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  • Stu WØSTU

PSK31 Varicode (G8C08)

The 2019-2023 General License question pools includes a question about the type of encoding used with the PSK31 digital mode:

G8C08: Which of the following statements is true about PSK31?

A. Upper case letters are sent with more power

B. Upper case letters use longer Varicode bit sequences and thus slow down transmission

C. Error correction is used to ensure accurate message reception

D. Higher power is needed as compared to RTTY for similar error rates

Phase Shift Keying 31 (PSK31) is a digital mode providing keyboard-to-keyboard text messages over the HF and higher bands. It’s very narrow bandwidth makes it highly power efficient and resilient in noisy conditions. The data rate is slow at 31.25 baud, or roughly 50 words-per-minute, but usually quite sufficient for most keyboard message operations. The PSK31 emissions use signal waveform phase reversals in consistent time intervals throughout a transmission to encode the 1s and 0s of alphanumeric and other characters. That is, with a binary PSK protocol, the waveform phase angle shifts 180-degrees within a time window to indicate one binary digit, such as '1,' and the absence of such a shift within a time window indicates the other binary digit, '0.' (Note: There is also a quadrature PSK31 that uses four phases instead of only two, improving error correction with practically no increase in bandwidth.)

A preamble and postamble sequence provide identifiable bookends for the message content. A sound card interface (SCI) is typically used to connect a computer to the amateur transceiver, and the computer keystrokes are translated into phase shifted audio by the SCI for transmissions. Received audio is decoded via the SCI back into characters displayed in a software utility window.

The four question responses allude to the code used by PSK31, while item D makes a claim about power efficiency relative to another digital mode, RTTY (radio teletype). The bandwidth of PSK31 is usually narrower than that of the lowest data rates of RTTY on the HF bands, so it is not likely that higher power is required for similar error rates between these two digital modes. Response D is eliminated.

PSK31 uses Varicode, in which different characters are assigned variable-length binary (1s and 0s) representations. More commonly used letters of the alphabet are assigned shorter length codes, while less commonly used letters have longer codes. This helps keep transmissions efficient, much like International Morse Code for CW operations, also a variable length code. Here are some salient examples of Varicode character code lengths:

e 11 v 1111011 t 101 x 11011111 o 111 j 111101011

E 1110111 V 110110101 T 1101101 X 101110101 O 10101011 J 111111101

It is easy to reason that the upper case characters do not impact signal strength, as claimed by response A. The same power level is used to transmit each 1 or 0 phase-shifted code digit. Easily eliminate response A.

Let’s consider the remaining responses, B and C. Response B seems to have some merit. Notice in our examples that just as B claims, the upper case letters have longer Varicode representations. If we examine a complete table of Varicode characters we will notice this trend generally across the set of characters. [ARRL Varicode Specification Table]

While the Quadrature Phase Shift Keying (QPSK) noted above may employ an error correction method as claimed by response C, the more common Binary PSK (BPSK) does not. Thus, response C is not universally applicable to PSK31 operations. It is eliminated, leaving the universally viable option B.

Since the upper case letters are less frequently used in normal text, it makes sense for efficiency to leave the longer code assignments to those big letters. Additionally, when upper case letters are transmitted, they require more of the transmission’s equally-timed segments to depict the character than do the lower case letters. As a result the transmission of upper case letters slows down the PSK31 message transmission.

The answer to General Class question G8C08, “Which of the following statements is true about PSK31?” is “Upper case letters use longer Varicode bit sequences and thus slow down transmission.”

-- Stu WØSTU


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