3
Digital Frequency Agile UHF Hand-held Transmitter
Rio Rancho, NM – USA
GENERAL TECHNICAL DESCRIPTION
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.
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.
Phase
Locked
Loop
Freq
Switches
11001001
A-D
Converter
Digital Signal Processor
Shunt
Limiter
Bicolor
Modulation
LEDs
Microprocessor
9V
Battery
Switching
Power
Supply
+3.3v
+1.8v
+9v
-3v
50
Isolator
Audio
Bicolor
Power
LED
Audio
Level
Input
Amp
LF
Rolloff
Digital
Modulator
RF
Amp
Dual
Envelope
Limiter
Encode
Encrypt
Voltage
Controlled
Oscillator
Encryption
Key Link
Preamp
Mic
Element
Preamp
Level
Tone
