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This text was first published in German on Golem on 16 August 2025: HDMI auf Gardena: Spül mir das Lied vom Tod.

How to transmit a Western film through a garden hose, and what a Soviet lunar probe, Daft Punk, and a fax machine have to do with it

Anyone who thinks HDMI solves all connection problems has clearly never tried sending a film through a garden hose. This experiment blends DIY humour, space history and a dash of absurd engineering: an HDMI-to-Gardena adapter – technically feasible, wildly impractical, but undeniably entertaining.

We love adapters

The urge to convert signals from one format to another is hardwired into the digital human. It is how we adapt to an industry that has spent decades bombarding its customers with new plugs, regularly adding new levels to the popular children's game of matching shapes and holes.

Most of us have the relics of this era buried in our drawers. Rummage around, and you'll uncover treasures from days gone by: old mobile phones, power supplies, CD-ROMs, batteries of unknown charge and condition, promotional USB sticks and, finally, an assortment of adapters and connectors. From USB-A to micro-USB, RCA to mini jack, and in deeper sediments you might even find a 20-pin Scart monster with an S-video connector at the other end.

Pre-USB veterans will recall the backs of earlier computers, each port being different: Keyboard (DIN), mouse (9-pin serial interface), printer (25-pin parallel interface), external storage media (various SCSI formats), monitor (VGA, 15-pin D-Sub), joystick (also D-Sub 15-pin, but different) and so on.

Looking through? Too easy!

No wonder, then, that adapters have become a staple of contemporary culture and thus a popular target for jokes. A classic is the adapter from HDMI to the Gardena coupler system for garden hoses. For the opposite direction, i.e. from Gardena to HDMI, there are even functional products available.

Let's go one step further and transmit images through a garden hose. The easy way would be to pull the hose straight, hold a screen at one end and peek through from the other. But we want something more elegant.

We find the technical solution to the problem in history. More precisely: in Soviet space history, because there was a mission that faced very similar transmission problems. And the solution from back then still works today–even if you replace a space radio link with gardening tools.

The dark side of the moon

The year is 1959, and the Cold War is raging. The space race between the USA and the USSR is in full swing. In the competition to be the first to reach the moon, the Soviet Union is ahead: Luna 1 is the first space probe to fly close to the moon (it was supposed to crash into it, but missed), Luna 2 at least hits the moon's surface, and Luna 3 has a particularly ambitious goal: To deliver the first-ever images of the Moon's far side – the area that is never visible from Earth – using two cameras.

Then, as now, launching an object into space was far easier than bringing it back intact to Earth. At the time of the Luna 3 mission in 1959, no spacecraft had ever returned in one piece. For Luna 3, too, a return from the moon was completely out of the question at that time. The first soft landing wouldn't happen until the following year, when a Vostok capsule returned intact from Earth orbit.

The world's biggest radio dead spot

The only way to obtain the images of the dark side of the moon taken by Luna 3 is to transmit them by radio. Radio technology, developed in the 1890s, is not the issue here. The technology for live image transmission also exists already and is being used on a large scale for television broadcasts. The real problem is the 73 trillion tonnes of moon that block the line of sight between the probe and the receiving stations at the time of recording: there is no place on Earth from which the probe could have been received.

The only solution is to take photos and send them at a later time, after the probe has emerged from the moon's shadow. However, this presents the next technical challenge: digital photography – and with it, simple methods for capturing, storing and encoding image data – has not yet been invented. The first CCD sensor will not detect the light of day until 1969. So light-sensitive film still has to be used and chemically developed.

Film material from the class enemy

For this reason, a compact, fully automated darkroom is being constructed and packed on board that is capable of developing, fixing, rinsing and drying the film. Spicy detail: the film material itself comes from the class enemy, repurposed from intercepted US spy balloons (part of the Genetrix project), hundreds of which floated over the Eastern Bloc, rising up into the stratosphere. The film material used in the cameras on board was specially designed by the CIA to withstand extreme temperatures and radiation. The unexposed film surpluses from captured reconnaissance balloons are then sent into space as part of the Soviet space programme.

But how does the developed picture reach Earth?

A fax via radio

To accomplish this, Soviet engineers turned to a technology known today as slow-scan television (SSTV) – in contrast to ‘fast-scan television’, i.e. regular TV. SSTV transmits image data over narrowband connections. Similar to a fax machine, an image is scanned line by line and the detected brightness value is modulated onto the radio signal.

In Luna 3, the developed film is stretched in front of a cathode ray tube. A moving light point scans the image line by line, while a photomultiplier mounted in front of the film measures the brightness and converts it into an electrical signal.

On Earth, two stations in Crimea and Kamchatka receive the radio signals. There, the signal is displayed on a cathode ray tube equipped with a particularly long-persistence phosphor screen: by the time the final line arrived, the previous lines – and thus the full image – are still visible. In a dark room, the transmitted image can then be photographed from this screen.

The idea for SSTV goes back to Russian television pioneer Semyon Isidorovich Kataev, who developed a system for narrowband transmission of television images in 1934. The technology was further developed for use in amateur radio by Copthorne ‘Cop’ Macdonald, a student at the University of Kentucky, starting in 1957, and was approved for this purpose by the Federal Communications Commission (FCC) in 1968.

SSTV transmissions via amateur radio use a frequency range also used for voice transmission. This means that image data encoded for SSTV can also be output as an audible signal. This precisely is the trick we will use for our HDMI-Gardena adapter.

Whistling in the hose

The signal source in our setup is the HDMI output of a laptop. The film playing is ‘Once Upon a Time in the West’: it could have been any other film, but I needed the title to use as headline. (Remark: In the original German version, I used the title "Spül mir das Lied vom Tod", which is a pun on the film's German title "Spiel mir das Lied vom Tod". "Spülen" means "to rinse" which seemed fitting for a garden hose.)

The first component is an HDMI grabber connected to the HDMI cable. In this case, it is a no-name product from Berrybase, because they were one of the few suppliers that guaranteed Linux compatibility. This grabber acts as an external monitor towards the signal source, i.e. towards the laptop.

The grabber's other end connects to the USB port of a netbook. The netbook's operating system, MX-Linux, recognises the signal grabber as a webcam, so no extra drivers are needed.

Garden hoses lack bandwidth

So far, we've piped the video stream from the HDMI source to the first part of our adapter. At the source, this stream still had full HD resolution at 30 frames per second and in colour. Since garden hoses aren't exactly designed for video transmission and lack bandwidth, we have to dial down the quality for the next steps.

There are various quality levels for image transmission via SSTV. The highest resolution offers 512 x 256 pixels and RGB colours, but transmitting a single image can take up to three minutes. That may be an acceptable frame rate when considered in relation to the duration of a space mission. However, it would significantly ruin the suspense of a film; even a Western by Sergio Leone, who was famous for his long takes.

If we want to maximise the frame rate instead of the image quality, we have to sacrifice resolution and colour: in the fastest mode, Robot 8, we transmit an image in greyscale with a resolution of 160 x 120 pixels in 8 seconds. Or, in other words, at 0.125 fps.

For encoding, we use the libsstv library (available on Github for self-compilation). Though it is actually a program library for encoding SSTV, it can also run stand-alone from the command line. Alternatively, there was also the PySSTV software, but it requires an image processing library no longer available in a 32-bit version. Since the netbook with its Atom processor can only run 32-bit software, this option was not viable.

A small script that runs in a continuous loop does the following:

1. The programme fswebcam captures the current frame from the video stream, converts it to greyscale, resizes it to 160 x 120 pixels, and saves it as a file.
2. libsstv encodes the image into an audio file.
3. aplay plays the audio file.

The feature film is now encoded in a whistle signal and can be fed into the garden hose.

Around the world, around the wo-orld

For this, we use a talkbox, an effects device popular with guitarists or keyboardists. It sends the instrument's sound through a tube into the musician's mouth, who can then change the sound spectrum by moving their mouth. Using a talkbox is like speaking or singing, except the sound is not produced by breathing and the movement of the vocal cords, but is introduced through the tube. The result, depending on how it's played, is a robotic-sounding vocal.

The talkbox was popularised in the 1970s by guitarist Peter Frampton, for example in Show Me the Way. Other iconic examples are Daft Punk's Around the World and 2Pac's California Love.

The talkbox connects to an audio source (in our case, the netbook's audio output), just like an active speaker. The amplifier is usually integrated; the other parts are a horn driver and the aforementioned tube. A horn driver is a specialized speaker that does not speak freely into the environment, but whose front is sealed airtight except for a small opening. Paired with a relatively large diaphragm, it generates a high alternating pressure at the opening.

The horn driver pumps the signal into the tube

When used in a horn loudspeaker, an acoustic horn is placed over the opening to amplify the sound. This design is far more efficient than if the loudspeaker were to speak directly into the environment. Such horns are familiar, for example, from PA systems at festivals or nightclubs, or from loudspeakers for train station announcements. In a talkbox, the horn is replaced by a tube, and the horn driver pumps the signal straight into it.

The musician inserts the other end into their mouth; they can shape the sound by moving their tongue, lips and jaw. The sound of the guitar or keyboard with modulated speech then emerges the musician's mouth and can be captured by a vocal microphone.

And instead of an electric guitar, we can pump any other signals into the tube – like, say, a film.

Charles Bronson as a comic book hero

To complete the HDMI-to-Gardena adapter, we attach a Gardena hose coupling to the other end, complemented by a standard water sprayer. This is where the acoustic signal blasts out, clearly audible as a loud chirping sound.

A microphone can pick up this signal and decode it. There is a simple way: the Robot 36 app (available for Android from F-Droid and Google Play Store) does exactly that on a smartphone or tablet. It uses the device's built-in microphone as a signal source, decodes the signal in real time and displays the resulting image. Depending on the incoming signal, every eight seconds another image gets visible.

Okay, we may have oversold it. Instead of a film, you get a slideshow of still images, and these are only in low quality. There is also no sound, but you could add that easily with a second hose and a second talkbox.

A slideshow with 0.125 fps

The biggest weakness of the setup is the low frame rate. A slideshow with one image every eight seconds – or even slower at higher quality – takes the excitement out of even the best Western.

One workaround would be to break the film down into individual images that are characteristic of the respective scene. With automatically generated subtitles, this would result in a kind of comic strip. Not a film yet, but at least the plot would make sense.

An alternative would be to pre-record the film on the netbook, split it into individual images, transmit them one by one through the tube, and reassemble them on the receiving end. It wouldn't be real time, but the result would be smoother, with better image quality. Let's do the math: a two-hour film contains 180,000 individual frames at 25 fps. In SSTV mode Martin M1, a 320 x 256 pixel colour image takes 114 seconds to transmit. This means that the entire film takes about 20.5 million seconds or 237.5 days, plus two hours for the soundtrack – or four for stereo.

With these limitations, it is unlikely that movie transmission via garden hoses will become widely accepted. At least now there is proof that an HDMI-to-Gardena adapter is technically possible, quite simple in design and easy to build.