WEBVTT NOTE Amateur Television, Fast and Slow 1 00:00:00.000 --> 00:00:02.827 Fast-scan analogue television. 2 00:00:02.827 --> 00:00:12.028 Question. How many horizontal lines make up a fast-scan (NTSC) television frame? 3 00:00:12.928 --> 00:00:16.339 Answer. 525. 4 00:00:16.939 --> 00:00:25.874 Question. How is an interlaced scanning pattern generated in a fast-scan (NTSC) television system? 5 00:00:26.774 --> 00:00:33.967 Answer. By scanning odd-numbered lines in one field and even-numbered lines in the next. 6 00:00:34.567 --> 00:00:41.004 Two fields of 262.5 lines each interleave to make one 525-line frame. 7 00:00:41.004 --> 00:00:59.847 Interlacing halves the bandwidth needed for a given apparent refresh rate, the picture updates 60 times a second while only 30 complete frames are sent. A 1930s compression scheme, and an effective one. 8 00:00:59.847 --> 00:01:05.007 Question. What is vestigial sideband modulation? 9 00:01:05.907 --> 00:01:14.867 Answer. Amplitude modulation in which one complete sideband and a portion of the other are transmitted. 10 00:01:15.467 --> 00:01:24.524 Question. Which of the following describes the use of vestigial sideband in analog fast-scan TV transmissions? 11 00:01:25.424 --> 00:01:35.864 Answer. Vestigial sideband reduces the bandwidth while increasing the fidelity of low frequency video components. 12 00:01:36.464 --> 00:01:48.497 A compromise between AM and SSB. Full SSB would be narrowest but makes the receiver's job much harder for a video signal with content down to DC. 13 00:01:48.497 --> 00:02:01.526 Vestigial sideband keeps one full sideband plus a stub of the other, so a simple envelope detector still works while most of the redundant sideband is saved. 14 00:02:01.526 --> 00:02:12.126 Question. What technique allows commercial analog TV receivers to be used for fast-scan TV operations on the 70-centimeter band? 15 00:02:13.026 --> 00:02:18.040 Answer. Transmitting on channels shared with cable TV. 16 00:02:18.640 --> 00:02:35.444 Cable channel frequencies overlap the 70 cm band, so a cable-ready receiver tunes them directly. That is why amateur fast-scan TV settled on the frequencies it did. Slow-scan television. 17 00:02:35.444 --> 00:02:45.924 SSTV sends a still image through a 3 kHz SSB channel, over seconds to minutes. Every aspect of the signal is a tone. 18 00:02:45.924 --> 00:02:56.708 Question. What kind of receiver can be used to receive and decode SSTV using the Digital Radio Mondiale (DRM) protocol? 19 00:02:57.608 --> 00:03:00.488 Answer. SSB. 20 00:03:01.088 --> 00:03:07.753 An ordinary SSB receiver plus software. No special hardware at all. 21 00:03:07.753 --> 00:03:16.100 Question. What aspect of an analog slow-scan television signal encodes the brightness of the picture? 22 00:03:17.000 --> 00:03:19.828 Answer. Tone frequency. 23 00:03:20.428 --> 00:03:36.188 Brightness is frequency: dark is around 1500 Hz, white around 2300 Hz. Which is why an SSTV signal sounds like a rising and falling warble, you are hearing the picture. 24 00:03:36.188 --> 00:03:43.908 Question. What signals SSTV receiving software to begin a new picture line? 25 00:03:44.808 --> 00:03:48.536 Answer. Specific tone frequencies. 26 00:03:49.136 --> 00:03:56.641 A sync tone at 1200 Hz, below the brightness range, marks each line start. 27 00:03:56.641 --> 00:04:02.939 Question. How is color information sent in analog SSTV? 28 00:04:03.839 --> 00:04:08.161 Answer. Color lines are sent sequentially. 29 00:04:08.761 --> 00:04:24.241 Red, green, and blue as separate lines in sequence. It is why a mistimed SSTV image shows colour fringing, the components have slipped relative to each other. 30 00:04:24.241 --> 00:04:33.916 Question. What is the function of the vertical interval signaling (VIS) code sent as part of an SSTV transmission? 31 00:04:34.816 --> 00:04:39.673 Answer. To identify the SSTV mode being used. 32 00:04:40.273 --> 00:04:55.261 There are many SSTV modes with different resolutions and timings. The VIS code at the start tells the receiving software which one is coming, so it configures itself automatically. 33 00:04:55.261 --> 00:05:00.673 Without it you would have to know in advance. Digital television. 34 00:05:00.673 --> 00:05:08.292 Question. Which types of modulation are used for amateur television DVB-T signals? 35 00:05:09.192 --> 00:05:13.992 Answer. QAM and QPSK. 36 00:05:14.592 --> 00:05:26.735 The digital modulations from the signals module, carried on OFDM subcarriers, which is what the "T" in DVB-T (terrestrial) implies. 37 00:05:26.735 --> 00:05:34.015 Question. In digital television, what does a coding rate of 3/4 mean? 38 00:05:34.915 --> 00:05:41.319 Answer. 25% of the data sent is forward error correction data. 39 00:05:41.919 --> 00:05:59.235 A coding rate of 3/4 means three quarters of the transmitted bits are payload and one quarter is forward error correction. Lower coding rates send more redundancy, robust on a poor path, slower on a good one. 40 00:05:59.235 --> 00:06:17.467 That trade is chosen deliberately: a marginal path gets a low coding rate and survives; a strong one gets a high rate and moves more picture. Check yourself. Why does NTSC interlace rather than scan every line in order? 41 00:06:17.467 --> 00:06:32.619 You hear a warbling tone that rises and falls, with a regular click. What mode, and what are you hearing? A DVB-T link uses a 1/2 coding rate. What does that tell you about the path? 42 00:06:38.619 --> 00:06:53.413 Interlacing halves the bandwidth for a given apparent refresh rate: two fields of alternating lines make one frame, updating 60 times a second while sending 30 frames. SSTV. 43 00:06:53.413 --> 00:07:01.538 The warble is brightness encoded as tone frequency; the regular click is the 1200 Hz line sync. 44 00:07:01.538 --> 00:07:13.339 Half the transmitted data is forward error correction, so it is configured for a poor or marginal path, robustness bought with throughput.