Every link in Loftskeyti is a real modem. Transmitters generate audio-rate signals at 48 kHz, the band from 0 to 24 kHz plays the part of the radio spectrum, and every receiver demodulates the actual samples that reach its antenna. Bit errors are counted, not looked up in a table.

Modems

FSK sends one of two tones per symbol, spaced so they are orthogonal over the receiver’s integration window. DBPSK and DQPSK move the carrier phase by 180° or by multiples of 45°, and the receiver compares each symbol with the one before. 16-QAM uses phase and amplitude, so the receiver estimates the channel from an 8-symbol preamble and equalises before slicing. Each frame carries 256 bits of payload, a 16-bit header and a CRC-16. Frames that fail the CRC are sent again.

Crystals

No radio is exactly on its frequency. Each station’s crystal is a little off, by how old its sets are: valve sets of the fifties by tens of hertz, and further while they warm up; transistor sets of the seventies by a few; synthesised sets of the eighties by a hertz or so. The error grows with the carrier, so a high carrier lands further off than a low one, and two stations’ errors add or cancel. Every receiver has an AFC that turns its own oscillator to meet the carrier as it arrives, reaching as far as two sets of its day can be apart: PSK and QAM read the offset off each frame’s preamble and data, FSK off how its louder tone turns within a symbol, slow-scan off its sync pulses, and a speech link off its pilot tone. A slow modem is the hardest to pull in, since the same offset is a bigger share of its symbol rate, and a strong neighbour can drag an AFC towards itself. Morse needs none: an operator hears the offset as the note’s pitch. The mission card says which sets a mission has, the carrier readout what the AFC is doing, and in the sandbox you choose the decade.

Slow-scan television

SSTV sends a picture the way radio amateurs do, as sound: each line is a sync pulse below black, a short porch, then the green, blue and red scans, every pixel a tone between black and white, 400 Hz either side of the carrier. It is analog: there are no frames to check, so noise shows as snow, an echo paints a ghost beside every edge, and a relay passes its noisy copy on, so the snow piles up hop by hop. The receiver reads the tones with an FM discriminator, which needs a strong signal: below about 10 dB the picture breaks into sparkles. The sender repeats the picture, and each pass paints over the last, line by line, as a slow-scan screen does: a cleaner pass replaces a noisier one, but a noisy link stays snowy however often it repeats. For a cleaner picture, build a better link or scan slower: half the pixel rate puts twice the energy into every pixel. Every pass starts with a VIS header, as real slow-scan does: a leader at the centre tone, a break, the leader again, then a start bit, seven bits naming the mode, a parity bit and a stop bit. It costs most of a second, and it is how every receiver finds the top of the picture: it listens for a header all the while, a new one starting a new picture, and finds each line by its sync pulse, held steady by a flywheel as a television’s line sync is, so noise moves a line a little early or late and a sync lost in a crash loses its line. The sync also tunes it: the lowest tone there is, read where it belongs, says how far off the carrier came in. The picture is drawn as the receiver swept it, soft where the tones glide from pixel to pixel and streaked along the line where noise goes by. At a 1900 Hz carrier the tones are the real thing: sync at 1200 Hz, black at 1500, white at 2300. Try it in the sandbox, whose message is a picture.

Spread spectrum

The sandbox’s Spread modem sends each bit as a run of 8 to 128 chips from the link’s code, a fixed pattern of plus and minus the same for every bit. The signal is as wide as the chip rate and as faint per hertz as it is wide: at 64 chips a bit, 18 dB below a narrow signal of the same power. The receiver multiplies everything it hears by the code, chip for chip, in step with the strongest arrival of its own transmitter. Its own signal folds back into one narrow line; whatever doesn’t carry the code in step is spread out the same way, and only one part in 64 of it lands in that line. Against plain noise that buys nothing: at the same bit rate the energy per bit is the same. Against a carrier in the band, an echo later than a chip, or another link on another code, it buys the whole factor, so links on different codes can share a carrier. Two links on the same code don’t collide either if they are a chip or more out of step, since each receiver hears only its own timing. But the factor is all it buys: a strong neighbour on another code still drowns a faint link, the near-far problem that makes mobile networks turn every phone’s power up and down. Set Hear to Despread to hear a spread link come out of the hiss.

By hand: Morse and telex

Telegrams by Morse or by telex never went in numbered frames with checksums, and they don’t here. The operator keys the telegram letter by letter (Morse opens with KA and closes with AR; the teleprinter at 50 baud sends ZCZC and NNNN, each character a start bit, five code bits and two stop bits), and goes through it again and again. The far end writes a word down when every letter of it came through clearly, or when two copies of it agree; doubtful words show faint until then. A relay writes the telegram down too and keys it on, with a question mark for any word it doesn’t have yet. The telegraph alphabet has no Icelandic letters, so they are spelt the way telegrams spelt them: Þ as TH, Æ as AE, Á as A. A telegram goes by hand the whole way or by data links the whole way, and pictures always need data links.

Morse

The receiver listens the way an operator does, to the energy of each unit through a ±100 Hz filter, with a threshold that follows the signal as it fades. It decides key down or key up for every unit, sorts the runs into dits, dahs and gaps, and reads the letters back; a letter only counts when the silence after it is heard, so a steady buzz writes nothing. Speed is in words per minute: 20 is a good hand, 320 is a machine sending from punched tape. Faster means less energy in every dit, so more words need a second copy.

Listening

Select a link and you hear its receiver. Select a place instead, a town or a mast, and the spectrum panel becomes that place’s receiver: pick a mode and drag across the waterfall to tune. AM hears the envelope of a ±2.8 kHz band, so the base radio’s carrier and its music. Single sideband (USB, LSB) sends one sideband and no carrier, in less than half the spectrum of AM, but the dial has to be within a few tens of hertz of the station before voices and music sound right; tune a little off and everything goes up or down in pitch. CW puts a narrow filter on a Morse signal and beats it to a 700 Hz note, and RTTY does the same for a teleprinter’s two tones. Under the waterfall, a decoder reads what the receiver hears: a link’s frames as its demodulator decoded them, CRC failures and all, or at a place, Morse and teleprinter text read from the antenna signal alone, its timing learnt from the signal itself. In noise it prints rubbish, as real decoders do. Wide is the whole band at once. Not everything on the air is on the chart.

Trouble is labelled the way radio amateurs say it: QRM for man-made interference, QRN for natural noise like static and aurora hiss, QSB for fading. (QSL means I confirm receipt, which is what a return channel is for.)

The music on the air is by Karl Helgason.

Through the air of your room

Sound is this game’s radio, so in the sandbox your station can send through the real air. Set its Speaker to The air and your loudspeakers play its signal, carrier and all, as its antenna would radiate it. Give any place’s radio the Microphone as its antenna and it hears your room instead of the chart’s air: the waterfall shows the room, and the radio and its decoders work on it. On one computer the signal leaves the speaker and comes back in through the microphone; with two it crosses the room from one to the other, as acoustic couplers once put modems on telephone handsets. The room is a real channel: its echoes are multipath, its hum and voices are the noise floor, and most speakers and microphones fade above about 16 kHz. Morse decodes well through it, and a voice in AM. Your station can send your voice live too, with Sends: Microphone. Drag from your station to another station and its programme goes down a link instead, as speech in single sideband (tune USB 1.5 kHz below the link’s frequency to hear it), and its broadcast falls silent while it is linked. A data link crosses the room the same way, with the Air antenna. Air is a strange antenna, but an antenna: at a link’s transmitting end it is your speakers, at its receiving end your microphone, and nothing of it goes on the chart or comes off it. Give one end the Air antenna and the other an ordinary one, or both ends on one computer, or one end on each of two. A return channel shares its end’s antenna, as through a duplexer, so with Air at both ends the answers cross the room too, the other way, and the sender sends again only what was lost: one computer’s Air receiving end answers into the room, the other’s Air transmitting end hears the answers. Through the room the sender cannot see the whole way, so when answers are slow in coming it waits twice as long the next time, as TCP does. The room is one band, as the chart is: any number of transmitters can play into it at once, each on its own carrier, and every receiver on the microphone picks out its own. One loudspeaker carries them all, so each is a little quieter for sharing it, and carriers too close together interfere for real. A relay on a computer that only listens passes on what it hears through the air, as it heard it. The room is a way, too, as much as any link: an Air transmitting end goes into it, and an Air receiving end comes out of it at its own station, however far apart the two links are on the chart, so a telegram can go from Seyðisfjörður into your room and out at Reykjavík. With nothing on the computer listening, the room is a way of last resort to anywhere, since another computer may hear it. The receiver then works only on what the microphone hears: it finds each frame’s preamble, keeps time with a Gardner timing loop (two sound cards never quite agree on 48 kHz), and lets each frame’s checksum judge it. Give both computers’ links the same modem, rate, coding and carrier. FSK and DBPSK cross a room well; DQPSK and 16-QAM decide on finer differences of phase, and the room’s echoes smear each symbol into the next. Spread spectrum has to find its code first: the receiver tries every half chip of a bit until one stands out, then holds on with an early-late gate, as a GPS receiver does, and a code with few chips can be fooled by another that happens to resemble it. Slow-scan listens for the VIS header, then finds every line by its sync pulse, which keeps two sound cards’ clocks from slanting the picture; echoes still paint their ghosts, fewer at a slower speed. Morse and telex cross the room by hand, as on the chart: the receiver finds each character by its gaps (Morse) or its start bit (telex), and writes the telegram down between its start and end signs, KA and AR or ZCZC and NNNN, a word at once when it came through clearly, else when two copies agree. A computer that only listens takes the far end’s telegram as its form; one that sends as well judges what it hears against its own. Every carrier can be typed in (10.7, or 10700 in hertz), so two computers can agree on one exactly. In the sandbox the Message row chooses what Seyðisfjörður sends: the puffins, a telegram of your own (up to 128 letters in four frames), or a picture of your own. Yours is cut to 96×72 and given sixteen colours of its own, chosen for it and dithered; its colour table goes first, in frame 0, as a GIF carries one, so a computer that only listens can paint whatever arrives, in greys until the table is in. By slow-scan it goes in full colour, since slow-scan sends colours rather than colour numbers. A computer that only listens shows whatever arrives. The browser asks before the game may use the microphone, and a published artifact’s page may not use one at all: open the game at loftskeyti.com, or saved to your computer, to listen with it.

Other people’s transmitters

Two radio amateurs near Reykjavík work each other in Morse on 14.05 kHz (the 20-metre band, scaled): a call, an answer with a signal report, names, places and the weather, then 73. Tune CW and the decoder reads it. A time station ticks every second on 10 kHz with a long tone on the minute, and once a minute and a quarter an ionosonde sweeps up through the whole band, sounding the sky. Lighthouses key their Morse call signs, boats send 45.45-baud RTTY in bursts as they move along the coast, the airport radar sweeps its beam around every six seconds, a military radio hops across 2–20 kHz, and the Loran-C mast at Gufuskálar sends groups of eight pulses every 79.7 ms. None of them will move for you. Far to the east, beyond the chart, the Soviet over-the-horizon radar that radio amateurs called the Woodpecker taps ten times a second; it arrives by sky wave, so the aurora silences it.

Acknowledgements

A link is one way. With nothing coming back, the sender cannot know what was lost, so it goes round the whole message again and again, least-sent frames first, and every relay does the same with what it holds; a lost frame waits for the next pass. Give a link a return channel and its receiving end can talk back on its own frequency through the same antennas, joined by a duplexer. But only the far end of a route knows what arrived, so its answers have to be passed back over every hop. One hop without a return channel, and the sender is on its own again.

When every hop has one, the far end puts a report of what it holds, a running count and a bitmap of the gaps, on every return frame (radio amateurs would say QSL: I confirm receipt). Each relay passes the newest report on toward the sender. The sender keeps a window of frames on the way, about one round trip’s worth at the pace of the slowest hop. A frame that is missing while a later one has arrived was lost, and goes again; so does one with no word for too long. A lost return frame only delays the news, because the next one repeats it. The return channel is a real transmitter: it costs a modem, a duplexer and a licence, it needs room near the forward carrier because the antennas must cover both, and if it is jammed the reports are lost and the sender repeats frames that may already have arrived. Some stations can only listen.

Resending from end to end has a price: a frame lost on the last hop crosses every hop again, so a bad hop needs error correction more than ever. Real networks have done it both ways. Packet-radio digipeaters and the internet’s TCP acknowledge end to end, as here; the old telegraph relay offices and NET/ROM packet nodes took each message over hop by hop and answered for it themselves.

Error correction

Hamming 7,4 adds three check bits to every four and fixes any single wrong bit in each group of seven. The convolutional code (rate ½, constraint length 7, generators 171 and 133) is decoded with a soft-decision Viterbi decoder, so it uses how sure the demodulator was about every bit. Both are interleaved: bits are written in rows of 24 and sent by columns, so a lightning burst lands on bits far apart in the code, where it can be fixed. The link panel shows bit errors before and after decoding.

Direction finding

A loop antenna hears well along its own plane and almost nothing broadside: it has two sharp nulls, back to back. Give a place’s radio a loop and turn it until a station fades away. The station lies on that line, ahead or behind, and a second place’s line crosses the first where it stands. But a loop only knows where the signal comes from. Off a sea cliff an echo can arrive stronger than the wave itself, and the null then points at the cliff; when two bearings agree and a third does not, doubt the third. Coast stations took bearings like this for ships lost in fog, and a ship could ask for its own (QTE: what is my true bearing from you?). A portable radio on a 2 m pole can be carried anywhere on land, onto a summit if you like, where it hears a transmitter directly, over the echoes. In the sandbox it can even be you, out with your phone: Where I am puts it where the phone’s GPS says you stand (in Iceland, on land) and keeps it with you, and Turn with me turns its Yagi, dish or loop as you turn the phone, corrected for magnetic north, which lies about 12° west of true in Reykjavík. Then hunt a station as fox hunters do. The browser asks first; the published artifact cannot have a compass or a location, the game at loftskeyti.com can.

Power

Every transmitter of yours is on one scale: an ordinary link transmitter puts 1 kW into its antenna, the amplifier makes it 4 kW, and a broadcaster carries 1, 4 or 16 kW in its carrier. Power goes with the square of the amplitude, so four times the power stands only twice as tall in the air: +6 dB, and sixteen times +12 dB, which is why doubling a transmitter rarely doubles what it reaches. A spark set draws 64 kW from the mains and radiates only the share its gap lets through, a tenth or a sixth with a plain gap. Each transmitter’s panel shows its power.

Spark

The first wireless stations had no carrier. A spark gap fires a charged condenser into a tuned circuit coupled to the antenna, and each spark rings both for a few milliseconds and dies away: a burst hundreds of hertz wide, in two humps when the circuits are coupled tightly, and with the arc’s harmonics and a crack across the whole band besides. A hundred sparks a second make the rough buzz you hear in the phones, at whatever speed the operator keys. That dirt is why spark sets were phased off the air from 1927 on. In Neistar it is 24 November 1905: the Marconi station at Rauðará, where Höfði stands, has copied Poldhu in Cornwall every night since June, the first wireless messages to reach Iceland (Poldhu sent the news to ships under the call ZZ; the bulletin you hear here is made up). Only the plain gap exists yet: Wien’s quenched gap is 1906, the rotary gap 1907. That year the Althing chose a telegraph cable over wireless, and farmers rode to Reykjavík to protest against it.

Broadcasting

A broadcast is one programme for everyone, on an AM carrier. In Útvarpið your transmitter takes the studio’s concert down a telephone line, and a relay transmitter carries it further: it listens to another transmitter with an ordinary set and sends on whatever comes out, hiss and fading included. Every town has a set switched on, and the game holds its loudspeaker up against the studio: the share of what it plays that is the concert, against the share that is noise, whistles and distortion (SINAD). Weighed against the music’s loud passages, as broadcast engineers quote noise against full modulation, a perfect set here plays the concert at about 16 dB, and analog hops add up: a relay leaves its towns about 12 dB, two good hops 11, a third rarely the 10 dB that counts as clear. A relay hears its own transmitter on the same mast too, only 20 dB down, so the more power it has the more it deafens itself; turn a Yagi or a dish to its parent and it hears its own carrier from behind. Two transmitters reaching one town must keep their carriers out of each other’s IF, and the band holds about four such channels: reuse them where the transmitters cannot hear each other. The real transmitter of December 1930 stood on Vatnsendahæð above Reykjavík: 16 kW of long wave. A transmitter of yours can draw its coverage on the chart, the way radio engineers plan: what a set with a 10 m aerial would hear of its carrier at every point of Iceland and the sea around it, on every mission whatever the chart shows, from 3 km apart down to the elevation model’s own 500 m, over the band’s background noise, the bands set where the towns’ sets judge reception clear (10 dB) and listenable (5 dB). It is a prediction from the ground and the curve of the earth, the surface wave and, on a quiet night, the sky wave; storms, hum, echoes and other stations come on top, and the towns’ sets and your portables judge what is really heard.

The mobile telephone

In Farsíminn you build NMT, the Nordic mobile telephone Iceland opened in 1986, and the call travels for real all the way. It leaves the exchange in Reykjavík down your radio links as speech: single sideband with the carrier suppressed, as the telephone companies’ radio links carried calls before they went digital, 300–2400 Hz of speech shifted up the dial with a pilot tone above it. Each hop’s receiver sets its gain and phase by the pilot and hands on everything it hears, the hiss with the speech, so every analog hop adds its own noise to what came before; a hop whose pilot sinks into the hiss is given up, and the base stations beyond it go out of service. A link carries the call outward, from the end nearer the exchange, whichever way you dragged it. The base station the telephone is best placed for puts the call on the air in FM, on the telephone channel at 20 kHz (a 0.75 m wave, about 400 MHz, so base stations belong on heights), pre-emphasised as NMT’s was: the treble lifted before the transmitter and lowered again in the telephone, which takes FM’s hiss, rising with frequency, down with it. The telephone’s own receiver hears the call the way the coverage map works it out, with the noise and anything else on the channel. FM’s threshold, where speech breaks into clicks, and the limiter that rounds off loud syllables come out of the signal itself. One call goes through the network at a time: the trawler’s, then the jeep’s, then the car’s, each at its speed of a sped-up drive, the whole Ring Road in under a minute, with the telephones’ timers sped up too. The score is a drive test’s: the call’s SINAD at the earpiece, the speech that was said over everything that wasn’t, all along each telephone’s way, clear at 12 dB. A judge holds what comes out against what was said through a filter of three taps, so a call turned in phase or a fraction of a sample late, which no listener hears, is not counted against it. Base stations transmit at 16 kW on the game’s one scale, which gives the big cells NMT 450 had. NMT’s users were fishermen out on the banks, travellers crossing the empty highlands and drivers, so a trawler, a jeep and a car make the test calls.

Two AM stations at once

An AM receiver’s envelope detector cannot pick one station out of two: whatever lies inside its filter comes out together. The base radio, on the old NATO base at Keflavík airport, is on 7.4 kHz, and Útvarp Faxaflói, from the masts on Úlfarsfell above Reykjavík, on 6.8 kHz. In Reykjavík both come in about as strong: tune 7.1 kHz in AM and both programmes play at once, and the two carriers beat into a 600 Hz whistle. Tune toward one and it grows louder, but the whistle stays as long as the other carrier is in the filter. That is why broadcasters were given channels apart, and why distant stations whistled at night.

Natural radio

Lightning crackles as sferics. A strong stroke also sends its noise out along the earth’s magnetic field and back, and the thin plasma up there delays low tones more than high ones, so it returns as a whistler: a tone falling from about 8 kHz to 1 kHz in a second or so. Each tone arrives at a time proportional to 1/√f, Eckersley’s law. The aurora adds its hiss and rising chorus.

Overload and intermodulation

Each receiver has an AGC and a front end that saturates. Thermal noise is added after it, inside the receiver. A strong signal therefore pushes the gain down and buries weak ones (desensitisation), and two strong carriers mix into phantom signals at 2f₁−f₂ that you can see on the waterfall. The +6 dB amplifier is driven into gentle saturation too: harmless for FSK and PSK, but it squashes the outer points of 16-QAM.

What you see and hear

The waterfall is a 2048-point FFT of the selected receiver’s antenna signal. What you hear is that signal through a band-pass around the selected carrier, with a little of the whole band underneath, and an AGC like a radio’s. The constellation shows the receiver’s decision values for recent symbols. SNR is measured on each frame’s preamble.

Propagation

A signal spreads out as any wave does in free space. Below the direct ray the sea or the ground reflects a second one, a fraction of a metre longer, with the reflection of that surface (the sea mirrors far better than heath, and rough ground scatters short waves), and the ground wave creeps along the surface. The two rays add at some carriers and cancel at others: the dashed curve over the waterfall shows where. Near the horizon the wave must bend round the curve of the earth, which costs more the shorter it is, and ridges in the way cost more again; low carriers bend over what stops high ones. Cliffs and mountain faces reflect, and the echoes arrive late, and every slope both ends can see scatters a little back, a faint reverb. For each link a full-wave solver has the last word: it marches the wave, as a parabolic equation, over the real terrain at several frequencies across the band, and the link follows its answer. Its field fills the terrain profile under Path.

Ground and roads

The ground is real: ÍslandsDEM, the national elevation model of Iceland (Náttúrufræðistofnun Íslands, CC BY 4.0), at about 500 m. Every town’s radio station and every relay stands on an 80 m mast, the way real stations put their antennas high to see over the curve of the earth. A relay needs land; it cannot stand on a glacier or inside a national park or nature reserve (friðland), it costs more on a mountain, and it needs an access road from the nearest main road, by the kilometre. The roads, the glaciers, the protected areas and the names of the summits are from OpenStreetMap (© OpenStreetMap contributors, ODbL). While you place or move a relay, the chart marks the high points: the named summits, each set on the highest ground near it, and the ground’s own tops, more of them the closer you zoom. A relay dropped near one goes onto its top; hold Alt (Option) to put it down exactly where you click. The broadcasters stand where the real transmitters do: Úlfarsfell above Reykjavík, Valhóll on Vaðlaheiði across the fjord from Akureyri, Arnarnes at the mouth of Skutulsfjörður, and the base radio on the old NATO base at Keflavík airport.

Nature’s mirrors

The echoes come from the real ground: 10,391 cliffs found in the 20 m elevation model: sea cliffs from Látrabjarg and Hornbjarg down to Dyrhólaey and Ingólfshöfði, the walls of the fjords and of Ásbyrgi, and the faces of the mountains, each with its true orientation, length and height. Walls lower than 30 m, like Almannagjá’s, are left out. A cliff sends a wave from one station to another only when it stands like a mirror between them: its face must look halfway between the two directions, and the reflecting spot must lie on the face itself, which for two low stations means a cliff that reaches down near their height. Gentle slopes throw the wave at the sky; the faint, smeared rest that every slope scatters back is the landscape’s reverb. No mirror works at every frequency: a face whose bumps approach an eighth of a wavelength scatters instead (the Rayleigh criterion), so high carriers lose the echo, and a face smaller than the first Fresnel zone, √(λ·d₁d₂/(d₁+d₂)), returns only its share of it, so long waves lose it too. A vertical whip’s wave meets a vertical cliff with its field along the face, so the reflection is total at grazing and about half face on. The strongest three cliffs per path are kept.

Aurora scatter

When the aurora is up it absorbs the sky wave, but it opens another road: the ionisation in the curtains, about 110 km up and north of you, scatters signals back down. Turn both antennas north, toward the same patch of sky, and a station hundreds of kilometres away comes through, best around 10 kHz (VHF in real life). The scattered signal arrives smeared over about half a millisecond and fluttering a hundred times a second, and it swells and fades with the aurora. That is why radio amateurs work the aurora in slow Morse, copied by ear: the receiver here listens to the energy of each dit, whatever its phase does. FSK with a wide enough shift survives with the convolutional code; phase keying loses track of the phase, and narrow FSK smears into its other tone. It sounds like a hoarse whisper.

Frequency matters

Low carriers bend over ridges and use the sky; high carriers need line of sight. In this game 1 kHz behaves like a 15 m radio wave and 10 kHz like a 1.5 m wave. Ground-wave absorption rises with frequency, and over salt water it is a fifth of what it is over land, which is why ships and coastal stations carry so far. A dish is a fixed aperture, so its gain grows 6 dB per octave and its beam narrows: nearly omni at 1 kHz, +15 dBi at 6 kHz, +25 dBi and pencil-sharp at 20 kHz. Yagis are treated as broadband.

Noise

The floor is white noise. Towns add 50 Hz mains buzz below about 2 kHz. Storms add low-heavy static and lightning sferics, which arrive as bursts and wreck whole frames even when the average SNR looks fine. The aurora adds hiss around 8.5 kHz and rising “chorus” chirps between 2.6 and 5.5 kHz.

Honest simplifications

On the chart, receivers of frames and telegrams know symbol timing exactly, as if a perfect timing loop had locked onto the strongest arriving path, and know which frame or word they are hearing, as if every header and start sign came through. A slow-scan receiver finds its own lines, but is told which picture it is. The crystals are off in their carriers only, never in their clocks, and by less than real ones were: a fifties set on the short waves was hundreds of hertz out, more than the game’s narrow modems could live with. Propagation delay is scaled up so that echoes land in the millisecond range, the way HF skywave multipath does. Reports ride on the return channel’s frames without taking any of their bits.

None of that holds in the air of your room. A receiver with the Air antenna is told nothing: it finds its own symbol timing, a spread code’s start, a picture’s top line and every Morse letter’s edges in the sound, and learns which frame it heard from the frame’s own header, which word from the start sign, which line from the VIS. A report that crosses the room is the return frame’s payload, written into its bits and read back out of them, one report a frame. And the delays are the room’s own: no scaling, only the speakers, the air and the microphone.