The Raspberry Pi is popular among retro computing and gaming enthusiasts. Video output from its emulators can be pixel-perfect when used with an HDMI television or monitor. However, some users miss the fuzzy blur of vintage RF-modulators! While some software filters exist, they are far from ideal. The latest project from Isotope Engineering's Adam Zeloof restores vintage computing's nostalgic RF displays. PiMod Zero is an RF modulator pHAT for the Raspberry Pi Zero.
"The PiMod Zero brings old tech back to life by displaying video (color and B&W) and audio from a Raspberry Pi Zero on a vintage television ... once the Pi is powered, the only other cable you need is a piece of coax to connect the HAT to your vintage TV."
PiMod Zero is capable of displaying both color and B&W images. The display format is NTSC. There are two VHF channels available, just like retro-hardware: 2 (55.25 MHz) and 3 (61.25 MHz). Audio is also supported, which a standard Pi Zero cannot do on its own!
The heart of the PiMod Zero is a New Japan Radio (JRC) NJM2519 chip. JRC designed this chip specifically for VHF-band RF-modulation. It also handles mixing the audio with the video together.
Since RF-modulation is no longer prevalent in consumer devices, Zeloof had to solve a couple of design challenges. The NJM2519 modulator, an already difficult to source component, relies on oscillators for the video and audio signals. Both required locating elusive inductors. The video depends on a SAW resonator, while the audio requires an LC tank.
Zeloof found an interesting behavior when tuning the audio's intercarrier frequency. The intercarrier method is how TV combines the video and audio signals into a single transmission. PiMod Zero's circuit worked well with a modern, presumably digital/SDR-based receiver. However, with a vintage receiver, Zeloof found the audio to be noticeably off. Fortunately, the inductor used in the circuit makes it possible to tune your specific receiver's frequency.
For analog audio, traditional Raspberry Pi boards generate audio by running a PWM signal through an RC filter. The Pi Zero, however, does not have this capability natively. For audio, the Pi Zero's GPIOs generate the digital signal, and PiMod Zero has an RC filter on-board.