WHY
RADIO STATIONS SOUNDED DIFFERENT
- AND LOUDER - IN THE 1970's
If you listened to AM
radio in the 1970's, then
you know the unique sound it had - from both the programming and
technical
point of view. 590/CKEY is here for you to listen to
today!
We here at
590/CKEY are proud of using the exact
equipment that was used when 590/CKEY was the top station in Toronto in
the 1970's. Our competition was 1050 CHUM with the Drake/PAMS
Top
40 format with its tight programming and jingles, and the very high
power
in the audio sidebands that brought out every watt the transmitter was
capable of.
Competing radio
stations scrambled in finding
the best equipment for being the loudest sounding station. An
added
benefit at the time was the fact that tube amplifiers and transformers
were in use in transmitters, which by their very nature having a warm
sound
responded very well to attempts at driving them harder to produce
louder
signals even when some distortion appeared. Attempts today to
use
multi-band digital audio processing with the transistor switching
modulators
now used in transmitters, while clean sounding, are not successful in
reproducing
that classic loud sound, even when playing the exact songs from the
1970's,
because they cannot be driven beyond the digital clipping level, which
is fixed in absolute voltage level with no headroom to go beyond it,
and
the multi-band processing has its own artificial artifacts that makes
the
songs contain a somewhat unnatural frequency distribution - a sound
similar
to CD's versus vinyl recordings: clean, but lacking something in its
character
because of the signal approximation as a result of the limited digital
resolution 16 bits and low sampling rate 44.1 kHz = artifacts of
omission
and sampling and aliasing errors, and a coldness due to the entire
absence
of beating of >20kHz frequency components to lower, audible
frequencies,
a natural phenomenon of mixing multiple instrumentation harmonics.
There is much
confusion about how the classic
analog loud sound was achieved, in what order the equipment was
connected,
and why it sounded the way it did. Here are the secrets
revealed.
In an AM
transmitter, unlike an FM transmitter,
the power of the carrier is modulated , or added to, by the audio
signal.
The more modulation power that is added, the louder the signal, for the
same carrier power and antenna radiation pattern. In theory
there
is no limit. In practice there are limits - but there are
many tricks
that can be used to get around them. Engineers found these by
trial
and error, and through experimentation.
WHY
USE SIGNAL PROCESSING?
Two reasons:
One:
Radio is different than listening
to a CD, record, or tape at home. At home, the environment is
quiet,
the CD plays directly into an amplifier and loudspeaker system.
Radio uses
electromagnetic waves radiating outwards
from an antenna...every doubling of distance from the transmitting
antenna
results in 1/4 the transmitted power available. There is
natural
and man made noise present in the atmosphere which the receiving
antenna
picks up. There is also noise in the radio
receiver. As the
transmitted signal gets weaker, these noises become greater in
proportion,
ie. the received signal to noise ratio gets poorer.
Two:
A transmitter can only handle
a certain peak power output before distorting or clipping or having
voltage
breakdown. Natural audio signals have a rather high
peak to
average ratio. Turning down the transmitted audio to
accomodate the
peaks so that they will just drive the transmitter to maximum peak
output
would mean the average signal would be much weaker...further reducing
the
transmitted signal to noise ratio discussed in reason one.
The psychology
of listening is such that listener
fatigue will set in while listening to a poor signal to noise
ratio.
The listener will soon tune to another radio station which has a better
signal to noise ratio, especially if similar programming is available
from
another station. From a marketing point of view, if the
transmitted
signal to noise ratio can be increased, more listeners will be
attracted
-- this is the basic premise of the "loudness war" in radio.
Broadcasting
stations in north america have a
limit of 50,000 watts carrier power output. The best way to
situate
the transmitting antenna is outside of a metropolitan area, preferrably
near the ocean or a lake, and then use multiple antenna towers to beam
the signal in the direction of the metropolitan area (at the expense of
other directions). This will result in the greatest possible
transmitted
field strength to the major listening area.
After the
obvious basic things that can be done
to improve signal to noise ratio, transmitter location, power output,
and
antenna gain, the only thing left is signal processing.
THE
AUDIO PROCESSING CHAIN SECRETS REVEALED
Firstly, the high
frequencies can be boosted.
Because most AM radios have IF amplifiers with limited bandwidth, audio
above 5kHz is severely attenuated. The FCC/DOC allows the
transmission
of up to 11 kHz sidebands, or a total of 22 kHz occupied bandwidth,
since
adjacent channels are not assigned in the same geographical area. The
lack
of high frequencies in a received signal in the presence of atmospheric
noise in an AM receiver results in a poor signal to noise
ratio.
Thus transmitter audio pre-emphasis boosts the high frequencies above
the
noise level to some degree. Since the IF amplifier in an AM
receiver
attenuates high frequencies, including noise, the resultant signal will
sound natural again, yet the noise has been reduced. This
gives the
impression that the signal is stronger, and also sounds more high
fidelity
at the same time. So the signal from the mixing console first
passes
through a high frequency booster or equalizer.
Secondly, to
ensure that the AVERAGE audio level
is as high as possible regardless of the program material, a compressor
is used. This compressor must have a medium attack time,
around 28
msec, to avoid a sudden loud sound from being obviously suddenly
reduced
in level. It must also have a slow decay time > 10
seconds to prevent
the average loudness from noticeably changing too quickly, which would
cause an unpleasant effect called gain pumping. All of this
ensures
that the average audio level is consistently as high as possible
without
leaving any obvious artifacts that the gain is actually
changing.
The Gates Sta-Level is such a compressor, using tubes, and increases
the
audio level by 2-15 dB compared to not using it. The actual
benefit
depends on the program material, the original loudness variations, and
the rate of syllabic variation.
Thirdly, in
1970, CBS Laboratories produced a
revolutionary product. The Volumax 4300. The first
analog signal
processor for maximum loudness. It contains two
circuits. A
special compressor, and a peak limiter. This device is
connected
after the main slow acting compressor. The special compressor
is
a FAST acting compressor. Unlike the main compressor, this
one has
a medium-fast attack time, around 8.5 msec, and a relatively fast
release
time, around 260 msec. Percussive sounds faster than 8 msec
do not
trigger the compressor and go straight through with their original
volume
level. The syllabic sounds are compressed and the gain is
re-adjusted
very quickly due to the 260 msec. This only works well if the
average
signal input is relatively constant - the job of the main slow acting
compressor.
The two compressors work together in preventing gain pumping, and allow
percussive sounds to sound loud because they don't reduce the gain of
the
second compressor (ie. their attack time is not altered), yet the
syllabic
variations are highly compressed and are brought up to maximum gain
quickly,
making the signal sound very loud, with a further increase of 6 dB or
4x
power compared to not using it.
Forthly,
because the transmitter uses power supplies
and amplifiers that have a limit to their power output capability
before
clipping, distorting, or causing illegal signal splatter to adjacent
channels,
the PEAK audio level must be instantly limited to the level before such
clipping, distortion, and splatter occurs. In the case of an
AM transmitter,
the negative going peaks are critical, since the transmitted carrier
cannot
go below 0 watts or -100% modulation - if it reaches 0 for any length
of
time, there is no output at all, and the non-sinusoidal character of
such
negative overmodulation results in illegal splatter and distortion of
the
transmitted signal. The positive peak, on the other hand, is
limited
only by the power supply and the modulator amplifier capability, as
well
as the voltage insulation of the transmitter components and can go as
high
in power as the modulator can cleanly produce without distortion, and
as
long as voltage breakdown does not occur in the transmitter carrier
amplifer.
This can be much more than +100% modulation that a symmetrical AM
sine-wave
signal is limited to. Thus unsymmetrical modulation of the
carrier
allows more sideband power in the signal compared to symmetrical
modulation.
A peak limiter is the equipment that is used for this
purpose. It
is a very fast acting device, with an attack time < 1
usec. In
this application, only the negative peaks would be clipped to keep them
at -100%, while the positive peaks would be allowed to go upwards
beyond
+100%. By using the two compressors discussed above, the
signal is
as loud as can be made before clipping cuts off the excessive peaks - a
much easier job to do compared to that if the signal loudness was
varying
all over the place. The peaks can be removed very close to
the average
signal - resulting in full utilization of the volume gain due to
compression.
Thus the second
circuit in the Volumax 4300 is
an unsymmetrical clipping circuit which takes care of the negative
peaks,
fixing them at the -100% modulation level. The 6dB
improvement from
this device alone is all totally useable since at no time will the
transmitter
be overmodulated in the negative direction, neither will it be
undermodulated,
as the average signal will always be close to the clipping
level.
Every top 40 radio station wanted one of these. One does not
really
hear any distortion because the energy in the clipped peaks is very low
- and what is left can fully modulate the transmitter toward
-100%
almost continuously!
Summary of
equipment used in audio chain, in order:
RCA BC-5B tube
mixing console
Frequency
response 30-15000 Hz +/- 1 dB ref. 1
kHz. This is the broadcasting standard.
Kahn
symmetra-peak, a device that reduces the
positive/negative asymmetry on voice signals, to manageable
values so that at -100% modulation the positive peaks are under +130%
modulation
level: average power gain +2 dB
(Without this
device, the highly unsymmetrical
nature of voice signals would require the negative direction to be peak
limited to less than 100% because the positive peaks would exceed the
transmitter
components insulation breakdown, which is around +130% and no
more.
The alternative is to clip the positive peaks more aggressively = too
much
distortion.)
Put only in the
voice channel -- NOT RECOMMENDED FOR MUSIC (certain bass/percussive
wave shapes less steep & weakened (subtle but noticeable) due
to non-linear phase rotation ~3.0 msec at 300 Hz, 3.5 mec at 100 Hz, 4
msec at 30 Hz, 0.5msec at 10000Hz )
Linear
pre-emphasis of frequencies above 1.2 kHz,
reaching +12dB at 11 kHz: average perceived on-air power gain
+3dB
Gates Sta-Level
tube compressor, attack time of
28 msec and dual constant release times of 2.35 sec and 10
sec: average
power gain +3dB
Volumax 4300
volume controller/peak limiter
compressor section,
8.5 msec attack time, 260 msec release time: average power
gain +6
dB
limiter section,
< 1 usec peak limiting of negative peaks for radio transmitter
Radio
Transmitter, capable of +130% positive modulation:
average power gain +2
dB
(on-air signal only)
Total on-air
power gain compared to not using
this equipment: +16dB (or 40x more power - thus a
50 kW station
sounds like 2000 kW!)
SUMMARY
OF EQUIPMENT
RCA 44-BX and RCA 77-DX
ribbon velocity microphones
ZaraRadio software
package mp3/wave file player/jingle&commercial
scheduler on HP notebook computer
RCA BC-5B tube broadcast
mixer console
Kahn symmetra-peak phase
rotator (only in voice channel)
Pre-emphasis from 1.2-11
kHz +1 to +12 dB
Gates Sta-Level tube
dual time constant slow
compressor
Volumax 4300 fast
compressor/negative peak limiter
Brick wall 11 kHz
splatter filter
SHOUTcast mp3 streaming
audio server on Packard
Bell 910C computer, Bell DSL internet line
NOTES
ABOUT DIGITAL SYSTEMS SUCH AS INTERNET
RADIO
Since the power level
represented by a digital
signal over the internet has fixed levels determined by the fixed
number
of data bits, this energy limited representation requires the positive
and negative peaks to be symmetrical +/-100% and to be represented by
data
bits 0 and 255 for maximum volume.
Thus the peak
limiting of positive peaks necessary
for the SHOUTcast digital audio service over the internet makes a
signal
that isn't quite as loud as the on-air signal, but it is still LOUD,
+13
dB or 20x more power. Most internet radio stations use some
processing
- many use excessive and artificial digital processing resulting in
distortion
on low frequencies and a listening fatigue due to an unpleasant
spectral
energy distribution - therefore, we are confident that our signal is
the
loudest - and clearest - on the internet.
Please note
that the 80 kilobits/second data stream
compression will create audible artifacts that would not be heard live
on the air. Plus, some of the "rare" commercials are poor
mp3s with
artifacts unfortunately...Other than these, the audio is virtually
distortionless
and clear.