Home  Listen  Engineering Links Contact
 

 
Processing has changed radically since the introduction of FM stereo in 1962.  The early days of FM and even AM made use of a person sitting in front of of a large volume control, reading scores of music.  His job, to slowly adjust the music volume as the music itself dictated.  He or she, 'rode the gain.'  In the days of classical music, this was just fine and dandy, as it was sufficient to prevent overmodulation.  But lets stop right there and explain exactly what the goal of broadcast audio processing is.

Processing in broacasting terms at least, has two ultimate goals:
1. Prevent overmodulation of the RF carrier.
2. Tailor the 'sound' of the station.

in the good old days, the only real goal was to prevent overmodulation.  overmodulation occurs when,in the case of FM, the carrier exceeds 100% modulation.  Because FM generates sidebands, in otherwords, groups of frequencies adjacent to the carrier when modulation is present, as modulation increases, the carrier and all of its modulation products, ie sidebands, begin to consume more bandwith.

This is problematic, especially if you have many stations around you.  Your uncontrolled modulation peaks will begin to overlap neighboring frequencies, causing crosstalk and distortion.  The solution is therfore to limit peak modulation to 100%.  This is where processing comes into play.

The second and most widely recognized use of broadcast processing is to talior a stations 'sound' to what the PD (programming director) wants.  In these instances, processing is used to 'beef up' the on air sound, adding loudness, presence, and even messing with the stereo image, theoretically widening the soundstage.  Many analog and digital processes are used to reach these goals, DSP technology leading the way in modern processors.

Early processing topologies consisted mainly of a pre-emphasized peak limiter, relying on the dj to consantly ride gain, presenting a consistant level to these early limiter circuits.  With the introduction of rock music in the mid 1950's, PD's began demanding more loudness from their stations.  Modern processing was born.

Here, we must take a sidestep, as during these early days, FM played largely orchestral music, and the loudness wars were heating up on the AM dial.  Engineers all over the world were trying to make the signal appear louder, while still remaining withing the modulation limits.  By reducing the dynamic range of an audio signal, the RMS lever, or loudness of a signal is increased.  The music sounds more consistant in volume, or denser.  An intresting problem arounse, however.  As the RMS level is increased, apoint is reached where the sonic impact is drastically diminished, and the sound turns into sonic mush, devoid and lifeless.  As time progressed, and the music got louder, engineers and PD's were looking for other ways to pump up the volume.

Enter the era of multiband processing.  Multiband processing works by concentrating on a small abnd of frequencies at a time.  Early systems typically employed two bands of compression, a bass band, and a high band.  By splitting the frequencies, much of the 'pumping' of wideband compression was eliminated.  The result was more loudness with fewer sonic penalties.

Companies such as Innovonics developed these early processors and took am processing to an all new level.  Their MAPP series of processors employed as many as eight bands of compression!  The extremely loud, dense sound of these early processors put innovonics on the map (dont mind the pun), and also lead to the slow downfall of AM audio quality.  Some listeners found the loud stations hard to listen to and fatiguing, a problem still faced today.

About this time, FM started taking on a whole new sound as well.  The classical formats of yesteryear gave way to pop music and rock.  FM now needed attention as well, so enter the CBS Volumax.

Basically a peak limiter, the volumax was the first step toword modern FM processing.  Stations could now be loud and legal, although I imagine audio quality left much to be desired.  this time, Durroughs came to the rescue with their DAP, or Discriminate Audio Processor.  Featuring three bands of processing, this little box made FM's sound good, as well as loud, and all in the analog domain.  About this time, Robert Orban introduced his Optimod 8000A, basically a Volumax with with a disstortion cancelled clipper and stereo generator in one box.  His 8100A introduced in the early 1980's was a true dual band processor with AGC, so it could ride gain all by itself.  This was the birth of modern multiband processing.

Today, all modern processors operate completely in the digital relm, makeing use of modern DSP technology and software.  Engineers and PD's now have near total control of their stations signature sound, be it classical or metal mania.

Now, lets take a look at how all of this comes together in the RF realm.

<<NEXT>>