DOOM Plays From a Nine Millimeter Antenna Plug Screwed Into a Television

RayTriangle built a game console small enough to live inside a 9.5 mm Goobay antenna plug, the sort of screw-terminal adapter once used to join old 300 ohm ribbon cable to a 75 ohm television socket. Battery included, the whole machine screws into the antenna jack and puts DOOM on the screen.

An ESP32-S3 does the job, as two circuit boards lie side by side atop an 8.5mm lithium-ion battery, the sort used in wireless earbuds, and a flexible cable connects the two together. Rigidflex would have made things much neater and more expensive, but this flexible connection was the compromise. Soldering the thing was a pain because the copper was uneven, and one of the traces tore during construction. He attached a bit of wire from a connection to serve as the 2.4 GHz Bluetooth antenna.

Video never needs to travel down the cable in perfect condition. A resistor ladder (later replaced by an 8-bit R-2R network) converts the data from the ESP32-S3 into a PAL composite video signal with 625 lines and a very stable 50 Hz. A printed lookup table with around 8k entries defines the colors. Each entry is a combination of color, phase shift, and brightness since PAL causes the colors to flip every line and the ESP32-S3 is already performing a lot of work and cannot be bothered calculating it on the fly. Each color line is covered by four samples, and the two brightness pixels share that line, resulting in a slightly squashed image. An LCD peripheral basically dumps the data it receives from the ESP32-S3 out of the device via DMA, which is not the most elegant method.

The PAL signal is then sent on a carrier near 48.25 MHz, the lowest frequency in the PAL B & G band that is still operational. The carrier is generated by the ESP32-S3 clock, which is then reduced in frequency using the I2S master clock output. A 2 transistor mixer simply slots the video onto the carrier. Sound is modulated on the same carrier by simply streaming a series of 1s and 0s, which cause the audio carrier to wobble. At around 172 ohms, the output impedance is close enough to connect directly into a 75 ohm socket.

This is a real DOOM port, not a canned demo. The video shows DOOM being played through a full level of Knee-Deep, complete with a frame counter and internal temperature indicator. It everything connects to an Xbox controller via Bluetooth Low Energy, keeping the controller outside the size limit. The signal strength is good, about 2 meters. The current draw with the radio turned on is roughly 120 milliamps, which was one of the reasons the earlier concept (an even smaller RCA plug with composite video but no radio) was abandoned, since the battery couldn’t feed the chip and it wasn’t obvious where the audio would go.

Picture quality matches the method. Sixteen voltage steps cover about a volt, resistor tolerance shows up as color bend, and negative modulation on this PAL variant flattens contrast. Crawling dots have now appeared all over the image. The audio is also bad, and even when you try to clean it up, it appears to leak into the picture. A second harmonic may also enter the FM broadcast band unless the boards are shielded. If you’re using this to watch PAL on an ancient VHF TV, that’s probably fine. If you tried to get it to function on anything that supports NTSC or a higher UHF channel, you’d have to completely rethink the carrier plan. RayTriangle has actually uploaded the files for the boards and the port to Github under “tv-doom,” and he has invited anyone who believes they can fit DOOM into anything smaller to give it a try.
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