Winter 2009 Issue
A Look Inside D-STAR Modulation
By Bob Witte, KØNR
For the Winter 2006 issue of CQ VHF, I provided an overview of D-STAR radio technology. At that time, the technology was just starting to emerge onto the amateur radio scene, and the amateur radio community was just starting to understand how the technology worked. By now there are quite a few D-STAR repeaters on the air, and many radio amateurs have experienced D-STAR communications on the VHF and UHF bands.
As D-STAR enthusiasts have gotten their hands
on the hardware, they have been experimenting with the various features.
In typical ham fashion, the techies are reverse engineering and tinkering
with the ICOM D-STAR equipment and in some cases even creating their own
In this column we’ll examine the modulation
scheme used in D-STAR, focusing on the signal-processing chain from the
microphone to the modulator (figure 1). For digital modulation, we need to
get the microphone audio into digital form and properly modulated onto the
carrier, similar to analog modulation. In Figure 1, we see that the
microphone audio is boosted by an amplifier to a suitable level for
driving an analog-to-digital (A/D) converter. The A/D converter samples
the microphone audio and converts it into a series of digital numbers that
represent the microphone waveform. The digitized audio is fed into the
vocoder, which processes the waveform, still in digital form, to reduce
the number of bits per second needed to represent the voice waveform. The
vocoder drives the digital modulator circuit, which for D-STAR is a GMSK
(Gaussian Minimum Shift Keying) modulator (more on that later). At the
output of the GMSK modulator, we see the modulated GMSK signal, which
drives the RF transmitter chain.
Before we dive into the topic of GMSK, we first need to review some other forms of digital modulation. One basic form of digital modulation is Frequency Shift Keying, or FSK. Radio amateurs have used FSK for decades, including using it to send radio teletype signals (RTTY) on the HF bands. The concept is simple: When the digital signal is a logical zero one frequency is output, and when the digital signal is a logical one a different frequency is output. The receiver on the other end has a demodulator circuit that detects the two frequencies and outputs the recovered digital signal accordingly.
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