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. 

Up until a few years ago, DDS was impossible and most transmitters used a VCO.  The DDS signal is derived on a very fast Digital to Analog converter and is outputted directly to the target frequency.  This is different than the VCO method where an oscillator is tuned to the desired frequency and then modulated by a device, usually a varactor diode.    

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!