| WHAT
HAPPENS TO MY RECORDING WHEN IT’S PLAYED ON THE RADIO?
By Frank Foti, Omnia Audio & Robert Orban, CRL/Orban
Few
people in the record industry really know how a radio station processes
their material before it hits the FM airwaves. This article’s
purpose is to remove the many myths and misconceptions surrounding
this arcane art.
Every
radio station uses a transmission audio processor in front of its
transmitter. The processor’s most important function is to control
the peak modulation of the transmitter to the legal requirements of
the regulatory body in each station’s nation. However, very
few stations use a simple peak limiter for this function. Instead,
they use more complex audio chains. These can accurately constrain
peak modulation while significantly decreasing the peak-to-average
ratio of the audio. This makes the station sound louder within the
allowable peak modulation.
Garbage
In—Garbage Out
Manufacturers
have tuned broadcast processors to process the clean, dynamic program
material that the recording industry has typically released throughout
its history. (The only significant exception that comes to mind
is
45-rpm singles, which often were overtly distorted.) Because these
processors have to process speech, commercials, and oldies in addition
to current material, they can't be tuned exclusively for “hypercompressed,”
distorted CDs. Indeed, experience has shown that there’s no
way to tune them successfully for this degraded material.
For 20
years, broadcast processor designers have known that achieving highest
loudness consistent with maximum punch and cleanliness requires extremely
clean source material. For more than 20 years, Orban has published
application notes to help broadcast engineers clean up their signal
paths. These notes emphasize that any clipping in the path before
the processor will cause subtle degradation that the processor will
often exaggerate severely. The notes promote adequate headroom and
low distortion amplification to prevent clipping even when an operator
drives the meters into the red.
About
three years ago, we started to notice CDs arriving at radio stations
that had been pre-distorted in production or mastering to increase
their loudness. For the first time, we started seeing frequently
reoccurring
flat topping caused by brute-force clipping in the production process.
Broadcast processors react to pre-distorted CDs exactly the same
way
as they have reacted to accidentally clipped material for more than
20 years—they exaggerate the distortion. Because of phase rotation,
the source clipping never increases on-air loudness—it just
adds grunge. The authors understand the reasoning behind the CD loudness
wars. Just as radio stations wish to offer the loudest signal on the
dial, it is evident that recording artists, producers, and even some
record labels want to have a loud product that stands out against
its competition in a CD changer or a music store’s listening
station.
In radio
broadcasting this competition has existed for at least the last 25
years. 25 years ago, radio stations used simple clipping to get louder,
and this 25-year-old technique has now migrated to the music industry.
The following graphic shows a section of a severely clipped waveform
from a contemporary CD. The area marked between the two pointers highlights
the clipped portion. This is one of the roots of the problem as described
in this paper; the other is excessive digital limiting that does not
necessarily cause flat-topping, but still removes transient punch
and impact from the sound.
The problem
today is that we now have sophisticated and powerful audio processing
for the broadcast transmission system and this processing does not
coexist well with a signal that has already been severely clipped.
Unfortunately, with current pop CDs, the example shown above is more
the norm than the exception. The attack and release characteristics
of broadcast multiband compression were tuned to sound natural with
source material having short-term peak-to-average ratios typical of
vinyl or pre-1990 CDs. Excessive digital limiting of the source material
radically reduces this short-term peak-to-average ratio and presents
the broadcast processor with a new, synthetic type of source that
the broadcast processor handles less gracefully and naturally than
it handles older material. Instead of being punchy, the on-air sound
produced from these hypercompressed sources is small and flat, without
the dynamic contours that give music its dramatic impact. The on-air
sound resembles musical wallpaper and makes the listener want to turn
down the volume control to background levels.
There
is a myth that broadcast processing will affect hypercompressed material
less than it will more naturally produced material. This is true
in
only one aspect—if there is no long-term dynamic range coming
in, then the broadcast processor’s AGC will not further reduce
it. However, the broadcast processor will still operate on the short-term
envelopes of hypercompressed material and will further reduce the
peak-to-average ratio, degrading the sound even more.
Hypercompressed
material does not sound louder on the air. It sounds more distorted,
making the radio sound broken in extreme cases. It sounds small,
busy,
and flat. It does not feel good to the listener when turned up, so
he or she hears it as background music. Hypercompression, when combined
with “major-market” levels of broadcast processing, sucks
the drama and life from music. In more extreme cases, it sounds overtly
distorted and is likely to cause tune-outs by adults, particularly
women.
A
Typical Processing Chain—What Really Goes On When Your Recording
is Broadcast:
A typical
chain consists of the following elements, in the order that they appear
in the chain:
Phase
rotator
The phase rotator is a chain of allpass filters (typically four
poles,
all at 200Hz) whose group delay is very non-constant as a function
of frequency. Many voice waveforms (particularly male voices) exhibit
as much as 6dB asymmetry. The phase rotator makes voice waveforms
more symmetrical and can sometimes reduce the peak-to-average ratio
of voice by 3-4dB. Because this processing is linear (it adds no
new
frequencies to the spectrum, so it doesn’t sound raspy or fuzzy)
it’s the closest thing to a “free lunch” that one
gets in the world of transmission processing.
There
are a few prices to play. In the good old days when source material
wasn’t grossly clipped, the main price was a very subtle reduction
in transparency and definition in music. This was widely accepted
as a valid trade-off to achieve greatly reduced speech distortion,
because the phase rotator’s effects on music are unlikely to
be heard on typical consumer radios, like car radios, boomboxes, “Walkman”-style
portables, and table radios.
However,
with the rise of the clipped CD, things have changed. The phase rotator
radically changes the shape of its input waveform without changing
its frequency balance: If you measured the frequency response of
the
phase rotator, it would measure “flat” unless you also
measured phase response, in which case you would say that the “magnitude
response” was flat and the phase response was highly non-linear
with frequency. The practical effect of this non-linear phase response
is that flat tops in the original signal can end up anywhere in the
waveform after processing. It’s common to see them go right
through a zero crossing. They end up looking like little smooth sections
of the waveform where all the detail is missing—a bit like
a
scar from a severe burn. This is an apt metaphor for their audible
effect, because they no longer help reduce the peak-to-average ratio
of the waveform. Instead, their only effect is to add unnecessary
grungy distortion.
There
has been a myth in the recording world that broadcast processing will
modify these clipped, over-compressed CDs less it will modify clean,
dynamic CDs. Thanks in part to phase rotation, this myth is absolutely
false. In particular, any clipping in the source material causes nothing
but added distortion without increasing on-air loudness at all.
AGC
The next stage is usually an average-responding AGC. By recording
studio standards, this AGC is required to operate over a very wide
dynamic range—typically in the range of 25dB. Its function is
to compensate for operator errors (in live production environments)
and for varying average levels
(in automated environments). Average levels vary mainly because the
peak to average ratio of CDs themselves has varied so much in the
last 10 years or so. Therefore, normalizing hard disk recordings (to
use all available headroom) has the undesirable side effect of causing
gross variations in average levels. Indeed, 1:1 transfers (which are
also common) will also exhibit this variation, which can be as large
as 15dB.
The price to be paid is simple: the AGC will eliminate long-term dynamics
in your recording. Virtually all radio station program directors want
their stations to stay loud always, eliminating the risk that someone
tuning the radio to their station will either miss the station completely
or will think that it’s weak and can’t be received satisfactorily.
Radio people often call this effect “dropping off the dial.”
AGCs
can be either single-band or multiband. If they are multiband, it’s
rare to use more than two bands because AGCs operate slowly, so “spectral
gain intermodulation” (such as bass’ pumping the midrange)
is not as big a potential problem as it is for later compression stages,
which operate more quickly.
AGCs
are always gated in competent processors. This means that their gain
essentially freezes if the input drops below a preset threshold, preventing
noise suck-up despite the large amount of gain reduction.
Stereo
Enhancement
Not all processors implement stereo enhancement, and those that do
may implement it somewhere other than after the AGC. (In fact, stand-alone
stereo enhancers are often placed in the program line in front of
the transmission processor.)
The
common purpose of stereo enhancement is to make the signal stand out
dramatically when the car radio listener punches the tuning button.
It’s a technique to make the sound bigger and more dramatic.
Overdone, it can remix the recording. Assuming that stereo reverb,
with considerable L–R energy, was used in the original mix,
stereo enhancement, for example, can change the amount of reverb applied
to a center-channel vocalist. The moral? When mixing for broadcast,
err on the “dry” side, because some stations’ processors
will bring the reverb more to the foreground.
Because
each manufacturer uses a different technique for stereo enhancement,
it’s impossible to generalize about it. The only universal constraints
are the need for strict mono compatibility (because FM radio is frequently
received in mono, even on “stereo” radios, due to signal-quality-trigged
mono blend circuitry), and the requirement that the stereo difference
signal (L–R) not be enhanced excessively. Excessive enhancement
always increases multipath distortion (because the part of the FM
stereo signal that carries the L–R information is more vulnerable
to multipath). Excessive enhancement will also reduce the loudness
of the transmission (because of the “interleaving” properties
of the FM stereo composite waveform, which we won’t further
discuss).
These
constraints mean that recording-studio-style stereo enhancement is
often incompatible with FM broadcast, particularly if it significantly
increases average L–R levels. In the days of vinyl, a similar
constraint existed because of the need to prevent the cutter head
from lifting off the lacquer, but with CDs, this constraint no longer
exists. Nevertheless, any mix intended for airplay will yield the
lowest distortion and highest loudness at the receiver if its L–R/L+R
ratio is low. Ironically, mono is loudest and cleanest!
Equalization
Equalization may be as simple as a fixed-frequency bass boost, or
as complex as a multi-stage parametric equalizer. EQ has two purposes
in a broadcast processor. The first is to establish a signature for
a given radio station that brands the station by creating a “house
sound.” The second purpose is to compensate for the frequency
contouring caused by the subsequent multiband dynamics processing
and high frequency limiting. These may create an overall spectral
coloration that can be corrected or augmented by carefully chosen
fixed EQ before the multiband dynamics stages.
Multiband
Compression and Limiting
Depending on the manufacturer, this may occur in one or two stages.
If it occurs in two stages, the multiband compressor and limiter can
have different crossovers and even different numbers of bands. If
it occurs in one stage, the compressor and limiter functions can “talk”
to each other, optimizing their interaction. Both design approaches
can yield good sound and each has its own set of tradeoffs.
Usually
using anywhere between four and six bands, the multiband compressor/limiter
reduces dynamic range and increases audio density to achieve competitive
loudness and dial impact. It’s common for each band to be gated
at low levels to prevent noise rush-up, and manufacturers often have
proprietary algorithms for doing this while minimizing the audible
side effects of the gating.
An advanced
processor may have dozens of setup controls to tune just the multiband
compressor/limiter. Drive and output gain controls for the various
compressors, attack and release time controls, thresholds, and sometimes
crossover frequencies are adjustable, depending on the processor design.
Each of these controls has its own effect on the sound, and an operator
needs extensive experience if he or she is to tune a broadcast multiband
compressor so that it sounds good on a wide variety of program material
without constant readjustment. Unlike mastering in the record industry,
in broadcast there’s no mastering engineer available to optimize
the processing for each new source!
Pre-Emphasis
and HF Limiting
FM radio is pre-emphasized at 50 microseconds or 75 microseconds,
depending on the country in which the transmission occurs. Pre-emphasis
is a 6dB/octave high frequency boost that’s 3dB up at 2.1kHz
(75µs) or 3.2kHz (50µs). With 75µs pre-emphasis,
15kHz is up 17dB!
Depending on the processor’s manufacturer, pre-emphasis may
be applied before or after the multiband compressor/limiter. The important
thing for mixers and mastering engineers to understand is that putting
lots of energy above 5kHz creates significant problems for any broadcast
processor because the pre-emphasis will greatly increase this energy.
To prevent loudness loss, the processor applies high frequency limiting
to these boosted high frequencies. HF limiting may cause the sound
to become dull, distorted, or both, in various combinations. One of
the most important differences between competing processors is how
effectively a given processor performs HF limiting to minimize audible
side effects. In state-of-the-art processors, HF limiting is usually
performed partially by HF gain reduction and partially by distortion-cancelled
clipping.
Clipping
In most processors, the clipping stage is the primary means of peak
limiting. It’s crucial to broadcast processor performance. Because
of the FM pre-emphasis, simple clipping doesn’t work well at
all. It produces difference-frequency IM distortion, which the de-emphasis
in the radio then exaggerates. (The de-emphasis is flat below 2-3kHz,
but rolls off at 6dB/octave thereafter, effectively exaggerating energy
below 2-3kHz.) The result is particularly offensive on cymbals and
sibilance (“essses” become “efffs”).
In the late seventies, one of the authors of this article (R.O.) invented
distortion-cancelled clipping. This manipulates the distortion spectrum
added by the clipper’s action. In FM, it typically removes the
clipper-induced distortion below 2kHz (the flat part of the receiver’s
frequency response). This typically adds about 1dB to the peak level
emerging from the clipper, but, in exchange, allows the clipper to
be driven much harder than would otherwise be possible.
Provided
that it doesn’t introduce audibly offensive distortion, distortion-cancelled
clipping is a very effective means of peak limiting because it affects
only the peaks that actually exceed the clipping threshold and not
surrounding material. Accordingly, clipping does not cause pumping,
which gain reduction can do, particularly when gain reduction operates
on pre-emphasized material. Clipping also causes minimal HF loss by
comparison to HF limiting that uses gain reduction. For these reasons,
most FM broadcast processors use the maximum practical amount of clipping
that’s consistent with acceptably low audible distortion.
Real-world
clipping systems can get very complicated because of the requirement
to strictly band-limit the clipped signal to less than 19kHz despite
the harmonics that clipping adds to the signal. (Bandlimiting prevents
aliasing between the stereo main and subchannel, protects subcarriers
located above 55kHz in the FM stereo composite baseband, and protects
the stereo pilot tone at 19kHz). Linearly filtering the clipped signal
to remove energy above 15kHz causes large overshoots (up to 6dB in
worst case) because of a combination of spectral truncation and time
dispersion in the filter. Even a phase-linear lowpass filter (practical
only in DSP realizations) causes up to 2dB overshoot. Therefore, state-of-the-art
processors use complex overshoot compensation schemes to reduce peaks
without significantly adding out-of-band spectrum.
Some
chains also apply composite clipping or limiting to the output of
the stereo encoder. The stereo encoder is the circuit that encodes
the left and right channels into the single multiplex signal that
drives the transmitter, and it’s actually the peak level of
this signal that government broadcasting authorities regulate. Composite
clipping or limiting has long been a controversial technique, but
the latest generation of composite clippers or limiters has greatly
reduced the interference problems characteristic of earlier technology.
Conclusions
Broadcast
processing is complex and sophisticated, and was tuned for the recordings
produced using practices typical of the recording industry during
almost all of its history. In this historical context, hypercompression
is a short-term anomaly and does not coexist well with the “competitive”
processing that most pop-music radio stations use. We therefore recommend
that record companies provide broadcasters with radio mixes. These
can have all of the equalization, slow compression, and other effects
that producers and mastering engineers use artistically to achieve
a desired “sound.” What these radio mixes should not have
is fast digital limiting and clipping. Leave the short-term envelopes
unsquashed. Let the broadcast processor do its work.
The result
will be just as loud on-air as hypercompressed material, but will
have far more punch, clarity, and life. A second recommendation to
the record industry is to employ studio or mastering processing that
provides the desired sonic effect, but without the undesired extreme
distortion component that clipping creates. The alternative to brute-force
clipping is digital look-ahead limiting, which is already widely available
to the recording industry from a number of different manufacturers
(including the authors’ companies). This processing creates
lower modulation distortion than clipping and also avoids blatant
flat-topping of waveforms. Compared to clipping, it is therefore substantially
more compatible with broadcast processing. Nevertheless, even digital
limiting can have a deleterious effect on sound quality by reducing
the peak-to-average ratio of the signal to the point that the broadcast
processor responds to it in an unnatural way, so it should be used
conservatively. Ultimately, the only way to tell how one’s production
processing will interact with a broadcast processor is to actually
apply the processed signal to a real-world broadcast processor and
to listen to its output, preferably through a typical consumer radio.
Printed with permsission.
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