Chief Engineer. Orban/CRL
In the early days of broadcasting, the
primary purpose of transmission audio processing was to protect the AM transmitters of the
time from damage due to modulator overload. Simple peak limiters using variable-mu tubes
in a push-pull configuration were employed. Because the gain-control signal was, in
essence, mixed with the audio signal, these early vacuum-tube devices required careful
balancing to cancel "thumps" representing feedthrough of the gain-control signal
into the audio. Dynamic range control was effected through careful manual gain-riding --
in classical music broadcasts, the "compressor" was a skilled operator reading
the musical score and using it to anticipate the required level adjustments. To this day,
no one has invented a more subtle or effective method of compression!
Later, simple compressors were placed
upstream from the limiters in situations where the budget did not permit skilled manual
gain-riding. These compressors were not gated and could exaggerate noise objectionably.
In the Region 2 countries, 75Ás
pre-emphasis is used in FM and television sound transmission. This pre-emphasis is up 17dB
at 15kHz and can cause severe over-modulation if its effects are not controlled. The
obvious solution - placing a wide-band peak limiter after the pre-emphasis filter -
proved unsatisfactory because high-frequency overloads would cause severe spectral gain
intermodulation: cymbal crashes would cause the sound to literally collapse. The Fairchild
"Conax" (originally designed for disk cutting) was often used to ameliorate the
problem. This device divided the audio into two bands with a 1kHz crossover and applied
pre-emphasis, clipping, and high-pass filtering to the upper band. The high-pass filter
reduced the difference-frequency intermodulation caused by the clipper, yielding
reasonably acceptable sound.
"Modern audio processing" could
be said to derive from the work of the design team at CBS Laboratories in the early 1960s.
Their "Audimax" (mispronounced by generations of engineers as
"audiomax"!) was a gated wideband compressor that successfully eliminated the
noise-breathing problem of earlier compressors. The "Volumax" was a clipper
preceded by a limiter with a moderate attack time. The moderate attack time prevented the
unit from punching holes in the program, while the clipper controlled the peaks that the
preceding limiter did not catch. The "FM Volumax" introduced a high-frequency
limiter to control overload due to the pre-emphasis curve. This high-frequency limiter was
a program-controlled 6dB/octave shelving filter placed between the limiter and clipper.
Once again, a moderate attack time was used and the overshoots were controlled by a final
clipper. The "Dynamic Presence Equalizer" measured the ratio of midrange energy
to wideband program energy and applied midrange equalization as necessary to correct the
midrange spectral balance of the program.
In the early 1970s, Dorrough Electronics
introduced the "Discriminate Audio Processor" ("DAP"). There were
versions for AM and FM. The DAP divided the audio spectrum into three bands with gentle
crossover slopes and compressed each band independently. The bands were recombined and
applied to a clipper with a very "soft" transfer characteristic. The DAP greatly
reduced spectral gain intermodulation by comparison to its wideband predecessors.
Additionally, many engineers adjusted the three bands for different gains, using the
device as a dynamic program equalizer as well.
In 1975 Orban Associates introduced
"Optimod-FM." This unit combined compressor, limiter, high-frequency limiter,
clipper, 15kHz low-pass filters, and stereo multiplex encoder into one box. This greatly
reduced the possibility of misadjustment of the processing chain. The unit's 15kHz
low-pass filters were non-linear filters without significant overshoot, and therefore
permitted higher average modulation by comparison to the linear low-pass filters used in
the stand-alone stereo encoders of the time.
In 1977 Orban Associates introduced
"Optimod-AM." This unit contained a high-slope receiver equalizer to
pre-compensate for the highly rolled-off radios of the time, and also included an 11kHz
low-pass filter to ensure that the unit complied with the occupied bandwidth requirements
of the 1978 FCC Rules. It also introduced the distortion-canceling clipper, which
substantially reduced difference-frequency intermodulation distortion caused by clipping.
In the late 1970s, Circuit Research
Laboratories introduced a processing system for AM whose most important novel features
were a phase rotator [the Kahn "Symmetra-Peak" being a fore-runner] prior to processing (to make voice more symmetrical, reducing
clipping distortion), and a subsonic equalizer after final peak clipping to pre-distort
the output waveform of the processor to compensate for low-frequency tilt in the
plate-modulated transmitters of the time. Compensating for this waveform tilt enabled the
better transmitters to be substantially louder by eliminating a factor that would
otherwise increase the peak-to-average ratio of the modulation. Although intuitively
inobvious, using a phase rotator to purposely eliminate the asymmetry in voice proved to
be far more effective than the older "polarity follower" [Pacific Recorders AM
"Modulimiter"] circuit. The older circuit preserved any natural waveform
asymmetry and switched its output polarity such that the side of the waveform with the
higher peak level modulates the carrier in the positive direction.
In the late 1970s, a number of
manufacturers made "composite clippers" designed to be placed between the output
of the stereo encoder and the input of the transmitter. These controlled the peak
modulation of the composite stereo signal unambiguously at the expense of introducing
harmonic and intermodulation distortion throughout the stereo baseband. Many
"hit-format" broadcasters thought that the increased loudness achieved by these
devices justified compromising the spectral purity of the baseband. Eventually, the FCC
judged these devices to be in violation of the FCC Rules of the time if they caused the
instantaneous 19kHz stereo pilot tone injection to be less than 8% modulation. In essence,
this meant that the pilot could not be clipped and must be injected after the clipper. In
1982, Modulation Sciences introduced a composite processor that did this, thereby
performing to the letter of the FCC Rules.
In 1982, Orban Associates introduced the
"Hilbert-Transform Clipper" as part of its Optimod-TV processor for stereo
television. The "Hilbert-Transform Clipper" was later adapted for use in
shortwave as well.
In general, transmission audio processing
in the 1980s refined and built upon the revolutionary developments of the 1970s without
introducing any radical novelties. Each manufacturer, for example, has a proprietary
technique for producing non-linear overshoot-free low-pass filters for FM and television
applications. Several manufacturers (including Inovonics and Circuit Research
Laboratories) introduced programmable processors whose subjective setup controls can be
changed by remote control to match the programming of the moment.
In the 1990s, the field must be considered
"mature." As in every other area of audio, digital signal processing (DSP) is
likely to eventually supplant analog circuitry. As of this writing, Orban, CRL, Valley
International, Gentner Electronics, and Audio Animation have introduced transmission
processors in which all processing is done in the digital domain. [The Valley, Gentner,
and Audio Animation units are no longer manufactured.] If properly designed, such a
processor can be readily reconfigured in milliseconds to change almost any aspect of its
topology, such as the number of bands in its multi-band compressor. Subjective setup
control settings can be stored and later recalled by local clock, remote control, or
computer to daypart processing. The processor can readily generate test and signalling
tones, facilitating tests of the transmission system and the generation of EAS alert
In a digital processor, achieving sound
quality equal to or better than its analog counterparts requires a marriage of art and
mathematical design more rigorous than anything in the genesis of its analog ancestors.
Many common analog processing functions (such as clipping) are much more difficult to do
competently in the digital domain. However, digital also presents the opportunity to do
things unachievable in analog, and digital's overwhelming advantages will ultimately
manifest themselves as clearly here as they have elsewhere in the audio processing arena.