Rue Mohr's 115kaudio Captures Audio as Serial Data — and Plays It Back with a Resistor and Capacitor

Captured via a Microchip ATtiny13, these data streams are fed into a two-component "1-bit" DAC to turn them back into audio again.

Gareth Halfacree
1 month agoMusic / HW101

Maker Rue Mohr has created what could be the most unexpected audio device, using an encoder to turn music into serial data and a simple resistor-capacitor (RC) filter to turn it back into recognizable sound once again: 115kaudio.

"I came up with an encoder that can turn audio into a 115,200 baud serial stream that when played back thru an RC filter, recovers the audio," Mohr explains of the project, which he further describes as a "1 bit ADC [Analog to Digital Converter] that frames [the] result as serial data."

This is a music player with a difference: you're hearing a raw serial data stream, with only a simple RC low-pass filter applied. (📹: Rue Mohr)

The hardware on the encoder side of the project is a Microchip ATtiny13, which is used to provide a real-time serial stream of the encoded audio. "The schematic is quite simple," Mohr writes, "the only reason I used a microcontroller was the knock out the start and stop bits for me."

The microcontroller, with additional hardware to provide a low-pass filter and to rebias the input to 2.5V, is connected to a host PC and a serial connection opened at 115,200 bits per second, eight bit, no parity, one stop and start bit — typically written as 115200 8N1. Audio captured by the encoder on a 3.5mm jack is streamed as serial data to the host, where it can be written to a file for later playback.

The hardware on the decoder side, by contrast, is as simple as recreating the low-pass filter used for the encoder — one resistor and one capacitor. When connected to the transmit pin of the host machine's serial port — or a USB to UART adapter, if no physical TTL serial pins are available — the captured audio data can be streamed to the decoder and heard coming through connected speakers.

"This is the first 8N1 1-bit audio stream ever made," Mohr claims, "'cause nobody was stupid enough to try it yet. Be aware that the levels coming out of this are 3V, which is REALLY hot for an audio signal, you will want to make sure that either the volume of what you play it to is quite far down, or that you use a voltage divider to scale it a bit."

Schematics for the project, along with the source and pre-compiled firmware for the ATtiny13 firmware driving the encoder, are available in Mohr's GitHub repository under an unspecified license; more information is available in the maker's Mastodon thread.

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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