Direct Digital Synthesis is the
next evolution of analog FM broadcast systems |
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![]() What is DDS and why should I care?
Direct Digital Synthesis
(DDS) is a way of generating a modulated FM signal
without using a voltage controlled oscillator or
VCO. DDS is superior to more common methods
for a lot of reasons, but sound quality is a huge
reason to switch.
All VCO/varactor diode based
systems have certain limitations that have plagued
the broadcast industry for
decades. The chief complains are
that the oscillators are microphonic, suffer from
AFC problems and add distortion.
If you bump into one hard
enough, it will make a sound that will be heard on
the air. If the exciter is used in a noisy
environment, rumble and other sounds may be faintly
heard on the radio. This is called being
microphonic. You may have seen some VCOs
potted in silicone or even put into sealed boxes on
rubberized standoffs in some transmitters in an
attempt to mitigate this problem.
Another serious issue occurs with the Automatic Frequency Control (AFC). AFC is required to make sure that the transmitter is operating on the correct frequency. To do this, there is a feedback circuit, called an AFC loop, that produces a correction voltage if the VCO tries to drift off of the assigned channel. This works well enough, but the AFC can be fooled by high modulation levels. When high modulation is applied, especially in low frequencies, this can simulate an AFC voltage change and cause the exciter to momentarily drift. This is sometimes known as AFC tearing because it can manifest itself on the air as a tearing sound in the demodulated audio. It also manifests as added multipath. The folks at Harris made a big stride towards reducing or eliminating this problem when they introduced the MS-15 exciter in the late 1970s. The MS-15 used a dual AFC loop operating at different frequencies. There are some variations of this technology and they're far more robust than the simple single speed AFC loop. Today, almost all commercially available transmitters using VCOs have multispeed AFCs. Still, there's an unforeseen drawback. The dual AFC loop prevents or distorts certain low frequency audio energy, preventing it from properly modulating the exciter. This is really just the old AFC unlock problem, reversed. Instead of the audio energy unlocking the AFC, the newly stabilized AFC is so stable that it suppresses some parts of the modulation. Oops! There's no real solution to this.
Last, the varactor diode always introduces distortion to the modulated carrier. Great VCO design and proper Varactor selection can minimize this a lot and produce some very good results, but some distortion always remains.
The DDS method doesn't use an
AFC or a varactor diode. Instead, the audio is
sampled by a high speed A/D chip and sent digitally
to a dedicated DDS encoder chip. We use the
Analog Devices AD9910, the best DDS encoder
available today. The DDS encoder digitally
creates a modulated FM carrier that is sonically as
pure as the original CD. There's no tearing,
multipath or distortion when the signal is
received. This creates a noticeably cleaner
sound than standard VCO FM transmitters. Even if
your old transmitter sounds pretty good, DDS is
always better! It's more expensive to use DDS,
but we think it's well worth it to maintain higher
sonic purity than many of our competitors. Our
customer's responses tell us that they have noticed
the difference. Every CW Broadcast transmitter
from 50W to 5kW is equipped with a DDS exciter.
In short, your message matters and at
CW Broadcast, we make sure that your message simply
sounds better! |