OPTIMOD-FM INTRODUCTION
1-3
ates in parallel with the FM peak limiting. The look-ahead limiting is optimized to make
the most of the limited bit-rate codec used in the iBiquity system’s digital channel. By
eschewing any clipping, the HD output prevents the codec from wasting precious bits
encoding clipping distortion products, allowing the codec to instead use its entire bit
budget to encode the desired program material.
15 kHz band-limiting on the digital output also optimizes the operation of the low bit-rate
codec. By not wasting bits encoding the 15-20 kHz frequency range that few radio listen-
ers can hear, the codec instead provides higher quality encoding of the crucial 20-15,000
Hz band.
8400HD also adds a second analog output to the 8400 system. This can be configured to
emit the 8400’s low-latency monitor signal, or to emit the signal that is processed for the
HD digital channel.
User-Friendly Interface
• A large (quarter-VGA) color liquid crystal display (LCD) makes setup, adjustment
and programming of OPTIMOD-FM easy. Navigation is by a miniature joystick, two
dedicated buttons, and a large rotary knob. The LCD shows all metering functions of
the processing structure in use (8400 only; 8400PD has no display).
• Use the Locate joystick to navigate through a menu that lets you Recall a preset,
Modify processing (at three levels of expertise), or to access the system’s Setup con-
trols (8400 only).
Absolute Control of Peak Modulation
• The 8400 provides universal transmitter protection and audio processing for FM
broadcast. It can be configured to interface ideally with any commonly found trans-
mission system in the world.
• The 8400 provides pre-emphasis limiting for the internationally used pre-emphasis
curves of 50μs and 75μs. Its pre-emphasis control is seldom audibly apparent, pro-
ducing a clean, open sound with subjective brightness matching the original program.
• The 8400 achieves extremely tight peak control at all its outputs—analog left/right,
AES/EBU left/right, and composite baseband.
• The stereo encoder has two outputs with independent level controls, each capable of
driving 75Ω in parallel with 47,000pF, (100ft/30m of coaxial cable).
• By integrating the stereo encoder with the audio processing, the 8400 eliminates the
overshoot problems that waste valuable modulation in traditional external encoders.
• The 8400 prevents aliasing distortion in subsequent stereo encoders or transmission
links by providing bandwidth limiting and overshoot compensated 15 kHz low-
pass filters ahead of the 8400’s audio outputs and stereo encoder.
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