Until the last few years, the narrow bandwidth (300kHz) of the
950MHz radio channels, which was adequate for the audio and technical
standards of the all-analog world, was insufficient to handle the much
greater bandwidth of the digital signals of newer generation,
AES3-compliant studio equipment. If stations wanted to use 950MHz radio
channels for their studio-to-transmitter link, they had to compress
their high-bandwidth digital audio to fit the limited bandwidth of the
Figure 1. On-off keying (OOK/s-ASK).
Figure 2. Binary phase shift keying (BPSK).
Neither of the two types of data compression schemes lossless and
lossy was very practical, however. While lossless data compression does
not degrade the quality of the received data in theory, current
lossless compression technologies used on personal computers are simply
not effective in reducing audio data in practice. Lossy-type data
compression schemes, which take advantage of perceptual masking to
discard some of the audio without being very noticeable to the
listener, can provide satisfactory results when used properly. However,
when a signal is compressed two or more times for example, for music
storage and to convey audio from the studio to the transmitter
listeners begin to notice reduced audio quality.
It was only with the introduction of 950MHz, frequency agile,
digital STLs, which use true digital modulation techniques to transport
uncompressed AES3 digital audio, that it was possible for broadcasters
to implement a digital studio-to-transmitter link without the
previously required use of lossy data compression. This new approach
prevents the possibility of cascading compression algorithms within the
STL link, which can cause audible artifacts in the recovered
Several digital modulation techniques are in use today, each with
its own set of advantages and disadvantages. Some methods are very
robust but require large amounts of bandwidth. Others are more
spectrally efficient but can be more prone to errors, requiring more
sophisticated forms of error correction. The goal of all of these
techniques is to transport unimpaired digital 1s and 0s from point A to
The amplitude and the phase of a high frequency carrier can be
represented using a phasor or vector diagram. The I (In-Phase) axis of
the phasor represents the in-phase component and its amplitude, and the
Q (Quadrature) axis represents a 90 degree phase shift from the
reference axis I. With this diagram, it is possible to represent a
carrier at different points of amplitude, phase or combinations of the
two. These points on the phasor are referred to as constellation
One of the most basic forms of digital modulation is called
Amplitude Shift Keying (ASK). In amplitude shift keying, the carrier
has two possible states, On and Off, On representing a digital 1, and
Off representing a digital 0. One bit of digital data may be
transmitted using ASK. Figure 1 shows ASK using the phasor diagram.
Bi-Phase Shift Keying (BPSK) allows the transmission of one bit of
digital information in both states 0 and 1 by shifting the phase of the
carrier from 0° (reference) to 180°. Figure 2 shows BPSK. The
0° reference represents a digital 1 and the 180° position
represents a digital 0.
Figure 3. Quadrature phase shift keying (QPSK).
Quadrature Phase Shift Keying (QPSK) allows the transmission of 2
bits of data within the same occupied bandwidth as BPSK. QPSK uses four
phase positions, 45°, 135°, 225°, and 315°, to
represent a 0 or 1 for two bits of digital data. Figure 3 graphically
represents QPSK. The individual phase positions represent the value of
the digital word as in BPSK. BPSK and QPSK vary the phase of the
carrier to represent more digital data bits.
A third method, Quadrature Amplitude Modulation (QAM), varies both
the phase and amplitude of a carrier to represent more symbol points
and thus more bits of digital data within the same occupied bandwidth.
Typical QAM formats are 16QAM, 32QAM and 64QAM, allowing the
transmission of 4-, 5- and 6-bit digital words, respectively. The
advantage of QAM over BPSK and QPSK is that more data may be
transmitted within the same RF bandwidth. The disadvantage of higher
rate QAM formats is that the signal becomes more delicate and prone to
errors as the number of symbol points increases. As the QAM rate
increases, from 16 to 64, more sophisticated modulation detection
circuitry and the addition of different forms of error correction are
required in order to keep the Bit-Error-Rate (BER) low. Figure 4 is a
graphic representation of 16QAM.
Digital RF STL systems today use various QAM rates and different
bandwidths (depending on the type of modulation and number of channels
of linear audio) to modulate a combination of main channel audio,
auxiliary audio and RS232 data subcarriers onto the RF carrier.
Different manufacturers offer different solutions. The various
transmitter input signals are combined into a baseband signal and then
coded using different forms of error correction algorithms. The final
PA in the STL amplifies the small signal output of the modulator. The
power amplifier typically employs linearization techniques that correct
for channel impairments and nonlinearities in the power amplifier, in
order to meet the channel RF mask requirements.
Figure 4. 16QAM.
With two watts of average power output and a receiver signal
threshold of -90dB (7mV), it is possible to operate with paths up to 50
miles using 8-foot parabolic dishes and still maintain an adequate fade
margin of about 40dB.
The receiver demodulates the RF carrier back to the coded baseband
signal. The encoded data stream is then applied to an inverse coding
algorithm, which rearranges the data back into its original order,
correcting errors incurred along the way. Most receivers will
simultaneously output AES3 digital audio, optical AES3 audio, analog
left and right audio signals, data channels and other various optional
There are many advantages to using an uncompressed all-digital path
from your studio to your transmitter, including the elimination of all
A/D and D/A conversions, which add distortion, noise and system costs
as well as full digital quality delivered to the on-air signal. Recent
technological advancements make all of this possible using standard,
Dave Agnew is a senior FM applications engineer at Harris
Corporation Broadcast Communications Division, Cincinnati.