The 100 system uses 20 kHz wide deviation for a high signal to noise ratio. The transmitter circuits are all regulated to allow full output power from the beginning (9 Volts) to the end (6.5 Volts) of battery life. The input amplifier uses a Motorola 33078 op amp for ultra low noise operation. It is gain controlled with a wide range input compressor which cleanly limits input signal peaks over 30dB above full modulation.
DUAL BAND COMPANDOR
Traditionally, compandors have been a source of distortion in wireless microphone systems. The basic problem with conventional systems is that the attack and decay times are always a compromise. If the time constants are fast, high frequency transients will not be distorted, but this will cause low frequency distortion. If the time constants are slower, low frequency audio distortion will be low, but high frequency transients will then be distorted. The 100 system introduces an entirely new approach to solving this basic problem, called “dual-band companding.”
There are actually two separate compandors in the 100 system, one for high frequencies and one for low frequencies. A crossover network separates the frequency bands at 1 kHz with a 6 dB per octave slope, followed by separate high and low frequency compandors. The attack and release times in the high frequency compandor are fast enough to keep high frequency transient distortion at a low level, and the low frequency compandor uses slower time constants, reducing low frequency distortion to well below that of a conventional compandor.
The signal to noise ratio of the 100 system is extended by using 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 complimentary manner or else the frequency response of the original audio signal will be altered. The dual-band compandor in the 100 system essentially provides a dynamic pre-emphasis/de-emphasis function with low distortion.
LONG BATTERY LIFE
High efficiency circuits throughout the design allow over 4.5 hours of operation using a single 9 Volt alkaline battery. (A 9V lithium battery will provide over 14 hours of operation.) The battery compartment is a unique mechanical design which automatically adjusts to fit any brand of battery. The battery contacts are spring loaded to prevent “rattle” as the unit is handled.
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 100kHz steps over a 25.5MHz range. This alleviates carrier interference problems in mobile or travelling applications.
At UHF frequencies, where wavelengths and antennas are shorter than at VHF frequencies, a resonant length wire is preferred over using the microphone cable as the antenna. The antenna on the UM100 consists of a flexible 1/4 wavelength bronze cable.