CRAC Class C (2025) — Questions 391–400
U0743. What amateur radio slow-scan television transmits is:
业余无线电慢扫描电视所传送的是：
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SSTV (slow-scan television) sends a still (static) picture, one scan line at a time, over several seconds (A). It is not moving video and typically not interlaced like fast-scan TV.
U0744. A certain amateur station intends to transmit several circuit diagrams with a resolution of 1024×768 pixels or somewhat higher over a short-wave voice channel. Appropriate modes of communication could be: (Choose all that apply.)
某业余电台拟通过短波话音信道传输几张分辨率为1024x768 像素或更高一些的电路图。适当的通信联络方式可以是：
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Over a short-wave voice (SSB) channel you can send moderate- resolution still images with digital SSTV (DSSTV, supports higher resolution) or radio facsimile (FAX) (A, B). Plain analog SSTV (D) is lower resolution and typically used on FM; ATV (C) needs several MHz of bandwidth and is not feasible on a narrow HF voice channel.
U0745. When receiving SSTV signals in modes such as Robot, Scottie, Martin, SC2, or PD, we hear a periodic, very brief "click, click" sound; they are:
接收Robot、Scottie、Martin、SC2或PD等方式的SSTV信 号时，我们会听到一种周期性的，非常短促的“咯、咯”声， 它们是：
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At the start of each scan line, SSTV sends a short visicorder/line sync tone — the periodic "click" you hear is the line synchronization signal (A). It is not a parasitic oscillation, a standard-frequency tone, or a beat note.
U0746. SSTV communication provides different transmission modes such as Robot, Scottie, AVT, and Martin. The specific differences are:
SSTV通信提供了Robot、Scottie、AVT、Martin等不同传输 方式。具体区别是：
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The SSTV modes differ mainly in their technical parameters — pixel resolution, how pixels are modulated, line frequency, and frame frequency — letting operators trade off picture quality and robustness to conditions (A). They mostly occupy similar audio bandwidth, and the differences are not fundamentally about receiver sensitivity or "difficulty".
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.