A signal code is a short table of agreed patterns that stands for a few messages, and it works when no pattern can be mistaken for another and the most urgent message is the simplest to send. If every pattern has the same number of short and long pulses and any two differ in at least two places, one mistaken pulse turns a pattern into something that is not in the table, so the receiver knows to ask again instead of acting on the wrong message; 5 pulses give 16 such patterns, of which 10 to 12 are worth using.
Part of Signal Systems and Communication
A code system assigns a pattern of flashes, blasts or beats to each message a community needs to send, and the design rules decide whether a tired or frightened operator can use it without error.
Why The Parent’s 10-To-12 Limit Is Not Arbitrary
The parent article says to keep the code book to at most 10 to 12 messages and to make the emergency signal one simple action. The first rule has a counting argument behind it.
Worked Example: How Many Patterns Survive An Error Check
Take a pulse that is either short (S) or long (L), as with horn blasts or lamp flashes.
- Patterns of exactly 3 pulses: 2 x 2 x 2 = 8. Patterns of 4: 16. Patterns of 5: 32.
- If any two patterns must differ in at least two pulse positions, then a single mistaken pulse cannot turn one valid pattern into another. One simple rule that achieves this is to use only patterns with an even number of long pulses. For length 4 that leaves the 8 patterns SSSS, LLSS, LSLS, LSSL, SLLS, SLSL, SSLL, LLLL.
- Length 3 gives 4 such patterns, length 4 gives 8, and length 5 gives 16 (half of the 32).
So to carry 10 to 12 messages with this check you need 5-pulse patterns. More messages need longer patterns, and every extra pulse is another chance for a tired operator to slip and another second before the alarm is complete. That is the reason to keep the code book small.
The check catches one mistaken pulse (a long sent as short or the reverse). It does not catch a pulse dropped or added, or two errors that cancel, so the receiver still counts the pulses and repeats back.
Principles For Building The Table
- List the messages first, then rank them by urgency. The most urgent gets the simplest pattern (the shortest, or one repeated action).
- One meaning per pattern, across every channel. The same pattern means the same thing whether it is blasted on a horn, flashed by lamp, beaten on a drum or whistled. Two tables for the same message is how the parent article’s two whistle codes collided.
- No pattern may be the beginning of another. A signal of one long blast is the start of one long plus two short, so the receiver must wait to learn which was sent. The parent’s code book has this fault four times: one long blast (ALERT) begins ALL CLEAR, VISITORS and RALLY, and one short blast (NEGATIVE) begins ACKNOWLEDGED. Fixed-length patterns avoid it.
- Same length for all non-alarm patterns, with a gap between repetitions.
- Authenticate and change it. See Authentication and Security.
- Write it down, number the version and pin it at every post and signalling position.
Method: Building And Testing A Code Book
Step 1: Write the messages the community will really need. Typically: alarm, assemble, attention, all clear, visitors, acknowledged, negative. Eight to ten is enough.
Step 2: Choose a pattern length and assign. Use Simple Code Tables as a starting point.
Step 3: Check every pair. For each two patterns confirm they differ in at least two positions and that neither starts the other.
Step 4: Test under stress. Run drills at night, after exercise, with a distraction. See Stress Inoculation. Record the errors; the pairs that get confused go back to Step 2.
Step 5: Train operators and publish the version. Everyone knows the alarm; at least 3 to 4 people know the full table, as the parent article advises.
Common Mistakes
- More messages than you can drill. An unpractised message is a false message.
- Different pitches for different messages. Pitch is easily lost at distance and in wind.
- Meanings by custom. “Everyone knows what that means” breaks when a new member arrives.
- Never changing the book. An outsider who has watched it long enough knows it.
What’s Next
- Learn next: Simple Code Tables — a worked table that follows these rules
- Combine with: Priority and Urgency Codes — which signal wins when two collide