The process

The five-band processor splits the program into five frequency bands. Each band imparts both dynamic compression and adjustable static gain to apply fixed equalization and other control of the program audio. Sub-bass program frequencies undergo independent dynamic compression, expansion, selective clipping and filtering for control over both static rumble and dynamic punch of the bottom-end components. Fast high-frequency limiting and distortion-cancelled clipping are utilized to provide independent amplitude control of program frequencies subjected to the FM pre-emphasis. The idea is to help preserve program brightness and clarity despite power bandwidth constraints inherent in FM broadcasting.

A user-programmable high-pass filter attenuates sub-audible noise that could compromise modulation efficiency. The filter is adjustable between 20Hz and 65Hz. The AGC is quasi-average-responding with a capture/correction range of ±18dB. Positive AGC gain may be truncated to any value between 0dB and +18dB, and the correction rate of the AGC is programmable.

The stereo enhancement effectively broadens the soundstage for the stereo program and the center-channel component independently. Up to 3dB of clipping may be applied to the composite/baseband signal. Clipping is performed before the injection of the stereo pilot and RDS subcarrier.

The unit includes the Inovonics exclusive Polarity-Independent Peak Processor (PIPP). With PIPP, the program audio waveform is split into positive-going and negative-going components. Each of these signals is then independently limited to a peak value that corresponds to 100 percent carrier modulation (in their respective directions), and the components then recombined. The purpose of this somewhat roundabout exercise is to optimize carrier modulation regardless of waveform asymmetry. The PIPP limiter can be disabled if desired.

Overall, I found the David IV to be a very functional processor. I really liked the fact that it starts passing audio almost immediately after booting up. That is a feature not seen in many other processors. I used the David IV on a country-formatted HD2 channel and it performed very well. It let me build the sound I wanted and did not color the audio at all. I'm sure many people will find a good use for the David IV in their audio facilities.

Smith is the chief engineer of WEMP 101.9, New York.