CRAC Class B (2025) — Questions 461–470

Forward Error Correction (前向纠错, FEC) adds redundant check bits so the receiver can detect and correct errors on its own without a round trip (A). B and C and D describe retransmission/repetition schemes (ARQ or brute-force repeat), not true FEC.

Automatic Repeat Request (自动重传请求, ARQ) requires a feedback link: the sender retransmits when it gets a re-request or no acknowledgment (A). B is the FEC definition; C/D are not the standard ARQ description (ARQ acts on missing ACK, not on the receiver echoing the whole message back).

To run RTTY with a PC and voice transceiver, the sound card generates an FSK audio tone pair (the digital baseband), which an SSB transmitter shifts up to the RF carrier (A). MT63 (B) and DRM (C) are different modes, and SSB alone (D) without the FSK audio cannot produce RTTY.

PSK31 is generated by the computer as an audio signal (the PSK baseband modulation) which is then fed into the microphone/audio input of a normal SSB transceiver; the SSB stage simply shifts that audio up to the RF carrier (频率搬移). MFSK16 (B) and GMSK (C) are different digital modes, and D is wrong because PSK audio must still be SSB-modulated onto RF.

PACKET (based on the AX.25 protocol) is sent by feeding an FSK audio tone pair from the PC into the SSB transceiver; the SSB stage shifts the audio baseband up to RF (频率搬移). THROBX2 and ALE1200 are not standard PACKET modems, and D omits the FSK baseband step.

PACKET/AX.25 is conventionally run at 300 baud on HF, 1200 baud on VHF (e.g., APRS), and up to 9600 baud on VHF/satellite links. These three rates (300/1200/9600) are the standard choices; the distractors mix in non-standard speeds.

On VHF, PACKET (including APRS) is almost universally modulated onto an FM carrier — the audio FSK tones (1200/2200 Hz) are transmitted via FM. SSB/AM/OFDM are not the standard VHF packet method.

APRS (Automatic Packet Reporting System) is built on PACKET and is primarily used to share position, status, and other telemetry — especially by mobile stations (GPS position beacons). It does not carry HD video (B), is not merely a monitoring channel (C), and is FSK-based, not chirp (D).

MFSK16, FT8, and Olivia all transmit several bits at once by using multiple tones simultaneously (parallel/orthogonal multi-frequency signaling), unlike RTTY/APRS which send one bit at a time. CW (D) is a single serial keyed tone, so it is not parallel.

MFSK16 encodes several bits per symbol, so its symbol (tone) duration is longer than RTTY's; longer symbols ride out short fades and impulse noise, giving better HF weak-signal performance. B and C are false generalizations, and D contradicts the given fact.