The second red LED indicates that the level is 10
dB or more into limiting. Occasional forays into the red are desirable for most applications, since the distortion
introduced by the limiter is so minimal, and full modulation is thus assured. We strongly recommend setting the gain
of the transmitter high enough so that the first red LED occasionally lights.
Generally speaking, some limiting is desirable in normal operation to improve the signal to noise ratio of the system.
The limiting action is not audible and does not create distortion. A highly trained ear would hear only the compres-
sion of the peaks in the audio signal, which is desirable with most recorders and many sound reinforcement systems.

WIDE-BAND DEVIATION

± 75 kHz deviation improves the capture ratio, signal to noise ratio and AM rejection of a wireless system dramati-
cally, compared to the more commonly used ±15 kHz deviation.

LONG BATTERY LIFE

Switching power supplies throughout the design allow over five hours of operation using a single 9 VDC alkaline
battery. (A 9 V lithium battery will provide over 10 hours of continuous operation.) The battery contacts are spring
loaded to prevent "rattle" as the unit is handled.

FREQUENCY AGILITY

The transmitter section uses a synthesized, frequency selectable main oscillator. The frequency is extremely stable
over a wide temperature range and over time.
Two rotary switches, located on the side panel of the unit, provide 256 frequencies in 100 kHz steps over a 25.5 MHz
range. This alleviates carrier interference problems in mobile or traveling applications.

ANTENNA

The antenna on the UM400 consists of a flexible 1/4 wavelength galvanized steel cable, detachable via an SMA
connector. The impedance of this connector is 50 Ohms.
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