2.11 Digital Transmission Modes and Bandwidth

2.11 Digital Transmission Modes and Bandwidth

ACMA Exam Focus: Syllabus item 5.5 asks you to recall that Morse code, Radio Teletype (RTTY), Frequency Shift Keying (FSK), Phase Shift Keying (PSK) and Packet radio are types of digital transmissions, and to recall that the bandwidth of a data transmission is dependent on the data transfer rate and the modulation type. Expect one question that names a mode and one that links keying speed to occupied bandwidth.

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1. What Makes a Transmission Digital?

An analogue mode such as SSB telephony varies the carrier smoothly and continuously in step with a voice waveform. A digital transmission does something different: it switches one property of the carrier - amplitude, frequency or phase - between a small number of discrete states. Morse code has two amplitude states, on and off. RTTY has two frequency states, mark and space. PSK31 has two phase states, 0 degrees and 180 degrees. Packet radio wraps bytes into addressed frames before keying them onto the carrier.

Two terms are easy to confuse and worth separating now:

• Baud (symbol rate) is the number of signalling states sent per second.
• Bit rate is the number of information bits per second. When each symbol carries one bit, as in ordinary two-tone FSK, the two numbers are equal. When each symbol carries several bits - as in the eight-tone modulation used by FT8 - the bit rate is higher than the baud rate.

The key physical fact behind syllabus 5.5 is that switching a carrier creates sidebands. The faster you switch, the further those sidebands spread either side of the carrier, and the wider the occupied bandwidth becomes.

2. Morse Code (CW) - A1A

Morse is the oldest digital mode. The transmitter's carrier is simply turned on and off by the key, which is why its emission designator is A1A: amplitude modulation, a single channel of digital information without a sub-carrier, telegraphy for aural reception.

Switching a carrier abruptly generates keying sidebands. A perfectly square on-off transition produces energy far either side of the carrier, heard by everyone else on the band as key clicks. Transmitters therefore shape the rise and fall of each element, typically over about 5 milliseconds, which keeps the occupied bandwidth to roughly 100 to 150 Hz at conversational speeds.

A useful approximation for CW bandwidth is:

$B \approx K \times \frac{\text{WPM}}{1.2}$

where $K$ is a shaping factor of about 3 for a well-filtered signal and about 5 for hard keying. At 20 words per minute the keying rate is $20 / 1.2 \approx 16.7$ baud, giving a bandwidth of roughly 50 Hz with good shaping and around 83 Hz with hard keying. Emission designators for CW are conventionally written as 150HA1A, allowing headroom for less-than-ideal keying.

3. Radio Teletype (RTTY)

RTTY sends the Baudot five-bit alphabet, with figures and letters shift characters to reach numbers and punctuation. Amateur practice has settled on a keying rate of 45.45 baud and a 170 Hz shift between the two tones. The higher radio frequency is mark and the lower is space. A receiver filter of about 250 Hz copies this comfortably; anything wider simply lets in more noise and more adjacent signals.

4. FSK Versus AFSK

There are two ways to produce the same RTTY signal, and the exam expects you to know the difference:

• FSK (Frequency Shift Keying) keys the transmitter's own frequency-determining stage directly. The radio has an FSK input; the carrier itself shifts between mark and space.
• AFSK (Audio Frequency Shift Keying) feeds two audio tones - commonly 2125 Hz for mark and 2295 Hz for space - into an SSB transmitter, which translates them to radio frequency.

The radiated signal is identical. A receiving station cannot tell which method you used. What differs is the risk. Because AFSK passes through the whole SSB audio chain, excessive audio drive pushes the transmitter into compression, the ALC clamps down hard, and intermodulation between the audio tones and their harmonics throws distortion products either side of the wanted signal. That is splatter, and it can widen a 250 Hz signal into several kilohertz. The remedy is to keep audio well below the level at which ALC action begins, and to switch off speech processing and compression.

5. Phase Shift Keying - PSK31

PSK31 carries information in 180 degree phase reversals of a continuous audio tone rather than in frequency changes. Its symbol rate is 31.25 baud, chosen to match comfortable typing speed, and it uses varicode: frequent characters get short codes and rare characters get long ones, so average throughput is far better than the raw baud rate suggests.

The occupied bandwidth is about 31 Hz, with the ITU emission designator 60H0J2B. To achieve that, the transmitted envelope is cosine-filtered so that phase reversals do not produce clicks. Because the PSK31 envelope is not constant, the transmitter must stay strictly linear - the same low-drive, no-compression discipline as AFSK RTTY.

6. Packet Radio

Packet radio wraps data into frames under the AX.25 protocol, complete with source and destination callsigns, a frame check sequence and automatic repeat of corrupted frames. A terminal node controller (TNC) or a software modem does the framing.

• 1200 baud packet uses AFSK tones (1200 Hz and 2200 Hz) fed through the microphone input of an FM transceiver. Because the whole FM channel is used, the occupied bandwidth is that of the FM channel, roughly 12 to 16 kHz. This is what APRS position beacons use.
• 9600 baud packet cannot pass through a microphone input at all. It is applied directly to the modulator and taken directly from the receiver discriminator, and it needs a wider, flatter channel of roughly 15 to 20 kHz.

7. Modern Weak-Signal Modes

FT8 is worth knowing even though it postdates most syllabuses. It uses eight-tone FSK with 6.25 Hz tone spacing, so the total occupied bandwidth is about 50 Hz. Transmissions are locked to 15-second sequences, of which roughly 12.6 seconds is actual transmission. The strict timing and heavy forward error correction are what allow contacts at signal-to-noise ratios far below the level at which a human could hear the signal.

8. Bandwidth Depends on Data Rate and Modulation Type

This is the sentence the examiner will test. For a two-tone FSK transmission:

$B \approx (K \times R) + \Delta f$

where $B$ is the occupied bandwidth in hertz, $R$ is the keying rate in baud, $\Delta f$ is the frequency shift in hertz, and $K$ is a shaping constant of about 1.2 for a well-filtered signal.

Worked example. Standard amateur RTTY runs at 45.45 baud with a 170 Hz shift:

$B \approx (1.2 \times 45.45) + 170 = 54.5 + 170 \approx 225\ \text{Hz}$

That is why a 250 Hz filter is the classic RTTY choice. Now change one variable at a time:

• Raise the data rate to 100 baud, same 170 Hz shift: $B \approx 120 + 170 = 290$ Hz.
• Widen the modulation to an 850 Hz shift at the original 45.45 baud: $B \approx 54.5 + 850 \approx 905$ Hz.

Both levers widen the signal, and neither one alone tells the whole story - hence data transfer rate and modulation type.

9. Why Bandwidth Matters in Australia

Bandwidth is not only a courtesy issue. The Radiocommunications (Amateur Stations) Class Licence 2023 attaches power spectral density limits to wide emissions: once an amateur standard station uses an emission mode whose necessary bandwidth exceeds the threshold specified for the band in question, the maximum power spectral density from the station must not exceed 1 watt per 100 kHz. Narrow digital modes such as CW, PSK31, FT8 and RTTY sit far below any such threshold, so in normal digital operating the limit that actually binds you is the station power limit, not the spectral density rule. The practical lesson for the exam is simply that wider emissions attract extra conditions, so you should choose the narrowest mode and the lowest data rate that will do the job.