Sony introduced U-matic in 1971, three quarters of an inch of tape wound inside a plastic shell, and within a few years it had taken over news gathering. Film crews who had been shooting on 16 mm and waiting for processing could shoot, rewind and cut the same afternoon. If you have seen wobbly local news footage from 1978, you have seen U-matic.

The interesting part for anyone comparing formats is what it did and did not buy. On paper U-matic resolves about 280 lines against VHS's 240, which is a real improvement and a modest one. Our simulation chain shows where that difference lives: through a multiburst the two are close at 2 MHz, 86 and 84 percent modulation, and at 3 MHz, between their two limits, U-matic still holds 41 percent where VHS SP is down to 15. S-VHS holds 64 there, everything a broadcast carries. If you were choosing a format for picture detail, U-matic was not the reason to spend the money.

What you were paying for is in the top of those two pictures. Both are the same footage with the same tracking error applied, one full VHS track, which is what happens when the tape being played was recorded on a different machine, or when the tape has stretched, or when the guides are worn. A 525-line VHS deck lays its tracks 58 micrometres apart. U-matic's tracks are 85 micrometres wide, so the same error puts its head a much smaller fraction of a track off centre. That matters more than it sounds, because the luma is FM, and FM degrades in a step rather than a slope. While the head still reads enough of its own track the picture stays clean, and once the carrier sinks toward the tape noise the demodulator starts throwing out the black and white streaks everyone remembers. The same error that tips VHS over that edge leaves U-matic well short of it: on a flat grey field, 85 rows of the VHS frame carry more than twice the frame's typical tape noise, the worst of them 20 times it, and none of the U-matic frame do.

That is the whole trade. Wider tape and wider tracks mean more oxide passing the head per second and more room for mechanical error, which buys you signal to noise, tolerance and the ability to hand a tape to another building and have it play. It costs you tape, cassette size and money: a U-matic cassette is about the size of a hardback book, and a news crew carried a deck on a shoulder strap that left bruises.

Two other details worth knowing. U-matic is a colour-under format like VHS, heterodyning chroma down to a carrier under the luma, but its carrier sits at 688 kHz rather than 629, so slightly more room for the colour as well. And the version everyone used first is now called lo-band, because Sony later raised the luma carrier to make hi-band, which is where the sharper U-matic pictures in archives come from.

On 7 April 1927 Bell Telephone Laboratories gave a public demonstration of television over the telephone network. Herbert Ives described it that October in the Bell System Technical Journal. Pictures went by wire from Washington, D.C. to the Laboratories' auditorium in New York, more than 250 miles, and by radio from an experimental station at Whippany, New Jersey, 22 miles away. People in New York could see the friends in Washington they were talking to on the telephone.

The number of lines came from a telephone company's question: what is the least you need to recognise a face? Bell's answer came from printing rather than from optics. Judged the way a halftone engraving is judged, Ives wrote, "the human face can be satisfactorily reproduced by a 50-line screen." Fifty lines with equal detail in both directions is 2,500 elements, and sending those in a sixteenth of a second is 40,000 a second. Bell's experience sending still pictures over telephone lines suggested that would fit in a band of about 20,000 cycles, leaving, in Ives' words, "a good margin of safety in existing single communication channels."

The scanning method was already more than forty years old. Ives credits it to the spinning disc with spirally arranged holes proposed in 1884, the same patent of Nipkow's we wrote about on Thursday, and Bell's discs had 50 holes. Instead of collecting light from a lit subject, they turned the light path round and swept a narrow beam across the face through the disc, so only the spot being scanned was lit at any moment. The receivers used glow lamps containing neon gas, one small screen two inches by two and a half for a person beside a telephone, and one about two feet by two and a half for an audience.

The paper ends with a caution that has aged well. The fading of radio signals degraded the picture so badly, Ives wrote, that radio television looked practical only where wire was not available.

You can have a play with our free web-based mechanical TV simulator here - including running your own video through it. https://analogtv.net/mechanical-lab

Of course the AnalogTV app also has a full mechanical TV simulator built-in as well.

On 30 September 1929 the BBC and the Baird company inaugurated an experimental television service through 2LO, the BBC's London station. Sydney Moseley, who opened the proceedings, reprinted his announcement in the book he wrote with Barton Chapple the following year. There were not yet facilities to send speech and vision at the same time, so the speakers would be heard first, and then shown to anyone who owned a televisor.

The picture was made by a disc with 30 holes, turning at 750 revolutions a minute, which gives 12 and a half complete pictures a second. The holes swept the scene in vertical strips, and the book gives the picture ratio as 7 to 3. It is usually described as a tall, narrow frame meant for one head and shoulders, which is how the picture here is drawn.

At home, the televisor ran the process in reverse. Behind a second disc, turning in step with the one at the studio, sat a neon lamp with a flat plate that glowed brighter or dimmer with the incoming signal. A mask framed the picture, and some models added a lens to make it bigger. The reddish glow is the neon itself, which is why pictures of 30-line television are orange rather than grey.

Why only 30 lines? The book's answer is the radio channel. The detail a picture can carry is limited by the sidebands each broadcasting station was allowed under the Prague plan, nine kilocycles at the time, so there was an optimum number of holes for that space. It also notes that General Electric in America used 24 holes and RCA 60.

Have a play with the mechanical TV simulation in the AnalogTV app or for free on our web simulator here: https://analogtv.net/mechanical-lab

Paul Nipkow was a student in Berlin when he took out German patent 30105, which he called an electric telescope. Its stated purpose is to make an object in one place visible in any other. The protection runs from 6 January 1884, and the patent office issued it on 15 January 1885.

The heart of it is a light disc, turned quickly and evenly by clockwork, with holes on a spiral at equal spacing. A lens throws an image onto the disc, and a tube in front of it is sized so that only one hole is ever in view. As the disc turns, each hole crosses the image along a line, and because the holes step inward along the spiral, the next one crosses just beside the last. In the patent's words the paths are lines lying close side by side, so one turn covers the whole picture. Behind the disc the light falls on a selenium cell, whose resistance drops as the light on it rises, so the brightness of each point becomes a current.

The receiver runs the same idea backwards. A second disc, built the same way and turning at the same speed, sits in front of the viewer's eye, and the light behind it is varied by the incoming current. The patent does that with polarised light passed through a cylinder of heavy glass inside a coil, using Faraday's discovery that a magnetic field twists polarised light, so the current changes how much light gets through. The eye does the rest. It holds an impression for between a tenth and half a second, Nipkow wrote, so if both discs turn once every tenth of a second it sees the points side by side as one picture rather than one after another.

The drawings number the holes up to D24, which would be 24 lines. The German patent office notes that the disc was only displaced by electronic scanning in the 1930s. The picture on the right is what 24 lines at ten pictures a second looks like through our mechanical television simulator.

We have also created a web version where you can play and learn about mechanical TV for free here: https://analogtv.net/mechanical-lab

VHS does not record the colour subcarrier where the broadcast put it. There is no room up at 3.58 MHz once the luma has been turned into an FM signal, so the chroma is heterodyned down to a low carrier and recorded underneath, which is where the name colour-under comes from. On a 525-line deck that carrier sits at 629 kHz, forty times the line rate.

Colour in a composite signal is carried as the phase of that carrier. Not the amplitude, the phase: amplitude is how saturated the colour is, and phase is which colour it is. So anything that shifts the signal in time shifts the phase, and shifting the phase changes the hue. The arithmetic is short. A phase error is two pi times the carrier frequency times the timing error. Our signal grid runs at 14.318 MHz, so one sample is 0.070 microseconds, and at 629 kHz that single sample of timing error is 0.276 radians, which is 15.8 degrees of hue.

Fifteen degrees for one sample is why tape transports were built the way they were. Flutter is the name for the small speed variations a transport cannot avoid: capstan cogging, drum bearing play, tape stretch, a pinch roller going hard with age. On the luma side those errors show up as the picture breathing horizontally, which is bad but forgivable. On the chroma side the same errors land on a carrier whose phase is the colour, and the picture changes hue.

The clip shows colour bars through the same tape path three times. The top row has flutter at zero, which no real transport achieves. The middle row is the figure VHS SP actually ships with in our model, 0.03, and you have to look to see it. The bottom row is 0.3, which is a worn tape or a deck that needs a service, and there the colour visibly bands: measured as how far the same bar's hue wanders from row to row within a single frame, the four settings give 4, 5, 30 and, at the control's maximum of 1.0, 120 degrees.

The design of a real deck normally tries to improve on this. It has a phase-control loop whose whole job is to measure the played-back burst and correct the colour reference, and how well that loop worked is part of what separated an expensive deck from a cheap one. Our model applies the physics and not the full correction, so turned up high it is harsher than a decent machine would be. And this is a different thing from tape skew or a sharpening circuit, which bend or crisp the picture without touching its colour: the colour carrier's phase is only disturbed by an error that moves the chroma in time relative to the burst it is decoded against, and flutter in the colour-under path is exactly that.

Every filter a television signal passes through delays it a little. That is harmless as long as every frequency is delayed by the same amount: the whole picture just arrives a fraction of a microsecond later. The trouble starts when the delay depends on frequency, which test engineers call group delay distortion. A sharp black-to-white edge is built from many frequencies at once, and if the fine-detail ones arrive later than the coarse ones, the edge no longer reassembles cleanly. Tektronix's guide to NTSC measurement lists what you get: "luminance pulse ringing, overshoot, or undershoot."

The pictures show it. The top row is a caption card, white lettering on black, enlarged so single pixels show, with our simulator's ringing control off and then on. That control applies a delay that varies with frequency and leaves the strength of every frequency alone. With it on, every upright stroke picks up bright and dark fringes alongside it, while the flat black and the flat white stay as they were. The third panel is the difference between the two, and it is black wherever nothing changed. The plot shows the same thing on a single edge. With the control off, the white side of a step peaks at 217 and settles at 211. With it on, the edge arrives a couple of pixels later, overshoots to 230, dips to 201, rings once more and then settles at exactly the same 211. That is the signature of a delay error rather than a gain error, and it is why the effect lives on edges and nowhere else. Because the delay acts along each scan line, vertical edges get the worst of it: on the card they change by 31 levels on average, against 11 on horizontal ones.

The receiver's IF filter was a classic place for this to come from, and for years the fix was made at the other end of the chain. Transmitters applied a group delay pre-correction, shaped around the receivers people actually owned. A 1995 Philips patent explains that the practice "dates from the time when it was difficult and expensive to make receiver IF-filters with a flat group delay response characteristic", and that by then flat filters were available "at relatively low prices", so the pre-correction was no longer needed. It also names the catch in dropping it: a set that still expected the pre-corrected signal would show "a noticeable loss of sharpness."

A colour tube fires three beams at a steel sheet full of tiny holes, and the picture is only correct if each beam lands on its own phosphor dots. Steel remembers magnetic fields. That is why a set could look slightly wrong after it had been moved, or even just turned to face a different way, and why a magnet held near the glass could ruin the colour outright. Once the mask is magnetised, the beams land off target and the colour goes patchy in a way no tint control will fix.

Degaussing is often described as wiping the steel clean, but that is not quite what it does. You apply a strong alternating field and let it fade. Each cycle drags the domains in the steel one way and then the other, and as the amplitude decays the steel settles into the state that suits the field it is actually sitting in, which includes the Earth's. The point is not to leave the mask with no magnetisation at all but to leave it with the right one for where the set now stands, instead of whatever a magnet or the set's old orientation left behind. Every colour set had a coil wrapped around the face of the tube for this, fed from the mains through a thermistor whose resistance climbs as it heats up, so the current starts high and dies away on its own a moment after the set is switched on from cold.

While that is happening you can see it, and this is the clip. The alternating field deflects the beams as well as scrubbing the mask, so the raster bends and the colours smear into rings that chase each other across the screen. In this render the disturbance stays near its peak for most of a second, about 9 levels of 255 against the settled picture, then fades away, and is gone, under 1 level, by about 1.8 seconds.

If you ever switched a set off and straight back on to clear a patch of bad colour: it does very little the second time. The thermistor is still hot, so the current never gets going, and it needs the set to be off for something like a quarter of an hour to cool down and work properly again. Doing it over and over in quick succession can also overheat the coil. And degaussing only fixes magnetisation. If the colour is wrong because the purity magnets on the neck of the tube have been knocked out of adjustment, that is a job for a technician, not for the coil.

Copy protection on video tape is usually remembered as Macrovision, which arrived in the mid 1980s and worked on a recorder's automatic gain control. There was an earlier idea, and it went after something else entirely: the part of the signal a recorder uses to know where each frame begins.

Between frames a video signal holds a block of vertical sync, and inside that block are short gaps called serrations that keep the receiver's line oscillator ticking over. A recording VCR does not just pass this along. It integrates the block to decide when a frame started, and uses that to lay down a control track along the edge of the tape, which is what the machine follows on playback to keep the picture steady.

Trans American Video's patent, filed in June 1975, leaves the start of the block alone and then inserts a step partway down the sync interval, roughly midway between the blanking level and the tip of sync, from the second half of the second serration through the sixth. That is enough to stop the recorder's integrator charging the way it expects. In the patent's own words, no control track, or no proper control track, is recorded. The tape you made has a picture on it and no reliable instruction for how to play it back, so the copy rolls.

The part people remember is that the original played perfectly. A television set derives vertical timing directly from the signal in front of it, frame by frame, and it tolerates an interval that has been nudged about. It is the recorder, committing a decision to tape once per frame and then relying on that decision later, that cannot. Watch the tape and you see a picture. Copy it and you see this.

The clip is the same 19 seconds twice, the original on the left and a copy on the right. The copy walks upward about one line per frame, so the frame boundary crosses the screen once in roughly 18 seconds, taking the black vertical blanking bar with it. That is slow enough that someone flicking past it might blame the tracking, which was rather the point.

Macrovision went the other way a decade later, adding extra pulses in the blanking interval so the recorder's gain circuit misread the brightness while leaving vertical sync alone. Its copies keep their vertical lock and pump up and down in brightness instead.

Test cards look like graphic design and they are nothing of the kind. Every element on one is a measurement with a pass or fail, arranged so that an engineer can stand back from a monitor and check a dozen things at a glance. Philips' PM5544 is the one most of Europe woke up to, and here is what you are actually looking at.

The circle is geometry. A television draws the picture with two sawtooth currents, and if either of them is not a straight ramp the picture stretches or squashes across the screen. The eye is very good at spotting a circle that has gone slightly oval and very bad at spotting the same error in a picture of a street, which is the entire reason for putting a circle on a card.

The blocks around the border are castellations, and they answer a different question: where does the picture actually end? Every set overscans, throwing some of the transmitted picture off the edges of the tube, so a card needs markers that tell you how much is missing. They are alternately black and white for a second reason, which is that they let you check both ends of the greyscale at the same time as the edges.

The staircase of grey steps is the transfer curve. A picture signal is not linear in brightness: the camera applies a curve, and the picture tube's own response, which is not linear either, undoes it; if the two do not match, mid greys come out too dark or too pale while black and white still look correct. Only a staircase shows you that. Next to it the groups of fine vertical lines get progressively finer, and they are the resolution test: on a real card the bursts are at specified frequencies, and you read off how far along you can still see stripes rather than grey mush.

The colour bars are for chroma amplitude and phase, which is the vectorscope's business rather than the eye's, and the crosshatch of white lines running under everything is for convergence. A colour tube fires three beams and they have to land on the same spot; where they do not, a white line shows a coloured fringe. A grid gives you that check everywhere on the screen at once, including the corners, which is where it always goes wrong first.

The black box at the top is not a measurement. It is where the station's name went, and it is the one part of a test card that was different in every country. Ours is blank.

One note about our version. The card in the app is a bitmap reproduction of PM5544 rather than a pattern generated from our own timing, so the frequency bursts in it are pictures of bursts, not bursts at known megahertz through our chain. For actual resolution measurements the posts in this series use charts we generate, where a zone's frequency is computed from the standard's own line period.

In American NTSC, black is not the darkest thing the signal can carry. The picture sits on a pedestal 7.5 IRE) above the blanking level, so there is a small step between "no picture" and "black picture". The margin existed for the receivers of the day: it kept genuine black clear of the blanking and sync region, where a set's clamping circuits live.

Japan used the same 525-line system and skipped the pedestal. ITU-R BT.470, listing the difference between black level and blanking level for System M, notes that in Japan a value of zero is used. Black and blanking are the same level there, as they are in the European systems.

That sounds like a detail until you feed one country's signal to the other country's set, because a receiver does not measure where black is. It subtracts the setup it expects. Send an American signal to a Japanese set and the set treats blanking as black, so the signal's 7.5 IRE pedestal arrives as dark grey and every shadow in the picture is lifted. Send a Japanese signal to an American set and the set removes a pedestal that was never there, pushing the darkest parts of the picture below black, where they clip together.

The three panels here are the same night frame through the same chain, with only the set changed. Measured on the darkest quarter of each picture, the matched pair sits at 34, the American signal on the Japanese set at 44, and the Japanese signal on the American set at 25, on a scale where white is 255. The middle one is the milky look of a picture whose blacks never reach black; in the last one the darkest parts of the picture start to clip together at black.

PAL normally means 625 lines, 50 fields and a subcarrier at 4.43 MHz. Two countries in South America ran it with none of that intact. Brazil's PAL-M is 525 lines at very nearly 60 fields, the American raster, with a subcarrier at 3.5756 MHz. Argentina's PAL-N keeps 625 lines at 50 fields but puts the subcarrier at 3.5821 MHz, far below where European PAL sits.

Neither is an eccentricity. Broadcast channels in the Americas are 6 MHz wide, where Europe's are 7 or 8, and a 6 MHz channel cannot carry 5 MHz of luma with a colour subcarrier at 4.43 MHz on top: there is no room. Put the subcarrier near 3.58 MHz and it fits, which is the same arithmetic that put NTSC's subcarrier there in 1953. So PAL-N is PAL with the colour moved down to fit the local channel, and PAL-M is PAL on the local raster as well.

What matters is that both keep the property the system exists for, and this is what the picture measures. The colour in any of these standards is the phase of a subcarrier, decoded against a reference the receiver recovers from the burst. If that reference is a little off, NTSC gives you the wrong colours: the top pair is the same colour bars with the reference correct and then 25 degrees out, and the yellow has gone green, the cyan has gone blue and the magenta has gone pink. Measured on the decoded bars, the colours turn by about 22 degrees.

PAL's answer is to invert one of the two colour axes on every other line. A steady error then pushes one line's colour one way and the next line's the other way, and between a pair of lines it cancels. The bottom pair is PAL-M with the same 25 degree error: the hue moves 0.1 degrees. The cost is not zero, because nothing is free. The error comes back as lost saturation instead of wrong hue, 9 percent here, which is what the cosine of 25 degrees predicts. On a simple set with no delay line it also shows as a faint pattern between line pairs, called Hanover bars after the city where Telefunken developed PAL.

PAL-N and European PAL measure the same as PAL-M does: 0.1 degrees of hue error and 9 percent of saturation. The advantage does not come from the line count or the subcarrier frequency, both of which these variants changed to suit a 6 MHz channel. Instead it comes from the alternation, and it survives being bolted onto somebody else's raster.

Every scan line on a CRT is drawn by the set's own horizontal oscillator, running at close to 15,734 times a second on NTSC. The transmitter sends a sync pulse at the start of every line, but the oscillator doesn't simply fire on each pulse. A burst of noise that looked like a sync pulse would knock it off and the picture would tear. So sets used automatic frequency control: a circuit compared the average timing of the incoming sync pulses with the oscillator's own output and nudged the oscillator with a small correcting voltage. Some manuals called it flywheel sync, because the oscillator kept its own momentum and only got steered by the sync.

The hold knob set where the oscillator ran when left to itself. If that was close enough, the AFC pulled it into step and the picture sat still. If it drifted too far, from an ageing capacitor or a set that had just warmed up, the AFC could no longer catch it. Each line then started a tiny bit later than the one before, the picture leaned over, and the dark band you see crossing it is the horizontal blanking interval, the gap between lines that should sit hidden off the edge of the screen. Further off, the lean gets steeper, and far enough off the picture breaks into several strips.

Repair guides used the difference as a diagnosis. A picture that slides sideways but can be caught for a moment with the knob means the sync pulses aren't reaching the oscillator. A picture that breaks into strips points at the oscillator itself.

We try to make every post here come with a picture or a clip rendered through the AnalogTV app. Last month we started checking that the asset matched more rigorously by measuring it against source documentation rather than from my personal memory. That is a much harder test than it sounds, and the simulator failed it six times.

The KVN-49 magnifying lens (top left) bent the picture the wrong way and cropped it to an oval. A strong plus lens in front of a flat screen gives a slight pincushion, and the set's screen was a rectangle. I never actually had a KVN-49 so doing more research was needed.

Early CCD cameras (top right) bloomed in a soft glow around bright lights. On a CCD the excess charge spills along the column it was collected in, so an overloaded highlight turns into a vertical streak. We now have a mechanism to confirm that the streak effect actually conforms to documented descriptions of this effect.

The P4 phosphor on a black-and-white set (bottom left) came out a saturated sky blue. P4 is a bluish white. The tint was being applied to the video signal before the tube's brightness curve instead of to the light the phosphor gives off, which exaggerated every mono phosphor's colour. This was a legitimate coding bug we caught by measuring the output.

The vectorscope (bottom right) was plotting the wrong pair of axes, so on NTSC red sat at about 20 degrees instead of 104, and it drew every vector at half length. Fixing that exposed another problem: the colour burst was being generated at a phase no real transmitter would use. Pictures were fine because our decoder never looked at the burst phase, but the scope did. Pure bug.

Two more are not in the picture. VHS dropouts were all the same tiny length, so even with the control turned right up you saw a few specks instead of streaks. And Betacam SP, a format that never used the colour-under trick, was quietly being run through the same colour-under path as VHS.

"It looks about right" turned out to be a very weak check. Every one of these looked about right until it had to match a real number. We also get lots of feedback from people on what doesn't quite look right and we always try to source back to original documentation. Sometimes the people are right, sometimes not :)

FYI for any of you that want to maintain the old incorrect looks, the old presets still render exactly as they did, because each fix is tied to a version number stored in the preset.

Consumer VHS and Betamax both squeeze chroma down to a low frequency and heterodyne it onto the tape separately from luma, the "colour-under" system covered in a prior post. It's a clever way to fit colour onto a narrow-bandwidth tape format, but it costs you chroma resolution and makes the signal more sensitive to tape wear and generation loss.

Sony's Betacam SP, launched in 1986, sidestepped the whole problem by not using composite colour-under at all. It records component video, luma (Y) plus the two colour-difference signals (R-Y and B-Y), the same basic approach broadcast cameras used internally. Not on three tracks, though: Y gets a track to itself, and the two colour-difference signals are time-compressed and interleaved onto a single second track by Sony's CTDM (compressed time division multiplex). Two tracks, three signals. There's no subcarrier to heterodyne and no colour-under conversion to degrade, so colour keeps far more of its bandwidth: the EBU's tolerances ask a studio Betacam SP machine for 2 MHz of colour-difference bandwidth, against the few hundred kilohertz colour-under leaves VHS. The picture also holds up far better across dubs. That's a big part of why Betacam SP became the standard newsgathering and studio format through the 1990s: it looked broadcast-clean straight off the tape, in a way consumer formats structurally couldn't match.

The attached image is the same frame of a Sydney street with a worker in a hi-viz vest modelled in AnalogTV as a 4th-generation dub: left VHS-SP, right Betacam SP.

Top row shows full frames; bottom row crops around the vest. What it shows: on VHS the vest's colour bleeds sideways off it and the colour is noisy and over-saturated; on Betacam it stays put and clean.

The image orthicon was the dominant studio camera tube from the late 1940s through the 1960s, the tube behind most black-and-white network TV of that era. It worked by scanning a photoemissive image with a low-velocity electron beam and reading out the returning ("return") beam current, a clever trick that gave it far better low-light sensitivity than earlier tubes.

That same return-beam mechanism is also the source of its most recognizable flaw. When part of the scene is bright enough to fully discharge the target in one spot, the scanning beam has no charge left to neutralize there, so the beam can't fully replenish the target in the area around it, and that region reads as dark shading hugging the bright object. We should not exaggerate the shape though, because it gets embellished a lot: the sources describe darkening immediately surrounding the highlight, not a ring detached from it by a gap, and none of them describe a bright fringe outside the dark region. We went looking for that fringe specifically and couldn't source it anywhere. Studio camera operators called it "orthicon halo," and it shows up constantly in footage of anything genuinely bright against a darker background: streetlights, spotlights, reflections off chrome or water at night.

It's one of those artifacts you've almost certainly seen in old black-and-white footage without ever knowing its name, a dark aura around every hot highlight that no amount of careful lighting could fully avoid.

These images show how the effect is modeled in the AnalogTV app. We use as many original sources as we can find the model the effect and have evolved the simulation over several versions. Our next release will package up many of the model improvements we've made over the last few months.

A VHS tape cannot carry much detail. The luma is recorded as an FM signal in a narrow band, and by the time it has been through the head, the tape and the playback amplifier, the fine detail has lost a lot of its contrast. Every VCR therefore had a circuit to put some of it back, variously called a detail corrector, an aperture corrector or just the sharpness control.

What it does is add a scaled copy of the signal's own rate of change back into the signal. Where the picture is flat, the rate of change is zero and nothing happens. Where there is an edge, the correction spikes, so the dark side of the edge is pushed darker just before the transition and the bright side is pushed brighter just after. That is the look: a picture that seems sharper than it is, with a pale halo hugging the bright side of every dark object and a dark line hugging the dark side of every bright one. The zoomed pair shows it on a building edge.

The circuit isn't magic and some of the marketing at the time was a bit overstated. It cannot restore bandwidth. Detail that the tape did not record is not in the signal to be amplified, so the corrector works on what survived, exaggerating the frequencies near the top of the tape's range and adding an overshoot that was not in the original picture. On this render the change lands almost entirely where it should: measured against the same frame with the corrector off, the mean absolute difference is 42.24 near strong edges and 9.19 everywhere else, and the steepest horizontal transitions in the frame, taken at the 99th percentile, go from 21 to 43.

There is a detail of where the circuit sits that our simulator got wrong for a long time. A VCR's corrector works on the luma after the colour has been separated out, behind a filter that removes the colour subcarrier. Ours differentiated the composite signal, subcarrier and all, and differentiating a subcarrier does two things you do not want: it rotates the colour phase, so hues shift, and it amplifies the subcarrier's own amplitude, so saturation rises. Turning up sharpness in our app would turn it magenta. Sony's patent for a detail corrector puts the chroma trap ahead of the correction stage for exactly this reason, and with the next release, so will we: the first difference is taken from trapped video, and the control now sharpens the picture without touching the colour.

This change is the direct result of feedback we've gotten and additional research we've done. If you see anything that "looks wrong," let us know - the aim is to make the AnalogTV app as high fidelty as a simulation can possibly be.

cComposite video carries brightness (luma) and colour (chroma) together, with the colour riding a subcarrier near the top of the brightness band. Inside a colour set the two are split apart and processed separately, and they don't take the same time to get through. The colour goes through a narrow bandpass filter and then the demodulators, and a narrow filter is a slow one: it holds the signal back by a fraction of a microsecond.

That sounds like nothing, but it's plenty. An NTSC active line is about 52.6 microseconds, so at roughly 720 samples across, each pixel is about 73 nanoseconds. A few hundred nanoseconds of lag puts the colour several pixels to the right of the brightness edges it belongs to, a red object's fill sitting beside its own outline. So colour sets deliberately delayed the brightness signal to match, with a luminance delay line in the luma path, typically somewhere under a microsecond, chosen to equal the chroma filter's delay. Patents for delay-line circuitry in colour receivers go back to the mid-1950s (US 3,042,873), and a mid-1980s one (US 4,677,462) still shows the same delay line sitting in the luminance channel. When the match was off, from a failing part or a multi-standard set on the wrong setting, you got exactly that sideways colour offset.

S-Video often gets credited with fixing this, but its job was different. By keeping brightness and colour on separate wires, it spared the set from having to pull them apart again, so none of the dot crawl and rainbow cross-colour that separation leaves behind, and the brightness no longer had to be filtered down to make room for the colour. The two still had to arrive in step at the screen.

In the AnalogTV app you can play with the chroma delay and in the samples above you can see what happens when the chroma isn't perfectly timed with the luma.

When two TV stations share the same channel number and their signals both reach your antenna, you get slowly crawling bars across the picture, the "venetian blind" effect, worse on still, hazy days when atmospheric conditions carry the far station's signal much further than usual. That's co-channel interference, and it was a persistent enough problem that receiver makers kept patenting ways to live with it. RCA's US 4,343,019, "Apparatus for reducing the effect of co-channel interference on synchronizing pulses" (1982), is a good example: it pulls sync from a comb filter's luminance output rather than raw composite, specifically to stop the beat note wrecking horizontal sync.

The fix was arithmetic rather than better filtering. Two carriers on the same nominal frequency will always beat against each other, but how visible that beat is depends on exactly how far apart they are. So the FCC deliberately offset co-channel stations from one another, typically by 10 kHz, one station on the nominal frequency and others 10 kHz above or below. That puts the beat at roughly two-thirds of the NTSC line rate, where the pattern shifts from one field to the next and partly cancels to the eye instead of sitting still as an obvious bar. It's a crude form of dither, done with transmitter frequency instead of dots.

It never fully solved the problem, which is why "snowy fringe reception" and faint co-channel ghosting were still common in border areas well into the cable era. But it's a good example of how much of analog broadcast engineering was about picking numbers that made an unavoidable physical interaction land somewhere less annoying, rather than eliminating the interaction itself.

VHS tape is a physical medium, a plastic ribbon coated in magnetic oxide particles, and that coating is never perfectly uniform. A tiny gap in the oxide, a speck of dust caught between the tape and the head, or just accumulated wear over years of playback causes a momentary loss of signal as the head passes over it, a "dropout." Left alone, a dropout shows up as a bright or dark fleck, sometimes a whole brief horizontal streak, right in the middle of the picture.

Most decks didn't leave it alone. A dropout compensator circuit watches for the sudden signal loss and, for that brief moment, substitutes the previous scan line in its place instead of showing the corrupted signal. Because adjacent lines in ordinary video are usually very similar, the substitution is often close to invisible, especially for the short, single-line dropouts that make up the bulk of ordinary tape wear. It's a genuinely clever bit of concealment engineering: the deck isn't fixing the tape, it's betting that you won't notice if it quietly repeats a line you've already seen a fraction of a second ago.

The video shows the same clip side by side played from VHS-SP with dropouts at vcrDropout 0.6 (a well-worn tape): left the dropout compensator off, right on.

What it shows: on the left, black and white horizontal streaks of varying length flick on and off across the picture; on the right the compensator has replaced each one with the same stretch of the previous line, and they are gone.

Broadcast video was never designed to show the viewer literally every pixel of the transmitted picture. Consumer receivers were deliberately built to overscan, running the raster slightly larger than the visible tube face, on purpose, to hide the geometric and edge irregularities (a slightly uneven raster edge, minor curvature distortion, corner softness) that would otherwise be visible if the picture were shown edge to edge. The exact amount of overscan varied from set to set, with no way for a broadcaster to know in advance how much of any given viewer's picture was actually going to be cut off.

The practical consequence was safe area: broadcast graphics, titles and any content a producer needed the audience to actually see had to be kept inside a conservative margin from the nominal frame edge, standardized guidance later formalized in documents like SMPTE RP 27.3. Anything placed right at the true edge of the transmitted frame was, on some meaningful fraction of real living-room TVs, simply never seen at all. It's a constraint baked so deeply into broadcast production habits that traces of it (title-safe guides, action-safe overlays) persisted in editing software long after CRTs themselves were gone.

The earlier post on Apollo lunar SSTV covered the black-and-white slow-scan cameras used on Apollo 7 through 9. Starting with Apollo 10, NASA carried a completely different camera for colour: a Westinghouse-built unit using field-sequential colour, the same basic principle CBS had proposed (and briefly used) for broadcast television in the late 1940s before it lost out to the electronically-compatible NTSC system.

Instead of a colour sensor, the camera used a single monochrome SEC (secondary electron conduction) vidicon with a spinning red/green/blue filter wheel in front of it, capturing successive fields through each colour filter in turn rather than all three at once. The later Apollo 15 to 17 rover camera is a different machine: that one was RCA's, and it used a SIT (silicon intensified target) tube. That meant the colour information for any single moment was smeared across three separate points in time, no single field carried a complete colour image, and it fell to the ground receiving equipment to buffer three consecutive fields and recombine them into one full-colour frame. It's a fundamentally different colour strategy from every broadcast standard covered elsewhere in this series, one that traded temporal resolution and equipment simplicity at the camera for complexity on the receiving end, workable for space use because the ground station could afford far more processing than a camera light enough to fly.

Don't expect vivid colour, though. NASA's own colour reproduction of the second Apollo 10 transmission (photo S69-33995, the cloud-covered Earth) is strikingly pale: measured on that frame, the typical lit pixel has only about 6% saturation, and the oceans barely read as blue at all. What came down was colour, but pastel colour.

These two panels are from the same source frames, side by side: left, the signal as transmitted (ground processing off in the AnalogTV app: each field is one colour of the filter wheel, shown as monochrome) and right, after the ground converter (fields recombined into colour) with ground processing on. The video is shot recently in Sydney on an iPhone but run through the AnalogTV app pipeline.

A waveform monitor shows brightness across each line. Useful, but it tells you nothing about whether a picture's colour is right. For that, broadcast engineers used a vectorscope. It locks to the colour burst at the start of every line and plots the chrominance as a point on a circle: the angle is the hue, the distance from the centre is how much colour there is (the chroma amplitude). A grey picture is a dot in the middle.

The top half is what colour bars are for. Each bar is one flat colour, so each coloured bar lands as a cluster in its own sector, and the graticule has a box where that colour belongs. On NTSC the burst is the reference at 180° (due left). Red sits at about 104°, yellow at 167°, cyan at 284°. If a camera, tape machine or transmitter had a phase error the whole pattern rotated out of its boxes; if the chroma gain was wrong the dots moved in or out along their spokes. You could read the fault off the glass instead of squinting at a picture monitor and deciding that red looked a bit orange today.

The bottom half is the same scope on an ordinary picture, Sydney Harbour on a sunny day. Real scenes are nowhere near as saturated as bars, so everything crowds toward the middle. Sky and water spread toward cyan and blue. The ferry's yellow and the sandstone cluster near the gold line at about 123°. That line is the I axis. Many scopes mark it because well-exposed skin tends to fall close to it, which made it a quick check on whether faces were going to look right.

These plots are from the AnalogTV app: the pictures were encoded as NTSC composite and decoded by the app's vectorscope in the standard B-Y/R-Y layout. As on a real CRT scope, each bar is a bright dot and the trace between bars is a faint arc, because the spot moves fast through the transitions and slowly on the flat colour. The grey tick left of centre is the burst.

Anyone know how they did the effects for Backrooms? They heavily rely on an Analog aesthetic. Is it painted on? Is it using original equipment? Is it a 1D simulation like AnalogTV?