CRAC Class C (2025) — Questions 401–410

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U0753. Sometimes, amateur stations use VSB to transmit analog ATV signals. Although this method belongs to AM, it sends only one complete sideband plus a small part of the other sideband near the carrier. When transmitting analog video, the advantages of VSB are: (Choose all that apply.)

有时，业余电台使用VSB来传输模拟ATV信号。这种方式虽然 属于AM，却只发送一个完整边带以及另一边带靠近载波的一小 部分。在传输模拟视频时，VSB的优点是：

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VSB (vestigial sideband) keeps one full sideband plus a vestige of the other, so its bandwidth is much narrower than full AM (A), and the retained carrier/vestige preserves the low- frequency (including DC/black-level) video components for faithful reproduction (B). Polarity (C) and power saving vs SSB (D) are not the cited VSB advantages. 数字通信方式 Digital Communication Modes

U0754. The class of emission refers to a set of characteristics of an emission designated by means of standard symbols, e.g., the modulation method of the main carrier, the modulating signal, the type of information sent, and other applicable signal characteristics. The class of emission denoting RTTY transmitted by single-sideband voice is:

发射类别（class of emission）是指用标准符号标示的某发射 的一组特性，例如主载波调制方式，调制信号，被发送信息的 类型以及其他适用的信号特性。表示用单边带话传输的RTTY信 号的发射类别是：

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RTTY over SSB uses an FM (frequency-shift) audio tone pair fed into an SSB transmitter: the ITU emission designator is F2B (F = FM on the subcarrier, 2 = digital selective, B = duplex telegraphy for automatic reception). A1A is CW; J3E is SSB phone; G2B is the PSK31 class. Per the GLOSSARY, emission class (发射类别) uses these standard symbols.

U0755. The class of emission refers to a set of characteristics of an emission designated by means of standard symbols, e.g., the modulation method of the main carrier, the modulating signal, the type of information sent, and other applicable signal characteristics. The class of emission denoting PSK31 transmitted by single-sideband voice is:

发射类别（class of emission）是指用标准符号标示的某发射 的一组特性，例如主载波调制方式，调制信号，被发送信息的 类型以及其他适用的信号特性。表示用单边带话传输的PSK31 信号的发射类别是：

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PSK31 over SSB is a phase-modulated (digital) signal on an SSB carrier — ITU designator G2B (G = phase modulation of the main carrier, 2 = digital selective, B = duplex telegraphy). F2B is RTTY; A1A is CW; J3E is SSB phone.

U0756. The class of emission refers to a set of characteristics of an emission designated by means of standard symbols, e.g., the modulation method of the main carrier, the modulating signal, the type of information sent, and other applicable signal characteristics. The class of emission denoting RTTY transmitted by FM voice is:

发射类别（class of emission）是指用标准符号标示的某发射 的一组特性，例如主载波调制方式，调制信号，被发送信息的 类型以及其他适用的信号特性。表示用调频话传输的RTTY信号 的发射类别是：

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RTTY is fundamentally an FSK (frequency-shift) digital signal; its emission designator is F2B regardless of whether the RF carrier is SSB or FM — the "F" denotes frequency modulation of the main carrier by the digital signal. So even over an FM voice channel the RTTY emission class is F2B (A). F3E is FM phone; J3E is SSB phone; F3F is not the RTTY designator.

U0757. The main considerations in choosing different modulation methods for amateur radio communication are: (Choose all that apply.)

业余无线电通信选用不同调制方式的主要考虑因素是：

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Modulation choice balances fidelity (how faithfully the information passes), robustness to interference (抗干扰能力), and spectrum efficiency (节省频谱资源) (A, B, C). Frequency stability (D) is a transmitter property, not a reason for picking a modulation type.

U0758. To satisfy the requirement of "technical characteristics of stations" in our Radio Frequency Allocation Regulations of the PRC (《中华人民共和国无线电频率划分规定》) that radio communication should "keep the bandwidth at the minimum value permitted by the technical state and the nature of the service", amateur radio operators should understand the necessary bandwidth of various communication methods. The factor that determines the necessary bandwidth is:

为了满足我国《无线电频率划分规定》“电台的技术特性”关 于无线电通信“把带宽保持在技术状态和该项业务的性质所允 许的最低值上”的要求，业余电台操作者应了解各种通信方式 的必要带宽。决定必要带宽的因素是：

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The necessary bandwidth (必要带宽) is set by the information rate and the noise/interference environment — higher data rate and more noise require a wider bandwidth (A). Transmitter power (B), receiver sensitivity (C), and distance (D) do not determine the necessary bandwidth; they affect link budget, not bandwidth.

U0759. Which of the following services use data modes for communication? (Choose all that apply.)

以下哪些业务使用数据方式进行通信？

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All four are data (digital) communication modes: RTTY and FT8 are classic amateur digital modes; wireless LAN (无线局域网) and 5G mobile communication are data services (A, B, C, D). The question tests recognition that "data mode" (数据方式) covers any digital/encoded transmission, not just ham modes.

U0760. Frequency-shift telegraphy refers to: a frequency-modulation telegraph technique in which the telegraph signal controls the carrier frequency to vary within a predetermined range. Which of the following amateur communications uses frequency-shift telegraphy:

频移电报技术（frequency-shift telegraphy）是指：电报信 号控制载波频率在预定的范围之内变化的调频电报技术。下述 业余通信使用的是移频电报技术：

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Frequency-shift telegraphy (频移电报技术) = FSK: the carrier shifts between two (or more) tones. RTTY is the classic FSK telegraph mode (A). CW is on/off keying (amplitude), PSK31 is phase-shift keying, and SSTV is analog FM tone imaging — none is FSK telegraphy.

U0761. The modulation method of RTTY is:

RTTY的调制方式为：

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RTTY uses binary frequency-shift keying — two frequencies representing mark/space — i.e. 2FSK (A). BPSK is PSK31's method; 2ASK is amplitude-shift (not RTTY); 2QAM is a quadrature scheme not used by standard RTTY.

U0762. Digital communication often involves the two parameters "symbol rate", which describes the change of signal states, and "data rate", which describes the information flow. The communication mode RTTY uses two frequencies, f0 and f1, as symbols to transmit the digits 0 and 1, and the symbol transmission duration is typically 0.02 s. The symbol rate and data rate of RTTY are respectively:

数字通信经常涉及描述信号状态变化的“符号速率”和描述信 息流量的“数据速率”这两个参数。通信方式RTTY使用f0和f1 共两个频率作为符号来传输数字0和1，并且符号的发送时长通 常是0.02秒。RTTY的符号速率和数据速率分别为：

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Symbol rate = 1 / symbol duration = 1 / 0.02 s = 50 baud. With binary 2FSK, each symbol carries one bit (0 or 1), so the data rate also equals 50 bits/s (A). RTTY is 1 bit per symbol, so the two rates match.