CRAC Class C (2025) — Questions 441–450

U0374. FreeDV is a digital voice mode. Its characteristics are: (Choose all that apply.)

FreeDV是一种数字语音方式。其特点是：

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FreeDV is an open-source, narrowband HF digital voice mode that lets hams experiment with DSP and build their own gear (A, B, C). D is false: FreeDV is designed for SSB transceivers, not FM handhelds, and "especially beautiful audio" is not a characteristic.

U0375. Digital amateur television (DATV) is a mode applying digital audio/video compression coding to real-time image communication. When conducting DATV contacts via a satellite transponder, amateurs generally choose the DVB-S2 protocol; the modulation may be: (Choose all that apply.)

数字业余电视（DATV）是将数字音视频压缩编码实验用于实时图像通信的一种方式。通过卫星转发器进行DATV联络时，爱好者们普遍选用DVB-S2协议，调制方式可以是：

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DVB-S2 (used for satellite DATV) supports several modulation schemes: QPSK, 8PSK, 16APSK and 32APSK. All four are valid DVB-S2 modulations, so all are correct.

U0376. When using the DRM protocol for digital slow-scan television (DSSTV) communication, amateurs always tend to transmit the DSSTV baseband (generated within the audio range) via single sideband rather than FM phone. The main reason is:

使用DRM协议进行数字慢扫描电视（DSSTV）通信时，爱好者总是倾向于通过单边带而非调频话方式来传输在音频范围内生成的DSSTV基带。主要原因是：

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SSB linearly passes the audio-range DSSTV baseband without FM's threshold/nonlinearity, preserving the digital signal and aiding error control in fading. B conflates cause (FM's threshold does widen bandwidth but the stated main reason is A's baseband fidelity); C/D mischaracterize FM's effect on quality.

U0791. An amateur radio station transmits in FM with an RF output power of 10 W, and its power-supply efficiency is about 80%. If the station transmits continuously for 10 seconds, the average transmit power during this period:

一部业余无线电台以FM方式发射时的射频输出功率为10瓦，电源效率约为80%。若该台连续发话10秒钟，则此期间的平均发射功率：

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FM is a constant-envelope mode, so the RF output stays at 10 W continuously while transmitting; the 80% power-supply efficiency only affects how much DC power is drawn (≈12.5 W), not the RF output. Average RF power ≈ 10 W.

U0792. An amateur radio station transmits in CW with a carrier output power of 10 W, and its power-supply efficiency is about 80%. If the station sends telegraphy continuously for 10 seconds, the average transmit power during this period:

一部业余无线电台以CW方式发射时的载波输出功率为10瓦，电源效率约为80%。若该台连续发报10秒钟，则此期间的平均发射功率：

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CW text consists of dots and dashes with spaces (off periods), so the carrier is not on 100% of the time; the time-averaged power is well below the 10 W key-down carrier level. The 80% efficiency is a distractor for the RF average.

U0793. An amateur radio station transmits in SSB with a peak output power of 10 W, and its power-supply efficiency is about 80%. If the station speaks continuously for 10 seconds, the average transmit power during this period:

一部业余无线电台以SSB方式发射时的峰值输出功率为10瓦，电源效率约为80%。若该台连续发话10秒钟，则此期间的平均发射功率：

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The 10 W given is peak envelope power (PEP) — the momentary voice peak. Normal speech averages only about 20–30% of PEP, so the average RF power over 10 s is far below 10 W. The 80% efficiency concerns DC input, not the RF average.

U0794. An amateur radio station transmits in SSB with a peak output power of 10 W, and its power-supply efficiency is about 80%. If it sends an FT8 message lasting 12.6 seconds using this mode, the average transmit power during this period:

一部业余无线电台以SSB方式发射时的峰值输出功率为10瓦，电源效率约为80%。若该台利用此方式发送一条时长12.6秒的 FT8消息，则此期间的平均发射功率：

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Although the SSB rig's PEP is 10 W, FT8 is a constant-amplitude (constant-envelope) 8FSK signal, so it drives the transmitter at full PEP continuously for the whole 12.6 s. Thus the average RF power ≈ 10 W (PEP = average here).

U0795. An amateur radio station transmits in SSB with a peak output power of 10 W, and its power-supply efficiency is about 80%. If it sends a PSK31 message lasting 12.8 seconds using this mode, the average transmit power during this period:

一部业余无线电台以SSB方式发射时的峰值输出功率为10瓦，电源效率约为80%。若该台利用此方式发送一条时长12.8秒的 PSK31消息，则此期间的平均发射功率：

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PSK31 (BPSK/QPSK) is phase modulation with constant envelope, so like FT8 it holds the transmitter at full PEP continuously; average RF power over the 12.8 s ≈ 10 W. The efficiency figure is irrelevant to the RF average.

U0796. An amateur radio station transmits in SSB with a peak output power of 10 W, and its power-supply efficiency is about 80%. If it sends a 10-second Robot 8 SSTV black-and-white image using this mode, the average transmit power during this period:

一部业余无线电台以SSB方式发射时的峰值输出功率为10瓦，电源效率约为80%。若该台利用此方式发送一段时长10秒的 Robot 8 SSTV黑白图像，则此期间的平均发射功率：

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Analog SSTV over SSB is a near-constant-amplitude FM-like signal (the picture modulates the audio tone continuously), so the transmitter runs at essentially full PEP throughout the 10 s scan; average RF power ≈ 10 W. DX 与竞赛操作 DX and Contest Operating

U0797. How do you call all DX stations?

如何呼叫所有DX电台？

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"CQ DX" is the standard general call specifically inviting any distant (DX) station to answer. "CQ AF/SA/OC" would restrict the call to a continent (Africa/South America/Oceania), not "all DX".