WEBVTT NOTE Identifying Modulation by Eye 1 00:00:00.000 --> 00:00:15.538 The skill. An experienced operator glances at a waterfall and names the mode. This is not intuition, it is four or five learnable visual signatures, and an hour of looking installs them permanently. 2 00:00:15.538 --> 00:00:17.342 The value is practical. 3 00:00:17.342 --> 00:00:32.880 Knowing what a signal is before demodulating tells you which demodulator to select, whether it is worth chasing, and, when something appears where it should not, what kind of device is producing it. 4 00:00:32.880 --> 00:00:45.133 The signatures. AM, a carrier with symmetrical shoulders. A strong, narrow vertical line with a lower, wider skirt either side of it. 5 00:00:45.133 --> 00:01:03.097 The line is the carrier; the skirts are the two sidebands, mirror images that move together as the audio changes. Total width about 6 kHz for shortwave broadcast, ten times that for FM broadcast. 6 00:01:03.097 --> 00:01:19.219 Look at 5 to 15 MHz in the evening, or the aviation band at 118 to 137 MHz, where AM is still standard. The giveaway is symmetry. Cover half the display and it looks the same. 7 00:01:19.219 --> 00:01:28.391 SSB, one lopsided sideband, no carrier. An irregular blob about 3 kHz wide with nothing at its centre. 8 00:01:28.391 --> 00:01:47.545 It appears and disappears with speech, and it is asymmetric: upper sideband extends upward from the suppressed carrier frequency, lower sideband downward. Look at 14.150 to 14.350 MHz (20 metres) in the afternoon. 9 00:01:47.545 --> 00:02:05.619 You will see a row of these, each 3 kHz wide, packed closely together. The giveaway is the missing carrier plus the one-sided shape. Once you have seen it beside an AM signal the difference is obvious. 10 00:02:05.619 --> 00:02:24.477 FM, a wide, flat-topped block. Nearly constant amplitude across its whole width, because FM carries information in frequency rather than amplitude. The block simply appears when someone transmits. 11 00:02:24.477 --> 00:02:43.239 About 15 kHz for a VHF repeater; about 200 kHz for broadcast FM. The giveaway is the flat top and hard edges. No carrier spike, no skirts, just a rectangle. CW, Morse code you can read on screen. 12 00:02:43.239 --> 00:03:01.520 A single very narrow line, about 150 Hz, switching on and off. On a waterfall scrolling at a reasonable rate, the dits and dahs are literally legible as short and long marks down the screen. 13 00:03:01.520 --> 00:03:13.547 Look at 14.000 to 14.070 MHz or 7.000 to 7.040 MHz. The giveaway is that it is the narrowest thing on the band and it blinks. 14 00:03:13.547 --> 00:03:37.224 FT8, the metronome. A dense forest of narrow tones, all starting and stopping in unison every 15 seconds. Each signal is about 50 Hz wide, and dozens share a 3 kHz slice. Look at 14.074 MHz, 7.074 MHz, or 50.313 MHz. 15 00:03:37.224 --> 00:03:59.147 The giveaway is the rhythm. Nothing else on the air is that synchronised. If you see a block of narrow signals that all begin and end together on a quarter-minute boundary, it is FT8 and nothing else. 16 00:03:59.147 --> 00:04:20.711 PSK31, a narrow warble. About 31 Hz wide, continuous while someone types, with a characteristic narrow fuzzy appearance rather than a clean line. Look around 14.070 MHz. Data bursts, packet, APRS, POCSAG. 17 00:04:20.711 --> 00:04:41.325 Short, wide, rectangular bursts. Present for a fraction of a second, then gone. APRS on 144.390 MHz in North America is the easiest to find: a burst every few seconds as stations report position. 18 00:04:41.325 --> 00:04:49.147 The giveaway is the duty cycle, brief, repeated, identical-looking bursts. 19 00:04:49.147 --> 00:05:13.032 The identification flowchart. Is it narrower than about 500 Hz? → CW (blinking), PSK31 (continuous warble), or FT8 (synchronised, in a group). Is it about 3 kHz and lopsided with no centre carrier? → SSB. 20 00:05:13.032 --> 00:05:32.467 Is it about 6 kHz and symmetrical about a strong carrier? → AM. Is it a flat-topped block 15 kHz or wider? → FM. Is it a brief wide burst that repeats? → a data mode. 21 00:05:32.467 --> 00:05:51.960 Do this now. Open a WebSDR on 20 metres in the afternoon and find, in this order: [ ] A CW signal below 14.070, read a character or two off the waterfall. [ ] The FT8 block at 14.074, watch the 15-second cadence. 22 00:05:51.960 --> 00:06:03.730 [ ] An SSB conversation above 14.200, note the missing carrier. [ ] An AM shortwave broadcaster below 12 MHz, note the symmetry. 23 00:06:03.730 --> 00:06:20.189 Then on VHF locally: [ ] An FM repeater on 2 metres, note the flat top and its 15 kHz width. [ ] The APRS bursts on 144.390 MHz. [ ] Your own GMRS handheld on a low-power channel. 24 00:06:20.189 --> 00:06:36.372 Seven observations, perhaps forty minutes. After it, the bandwidth table from the signals module is something you have seen rather than something you memorised. Check yourself. 25 00:06:36.372 --> 00:06:52.004 A signal is 3 kHz wide, comes and goes with speech, and has no carrier at its centre. Mode? Twenty narrow signals all start at the same instant and stop 12 seconds later. 26 00:06:52.004 --> 00:07:02.486 What are you looking at, and what frequency are you probably on? What visual difference distinguishes AM from SSB? 27 00:07:08.486 --> 00:07:27.736 SSB. Which sideband depends on whether it extends above or below the suppressed carrier. FT8, nothing else is synchronised to a 15-second boundary. Likely 14.074, 7.074, or 50.313 MHz. 28 00:07:27.736 --> 00:07:37.152 AM is symmetrical about a strong carrier; SSB has no carrier and extends to one side only.