Lectrosonics Inc UH400E Manuel D’Utilisation

5

Input Limiter

The Digital Hybrid Wireless

™

 transmitters employ a 

digitally-controlled analog audio limiter just before the 

analog-to-digital 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 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 dy-

namic changes.
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 compression of the peaks in the audio signal, which 

is desirable with most recorders and many sound rein-

forcement systems.

No Pre-Emphasis/De-Emphasis

The signal to noise ratio of the Digital Hybrid Wireless

™

 

system is high enough to preclude the need for conven-

tional 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 pro-

vides 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 Digi-

tal Hybrid system.

Pilot Tone Squelch

The Digital Hybrid Wireless

™

 system uses 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.             

Wide-Band Deviation

A ±75 kHz deviation improves the capture ratio, signal 

to noise ratio and AM rejection of a wireless system 

dramatically, compared to the more commonly used ± 

15 kHz deviation.

Long Battery Life

Switching power supplies throughout the design allow 

over 4.5 hours of operation using a single 9 Volt alka-

line battery.  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.