20.2 Signal Processing, Software & Codes

20.2 Signal Processing, Software & Codes

Quick Answer: SDR = Software Defined Radio. DSP filters, demodulates, and can steer frequency / modulation / detection—but not SWR. DSC code = 10-bit error-correcting characters (data bits + 3-bit ECC). SITOR characters = 7 bits with 4 zeros + 3 ones. Idle Signal (a) = 0000111 = FEC Phasing Signal 1. Modes: Morse, RTTY, ASCII, packet ride on RF; selective calling addresses stations digitally.

Key topic 084 is the software and coding layer of Topic 3-M. If §20.1 explained how channels are shared, this section explains how bits and algorithms process the signal once it is digitized.

Software Defined Radio (SDR)

Definition

SDR means Software Defined Radio—not “software deviation ratio,” “SWR meter,” or “static dynamic RAM.” In an SDR, many functions that older designs built with crystals, discrete IF filters, and analog detectors are implemented as software (embedded firmware, FPGA fabric, or a PC host) operating on digitized samples.

Why GROL techs still matter

SDR does not eliminate RF craftsmanship:

1. The antenna, feedline, and front-end still determine noise figure, IMD, and SWR.
2. Clock quality, ADC overload, and shielding still cause “DSP ghosts.”
3. Illegal emissions still come from bad amplifiers and mixers—even if the waveform was “perfect” in software.

When a customer says “the software radio is broken,” separate RF path failures from firmware/configuration failures before swapping expensive boards.

Digital signal processing (DSP) — what it does and does not do

DSP samples an analog waveform (or processes already-digital baseband) and applies algorithms: filtering, noise reduction, demodulation, AGC, squelch logic, and modulation synthesis.

Pool fact: what DSP does not control

In a modern DSP radio, DSP can be described as controlling:

• Frequency (tuning / LO programming concepts),
• Modulation, and
• Detection (demodulation),

but DSP does not control SWR. Standing-wave ratio is a property of the antenna system and feedline match, outside the DSP core. No amount of clever filtering will turn a disconnected whip into a 1:1 load.

Filtering and demodulation literacy

Think of the classic receiver chain (Ch12) with a digital middle:

1. RF front end still filters images and strong out-of-band signals.
2. ADC digitizes IF or baseband.
3. DSP implements sharp filters that would be bulky as analog crystal banks.
4. Demodulator code recovers audio or bits.
5. DAC (or digital audio link) drives the speaker or data port.

Advantage of DSP filters vs pure analog: flexible bandwidth, adaptive notches, and reproducible performance without aging inductors. Limitation: garbage-in from a saturated front end is still garbage-out.

High-level data modes: Morse, RTTY, ASCII, packet

Element 3 does not require you to send perfect Morse on exam day, but it expects you to place codes on the right “layer” of the system.

Troubleshooting map

Selective calling codes — DSC and GMDSS digital calling

Digital Selective Calling (DSC) is the automated digital calling system used with GMDSS radios (VHF Ch-70, MF/HF DSC frequencies). Element 3 focuses on the character code, not the entire SOLAS carriage matrix.

DSC code structure (pool wording)

The code used for GMDSS-DSC transmissions is best described as a 10-bit error-correcting code starting with bits of data followed by a 3-bit error-correcting field.

Reject distractors:

• 10-bit code that starts with the 3-bit ECC then 7 data bits (order reversed),
• Simple 8-bit with one parity bit only,
• 7-bit code merely “transmitted twice.”

Selective calling in the larger picture

Selective calling lets a coast station or ship address a specific MMSI (or group/all-ships categories) without requiring continuous voice watch on every possible working channel. For the GROL maintainer:

1. Verify MMSI programming and GNSS position input to the DSC controller.
2. Confirm the Ch-70 or MF/HF DSC RF path (antenna, duplexers, dedicated DSC receivers).
3. Remember: DSC is not the same as SITOR telex and not the same as NAVTEX—even though all are “digital maritime.”

SITOR character coding (bridge into marine NBDP)

SITOR (Simplex Teletype Over Radio) / NBDP (Narrow-Band Direct Printing) is the maritime telex family. Element 3 stores two hard coding facts under 084:

Character composition

The code used for SITOR-A and SITOR-B transmissions: each character consists of 7 bits with 4 zeros and 3 ones (constant-ratio code). Receivers use the 4/3 balance as a first error check.

Idle Signal (a) and FEC phasing

Which statement is true in the pool’s control-signal set? Idle Signal (a) (0000111) is used for FEC Phasing Signal 1.

Memorize the bit string 0000111 with the label Idle Signal (a) / FEC Phasing Signal 1. Do not invent that Signal Repetition or Control Signal 1 “is always” the FEC phasing answer when the stem asks for Idle Signal (a).

Codes and modes at a glance

Maintainer workflow: software radio + coded emissions

1. Confirm RF health first — power, frequency, SWR, antenna. DSP cannot “software away” a bad match.
2. Confirm mode and code — J3E voice vs F1B/J2B telex vs DSC; wrong mode yields perfect RF and unreadable traffic.
3. Confirm identity programming — MMSI, answerbacks, EPIRB hex ID (survival gear in §20.4).
4. Use built-in self-tests for DSC/controllers; avoid live distress alerts for curiosity.
5. Document firmware versions on SDR platforms—field “regressions” are often config loads, not blown finals.

Exam-day checklist (3-M 084)

1. SDR = Software Defined Radio.
2. DSP: frequency / modulation / detection yes; SWR no.
3. DSC: 10-bit error-correcting code; data bits followed by 3-bit ECC.
4. SITOR: 7-bit characters with 4 zeros and 3 ones.
5. Idle Signal (a) 0000111 → FEC Phasing Signal 1.
6. Place Morse / RTTY / ASCII / packet on the correct functional layer when troubleshooting.
7. Selective calling = addressed digital call (DSC), not voice shouting a ship’s name on Ch-16 (though voice remains the follow-on).

When software, DSP limits, and the DSC/SITOR bit rules are cold-memorized, Topic 3-M is complete. Topic 3-N applies the same thinking to marine VHF, MF/HF SSB, and SITOR operations.