Chapter 6 Digital Modes Bit – fundamental unit of data; 0 or 1 Bit rate – number of digital bits per second sent from one computing system to the other Symbol – a characteristic of the transmitted signal that represents data Baud or bauds – number of symbols sent per second (symbol rate)

Duty cycle – the ratio of time that the transmitter is on to the total of on time plus off time Protocol – rules that control the method used to exchange data Mode – the combination of a protocol with a modulation method such as RTTY or PSK31

Frequency shift keying (FSK) is a shift in the transmitter’s carrier frequency to represent the binary data. A more common approach is audio frequency shift keying (AFSK). This method uses two audio frequencies to represent the binary data. The advantage of AFSK is that the tones can be fed directly into the microphone input of a transmitter. LSB is used for AFSK. As the symbol rate increases, the two tones must be spaced further apart.

Radioteletype (RTTY) is an old form of digital communications which uses a mark (1) and space (0). The initial bit is a start bit and the characters are separated by a stop bit. RTTY uses the 5 bit Baudot code. On HF, most RTTY signals are 45 baud and use a 170 Hz shift between mark and space.

Multiple-frequency shift keying (MFSK16) uses 16 separate tones separated by 15.625 Hz. This fits within a 500 Hz CW bandpass filter. MFSK16 provides good weak signal performance. MFSK16 does not provide error correction. Phase Shift Keying Phase shift keying (PSK) is when the phase of the tones or signal are varied to create the binary signal.

PSK31 is a popular mode that uses a 180̊ phase shift of the tone to create the binary signals. The baud rate is 31.25 Uses a varicode where the characters do not have a fixed length (similar to Morse code).

Packet Modes A packet is a structured data group. An entire packet must be received to be processed correctly at the receiving end.

Errors can be detected by calculating a checksum Errors can be detected by calculating a checksum. This is similar to an ARRL radiogram with a letter count. Like a letter count, a checksum is not robust and may incorrectly say a message has no errors. A more sophisticated count is the cyclic redundancy check (CRC). If an error is detected, the receiving system responds with NAK (not acknowledged) message and the packet is resent. This protocol is called Automatic Repeat Request (ARQ).

Forward error correction (FEC) includes some redundant information in the packet to allow for some error correction at the receiving end without a resend. Packet radio is used in VHF and UHF bands at baud rates of 1200 or 9600 baud. PACTOR I uses ARQ and FSK for reliable HF communications. PACTOR II and PACTOR III use PSK. WINMOR also uses ARQ and FSK or PSK for HF communications. Winlink 2000 uses either PACTOR or WINMOR.

Finding Digital Activity Band Notes (Meters) (MHz) 160 1.800 - 1.810 80 3.570 - 3.600 60 Data emission not permitted 40 7.080 - 7.125 RTTY DX calling frequency 7.040 MHz 30 10.130 - 10.150 20 14.070 - 14.0995 and 14.1005 - 14.112 PSK31 calling frequency 14.070 MHz 17 18.100 - 18.110 15 21.070 - 21.110 12 24.920 - 24.930 10 28.070 - 28.150