CRAC Class B (2025) — Questions 441–450

U0747. The method of conducting SSTV contacts using a personal computer (PC) and an ordinary voice transceiver is: (Choose all that apply.)

利用个人电脑（PC）和用于话音的普通收发信机进行SSTV通联的操作方法为：

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The PC generates the SSTV audio (FM-tones carrying the picture) as baseband, then: on HF you drive an SSB transceiver which translates that baseband up to the carrier (A); on VHF/UHF you drive an FM transceiver which applies composite FM modulation (B). FSK (C/D) is not how SSTV audio is produced.

U0748. The baseband signal of slow-scan television (SSTV) is normally generated within the audio range and transmitted by means of single sideband or FM voice. Regarding the SSTV baseband, which of the following descriptions is correct?

慢扫描电视（SSTV）的基带信号通常是在音频范围内生成并通过单边带或调频话方式进行传输的。关于SSTV基带，以下描述正确的是：

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In the standard FM (tone-based) SSTV baseband, brightness is encoded as frequency: white ≈ 2300 Hz, black ≈ 1500 Hz, and sync is the lowest at ≈ 1200 Hz (A). The picture information is carried by tone frequency, not by amplitude levels as the AM- style options claim.

U0749. If the received SSTV or ATV image shows thick horizontal black stripes and the upper and lower half-frames are misaligned, it is because:

如果收到的SSTV或ATV图像出现水平粗黑条纹、上下半帧错位的现象，则是因为：

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Thick horizontal black bands with half-frame misalignment mean the line and frame rates match but the frame-start phase is offset — a frame-sync timing mismatch (A). Unequal line frequency (B) would shear the image diagonally; wrong tuning (C) or polarity (D) produce different defects.

U0750. If the received SSTV or ATV image is tilted left-right, appearing as a parallelogram or slanted thick-stripe pattern, it is because:

如果收到的SSTV或ATV图像左右倾斜，呈平行四边形或斜粗条纹图样，则是因为：

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A left-right slant / parallelogram distortion is caused by the two stations' line-scan frequencies not matching — one side draws lines at a slightly different rate (A). Frame-sync phase offset (B) causes horizontal banding, not slanting; tuning/polarity errors cause other symptoms.

U0751. Why can't regular ATV communication be conducted in amateur bands below 1.2 GHz?

为什么不能在低于1.2GHz的业余频段进行常规的ATV通信？

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ATV (amateur television) is wideband — it needs over 5 MHz of bandwidth, far wider than the narrow segments available below 1.2 GHz, so those lower bands simply cannot hold a regular ATV channel (A). The interference/yielding arguments (B, C) are secondary etiquette concerns, and D's stability claim is not the reason.

U0752. Sometimes, amateur radio operators use FM instead of the classic VSB method commonly used in TV broadcasting to transmit analog ATV signals. The advantage of doing this is:

有时，业余无线电爱好者使用FM而非电视广播常用的经典VSB 方式来传输模拟ATV信号。这样做的优点是：

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With low-power ATV, FM's key advantage is its immunity to amplitude noise: the limiter/discriminator discards AM noise riding on the signal, giving cleaner pictures (A). FM's bandwidth is generally wider, not narrower, than VSB (B false); receiver sensitivity is not an order of magnitude better (C false); and broad compatibility with handheld FM voice gear (D) is not the real technical benefit cited.

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.