Winter 2009 Issue

FM

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 D-STAR designs.

Block Diagram

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.

GMSK

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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