2020 Extra Class study guide: E2E – Operating methods: operating HF digital modes

Perhaps the most popular digital mode these days is PSK31. PSK stands for “phase shift keying.” One of its main advantages is that it had a very narrow bandwidth—only 31 Hz. One of the reasons that PSK31 has such a narrow bandwidth is that it uses variable length coding. That is to say, characters have different numbers of bits, depending on how frequently they appear in normal text.

QUESTION: Which of these digital modes has the narrowest bandwidth? (E2E10)
ANSWER: PSK31

QUESTION: Which of the following HF digital modes uses variable-length coding for bandwidth efficiency? (E2E09)
ANSWER: PSK31

 

Another type of modulation commonly used on the HF bands is frequency-shift keying, or FSK. FSK is a type of modulation that is common for data emissions below 30 MHz. RTTY, for example uses FSK modulation. It shifts between two different frequencies, which denote “mark” and “space,” which then translate to one and zero.

Amateur transceivers use two different methods to modulate a signal using FSK: direct FSK and audio FSK. In direct FSK, a digital signal is connected to the transceiver to shift the frequency. When using audio FSK, the transceiver is set up to transmit an SSB signal and an audio signal, typically from a computer sound card, is used to shift the frequency of the transmitted signal. The biggest difference between the two methods is that when using direct FSK, the FSK signal occupies less bandwidth.

Which of the following types of modulation is common for data emissions below 30 MHz? (E2E01)
ANSWER: FSK

QUESTION: What is the difference between direct FSK and audio FSK? (E2E11)
ANSWER: Direct FSK occupies less bandwidth

To tune an FSK signal, one often uses a crossed-ellipse display. You have properly tuned a signal when one of the ellipses is as vertical as possible, and the other is as horizontal as possible. When one of the ellipses in an FSK crossed-ellipse display suddenly disappears, selective fading has occurred.

QUESTION: What is indicated when one of the ellipses in an FSK crossed-ellipse display suddenly disappears? (E2E04)
ANSWER: Selective fading has occurred

 

PACTOR is another digital mode that uses FSK. It uses the ARQ protocol to detect errors, and because of this, you can use PACTOR to transfer binary files error-free. One of the disadvantages of PACTOR is that you can’t use it for keyboard-keyboard communications like you can RTTY and PSK31.

QUESTION: Which of the following HF digital modes can be used to transfer binary files? (E2E08)
ANSWER: PACTOR

QUESTION: Which of these digital modes does not support keyboard-to-keyboard operation? (E2E05)
ANSWER: PACTOR

Another way to detect and correct errors in a data transmission is forward error correction (FEC). Digital transmission systems that use forward error correction send redundant data that not only allows a receiver to detect errors, but also correct some errors.

QUESTION: What do the letters FEC mean as they relate to digital operation? (E2E02)
ANSWER: Forward Error Correction

No matter what type of modulation you use, data transmission over an HF radio link is very slow. 300 baud is the most common data rate used for HF packet communications. In fact, due to bandwidth limitations, 300 baud is the maximum symbol rate. Under clear communication conditions, 300 baud packet has the fastest data throughput on the HF bands.

QUESTION: What is the most common data rate used for HF packet? (E2E06)
ANSWER: 300 baud

QUESTION: Which of these digital modes has the fastest data throughput under clear communication conditions? (E2E13)
ANSWER: 300 baud packet

Some HF digital modes operate automatically, that is to say the software running these digital modes automatically initiate and maintain the connection. One technique for doing this is called Automatic Link Enable (ALE). ALE stations establish contact by constantly scanning a list of frequencies, then activating a radio when it receives a designated call sign. There are many reasons that you may not be able to initiate a contact with an ALE or other digital station, including using the wrong transmit frequency, using a protocol not supported by the digital station, or another station, that you may not be able to hear, is using the frequency.

QUESTION: How do ALE stations establish contact? (E2E12)
ANSWER: ALE constantly scans a list of frequencies, activating the radio when the designated call sign is received

QUESTION: Which of the following is a possible reason that attempts to initiate contact with a digital station on a clear frequency are unsuccessful? (E2E07)
ANSWER: All these choices are correct

• • Your transmit frequency is incorrect
  • The protocol version you are using is not supported by the digital station
  • Another station you are unable to hear is using the frequency

 

One digital mode that is becoming quite popular is FT4. Part of the WSJT suite, it is similar to FT8 in that uses fixed-length transmissions, structured messages with formats optimized for minimal QSOs, and strong forward error correction. The biggest difference is that FT4 has a 7.5 s transmit/receive cycle instead of a 15 s cycle. The tradeoff is that FT4’s weak-signal performance is not quite as good as FT8’s.

QUESTION: How is the timing of FT4 contacts organized? (E2E03)
ANSWER: Alternating transmissions at 7.5 second intervals