Digital communications

Hams have developed techniques for transforming 1’s and 0’s into tones into the same frequency range as human voice.

So now a radio can transmit and receive digital data.

The addition of information to a radio wave is called modulation.

AM= Amplitude modulationFM= Frequency modulation

The rules by which digital data is packaged and exchanged is called protocol.

A modem (modulator-demodulator) changes data signals to and from audio signals.

If the modem combines computer software that performs the protocol rules and a soundcard, it is called a terminal node controller (TNC)

The TNC is connected between the transceiver and the computer in a packet radio station. T4A06

Some examples of digital modesT8D01

On HF BandsRadioteletype (uses 5-bit Baudot code)Winlink 2000 (uses PACTOR or WINMOR protocols)

PSK31 keyboard to keyboard MFSK

On VHF/UHFPacket radio (uses AX.25 protocol)

CW (uses Morse code) (T8D09)

Since Radio signals experience many disruptions as they travel from transmitter to receiver; it is likely that errors may be introduced into the stream of data.

This is measured by the bit error rate (BER) (T7B12)

Some protocol codes include elements to allow the receiver to detect and sometimes even correct the errors.

This special type of code is called a parity bit (T8D11)

Packet radio is the most common digital mode on VHF and UHF.

Packets are transmitted in noise-like bursts. Frequency shift keying (FSK) is used to transmit the individual characters as a series of rapidly alternating tones.

The receiving TNC reassembles the data from the packets.

Each “packet” contains a header with info about the packet and the call sign of the destination station The header also contains checksum (the added element to allow the receiver to check for errors)

If an error is detected, the receiver automatically requested a retransmission. This is called ARQ for automatic repeat request. (T8D08)

Digital modes designed for real-time person to person communication are called keyboard to keyboard modes.

Most popular digital mode is PSK31 (phase shift keying, 31 baud) (T8D06); even though it sends data at a low rate, it works well in noisy conditions (T8D07).

The Automatic Position Reporting System (APRS) uses packet radio to transmit the location of a mobile or portable station. (T8D02)

The portable station is basically a packet radio station with a Global Positioning System (GPS) receiver attached to the TNC. (T8D03)

Traveling hams can send e-mail “over the air” by using Winlink. Winlink combines e-mail with a digital modes.

The radio station that makes connection between the traveling radio and the internet is called a gateway. (T8C11)

T7B12What does the acronym “BER” mean when applied to digital communications systems?A. Baud Enhancement RecoveryB. Baud Error RemovalC. Bit Error RateD. Bit Exponent Resource

T8D01Which of the following is an example of a digital communications method?A. PacketB. PSK31C. MFSKD. All of these choices are correct

T8D08Which of the following may be included in packet transmissions?A. A check sum which permits error detection

B. A header which contains the call sign of the station to which the information is being sent

C. Automatic repeat request in case of error

D. All of these choices are correct

T8D09What code is used when sending CW in the amateur bands?A. BaudotB. HammingC. International MorseD. Gray

T8D11

What is a “parity” bit? A. A control code required for automatic position reportingB. A timing bit used to ensure equal sharing of a frequencyC. An extra code element used to detect errors in received dataD. A “triple width” bit used to signal the end of a character

T8D06What does the abbreviation PSK mean?A. Pulse Shift KeyingB. Phase Shift KeyingC. Packet Short KeyingD. Phased Slide Keying

T8D07What is PSK31?A. A high-rate data transmission modeB. A method of reducing noise interference to FM signalsC. A method of compressing digital television signalD. A low-rate data transmission mode

T8D02What does the term APRS mean?A. Automatic Position Reporting SystemB. Associated Public Radio StationC. Auto Planning Radio Set-upD. Advanced Polar Radio System

T8D03Which of the following is normally used when sending automatic location reports via amateur radio?A. A connection to the vehicle speedometerB. A WWV receiverC. A connection to a broadcast FM sub-carrier receiverD. A Global Positioning System receiver

T4A06Which of the following would be connected between a transceiver and computer in a packet radio station?A. TransmatchB. MixerC. Terminal node controllerD. Antenna

T4A07How is the computer’s sound card used when conducting digital communications using acomputer?A. The sound card communicates between the computer CPU and the video displayB. The sound card records the audio frequency for video displayC. The sound card provides audio to the microphone input and converts received audio to digital formD. All of these choices are correct

T8C11What name is given to an amateur radio station that is used to connect other amateur stations to the Internet?A. A gatewayB. A repeaterC. A digipeaterD. A beacon