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E2B

OPERATING PROCEDURES

Television practices: fast-scan television standards and techniques; slow scan television standards and techniques

E2B011 of 12

In digital television, what does a coding rate of 3/4 mean?

Why Forward error correction adds redundant bits so the receiver can repair errors without a retransmission. The code rate is the ratio of useful data bits to total transmitted bits, so 3/4 means that for every 4 bits sent, 3 carry payload and 1 is added FEC overhead. One bit out of four is 25% of the transmission devoted to error correction. Lower code rates like 1/2 are more robust but carry less data in the same bandwidth.

Watch out The guard interval answer describes a different OFDM parameter, the fraction of each symbol period left blank to absorb multipath echoes, which is quoted separately as 1/4, 1/8, 1/16 and so on. Compression ratios are also specified independently of the coding rate.

Code rate = payload/total. 3/4 means 3 data bits per 4 sent, so the leftover 1 in 4, 25%, is FEC.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E2B022 of 12

How many horizontal lines make up a fast-scan (NTSC) television frame?

Why Analog fast-scan TV in the US uses the NTSC standard, which builds each complete frame from 525 horizontal scan lines. The frame is sent as two interlaced fields of about 262.5 lines each, with the fields alternating at 60 per second so that complete frames appear at 30 per second. Amateur FSTV on 70 cm and higher bands follows these same NTSC timings so ordinary analog TV receivers can display the signal.

Watch out The numbers 30 and 60 are the frame rate and field rate in hertz, not line counts, and 1080 belongs to modern HDTV digital formats rather than NTSC.

NTSC: 525 lines, 30 frames, 60 fields. Only 525 is a line count; 30 and 60 are rates.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E2B033 of 12

How is an interlaced scanning pattern generated in a fast-scan (NTSC) television system?

Why NTSC fast-scan TV builds each 525-line frame out of two fields transmitted one after the other: the first field paints the odd-numbered lines, the second field fills in the even-numbered lines between them. Each field takes 1/60 second, so a complete frame appears 30 times per second while the screen is refreshed 60 times per second. Interlacing this way gives flicker-free motion without doubling the required video bandwidth.

Watch out Scanning two fields simultaneously is impossible in a single-beam raster system; the fields are sequential, which is exactly what makes the pattern 'interlaced'. Scanning direction is always left-to-right and top-to-bottom in both fields.

Odds first, evens second: 2 fields at 60/sec interlace into 1 frame of 525 lines at 30/sec.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E2B044 of 12

How is color information sent in analog SSTV?

Why Analog SSTV transmits an image line by line as an audio-frequency FM tone, where the instantaneous frequency represents brightness (roughly 1500 Hz black to 2300 Hz white). There is no room for a separate color subcarrier in a 3 kHz SSB channel, so color modes like Scottie and Martin send each scan line three times in sequence, once for each color component (red, green, blue, or luminance plus two color-difference signals). The receiving software reassembles the sequential color lines into one full-color line.

Watch out The color burst idea comes from analog fast-scan NTSC television, which uses a 3.58 MHz color subcarrier with a burst on the back porch of each line; SSTV has no such subcarrier.

SSTV is slow because it sends each line three times: red, then green, then blue, one color at a time.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E2B055 of 12

Which of the following describes the use of vestigial sideband in analog fast-scan TV transmissions?

Why Analog NTSC video extends to about 4.2 MHz, so double-sideband AM would need roughly 8.5 MHz of spectrum. Filtering out one sideband completely is impractical because the low video frequencies sit right next to the carrier and a sharp filter there would distort them and lose phase information. The compromise is vestigial sideband: the upper sideband is transmitted in full while only a vestige of the lower sideband (about 0.75 MHz) is kept, which trims the channel to 6 MHz yet preserves the low frequency video content that carries the large-area brightness of the picture.

Watch out The audio is a separate FM carrier 4.5 MHz above the video carrier, and chroma rides on the 3.58 MHz color subcarrier in the main sideband, so neither lives in the vestige.

Vestigial = keep a sliver of the second sideband so the LOW video frequencies survive; 6 MHz channel instead of 8+.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E2B066 of 12

What is vestigial sideband modulation?

Why Vestigial sideband (VSB) is amplitude modulation in which one sideband is sent complete and only a small remnant, the "vestige," of the other is left in place. Analog fast-scan TV used it because a full double-sideband video signal would need roughly 8 MHz, while filtering away most of one sideband squeezes the channel down to 6 MHz. The vestige is kept rather than removing the sideband entirely so the very low video frequencies and the carrier survive for simple envelope detection at the receiver.

Watch out The narrow-band FM description is wrong on two counts: VSB is an amplitude modulation scheme, not FM, and the filtering is done on the modulated RF, not on the audio beforehand.

Vestigial = vestige = leftover stub. One full sideband plus a stub, which is how analog TV fits video into 6 MHz.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E2B077 of 12

Which types of modulation are used for amateur television DVB-T signals?

Why DVB-T (Digital Video Broadcasting - Terrestrial) is the digital ATV standard borrowed from European broadcast TV. It uses COFDM, thousands of closely spaced subcarriers, and each subcarrier is modulated with a phase/amplitude scheme: QPSK, 16-QAM or 64-QAM depending on the robustness and data rate you want. So the modulation on the individual carriers is quadrature phase shift keying or quadrature amplitude modulation.

Watch out The FM choice describes analog fast-scan ATV on the higher microwave bands or FM broadcast audio, not DVB-T; AM with vestigial sideband is the old analog NTSC-style ATV video scheme, and on-off keying is basically CW.

DVB-T = digital = COFDM carriers keyed with QPSK/QAM. Digital standard, digital modulation.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E2B088 of 12

What technique allows commercial analog TV receivers to be used for fast-scan TV operations on the 70-centimeter band?

Why Cable TV systems assign channel numbers to frequencies that do not match over-the-air broadcast channels, and several mid/superband cable channels fall inside the 420-450 MHz amateur allocation. A cable-ready analog TV set can therefore tune an amateur fast-scan ATV signal directly if the transmission is placed on one of those shared cable channel frequencies (cable channels 57-61 land in the 70 cm band). No converter or downconverter is needed, just the set's cable tuning mode.

Watch out Converted satellite dishes are for microwave reception and do nothing to make a TV tune 70 cm; the USB-and-sound-card method describes slow-scan TV (SSTV) on HF, not fast-scan ATV.

70 cm ATV = cable channels. A cable-ready TV already tunes 420-450 MHz.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E2B099 of 12

What kind of receiver can be used to receive and decode SSTV using the Digital Radio Mondiale (DRM) protocol?

Why Digital SSTV using the DRM protocol packs a digital multi-carrier (OFDM) signal into about 2.3 kHz of audio bandwidth, which is exactly what an HF SSB channel provides. The receiver demodulates the sideband signal to audio, and software on a computer or sound card interface decodes the OFDM carriers back into the image file. So any standard SSB transceiver, tuned as for voice and with stable tuning, is all the hardware you need.

Watch out AM is tempting because commercial shortwave DRM broadcasting occupies AM broadcast channels, but ham digital SSTV is carried in an SSB passband; CDMA is a cellular access method and AREDN is an amateur microwave mesh networking system, neither of which decodes HF images.

Digital SSTV rides in a 2.3 kHz voice channel, so the answer is the same rig you use for HF voice: SSB.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E2B1010 of 12

What aspect of an analog slow-scan television signal encodes the brightness of the picture?

Why Analog SSTV is an audio-frequency FM-style mode: each pixel's luminance is sent as a tone whose frequency varies between about 1500 Hz for black and 2300 Hz for white, with sync pulses at 1200 Hz. Because the information is in the frequency, the signal can be fed straight into an SSB transmitter and survives fading and level changes on the path. The receiving software measures the instantaneous tone frequency to rebuild each line of the picture.

Watch out Tone amplitude sounds plausible because brighter usually means stronger, but amplitude varies with propagation and audio gain, so it would make the picture brightness depend on band conditions; amplitude carries no picture data in SSTV.

SSTV is FM by ear: 1200 Hz sync, 1500 Hz black, 2300 Hz white. Higher pitch equals brighter pixel.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E2B1111 of 12

What is the function of the vertical interval signaling (VIS) code sent as part of an SSTV transmission?

Why Analog SSTV has many incompatible modes (Scottie S1/S2, Martin M1/M2, Robot 36, PD120 and so on) that differ in line timing, number of lines and color encoding. Before the picture lines begin, the sender transmits a short digital header of audio tones called the Vertical Interval Signaling code, a 7-bit number plus a parity bit, that names the mode. Receiving software reads the VIS code and automatically switches to the right decoder, so the picture comes out with correct geometry and color instead of skewed or off-color.

Watch out Vertical synchronization is a separate function handled by the 1200 Hz sync pulses at the start of each line and frame, not by the VIS header, and nothing in the VIS code carries a call sign or locks a color burst oscillator.

VIS = "Video Identifies Setup": it announces the SSTV mode, not sync, not your call sign.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E2B1212 of 12

What signals SSTV receiving software to begin a new picture line?

Why Analog SSTV is an FM/audio-subcarrier mode: everything, including timing, is encoded as tone frequency. Brightness runs from 1500 Hz (black) to 2300 Hz (white), and a 1200 Hz tone burst below the video band serves as the horizontal sync pulse that tells the decoder to start the next scan line. Because 1200 Hz never appears as picture data, the software can reliably detect it and stay in step with the transmitter.

Watch out Elapsed time alone is tempting since each line has a fixed duration, but drift between the sending and receiving sound cards would slant the picture without periodic sync tones; amplitude carries no information at all in SSTV, since the mode is frequency-based.

SSTV speaks in pitch, not volume: 1200 Hz = new line, 1500-2300 Hz = black to white.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

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