The demo code for [XTronical]’s ESP32-based SD card music player is not even 40 lines long, though it will also require a few economical parts before it all works. Nevertheless, making a microcontroller play MP3s (and other formats) from an SD card is considerably simpler today than it was years ago.
Part of what makes this all work is I2S (Inter-IC Sound), a format for communicating PCM audio data between devices. Besides the ESP32, at the heart of it all is an SD card reader breakout board and the MAX98357A, which can be thought of as a combination I2S decoder and Class D amplifier. The ESP32 reads audio files from the SD card and uses an I2S audio library to send the I2S data stream to the MAX98357A (or two of them for stereo.) From there it is decoded automatically and audio gets pumped though attached speakers.
It’s amazing how much easier audio is to work with when one can take advantage of shuffling audio data around digitally, and the decoder handles multiple formats with an amplifier built in. You can see [XTronical]’s ESP32 player in action in the video embedded below.
Of all the people I was looking forward to meeting at Supercon, aside from my Hackaday colleagues with whom I had worked for five years without ever meeting, was a fellow from Germany named Matthias Balwierz. The name might not ring a bell, but he’ll certainly be familiar to Hackaday readers as Bitluni, the sometimes goofy but always entertaining and enlightening face of “Bitluni’s Lab” on YouTube.
I’d been covering Bitluni’s many ESP32 hacks over the years, and had struck up a correspondence with him, swapping ideas and asking for advice on the many projects I start but somehow never finish. Luckily for us, Bitluni is far better on follow-through than I am, and he brought that breadth and depth of experience to the Design Lab stage for that venue’s last talk of the 2019 Superconference, before the party moved next door for the badge-hacking presentations.
Last month we marked the 40th birthday of the CD, and it was as much an obituary as a celebration because those polycarbonate discs are fast becoming a rarity. There is one piece of technology from the CD age that is very much still with us though, and it lives on in the standard for sending serial digital audio between chips. The protocol is called I2S and comes as a hardware peripheral on many microcontrollers. It’s a surprisingly simple interface that’s quite easy to work with and thus quite hackable, so it’s worth a bit of further investigation.
It’s A Simple Enough Interface
Don’t confuse this with the other Philips Semiconductor protocol: I2C. Inter-Integrated Circuit protocol has the initials IIC, and the double letter was shortened to come up with the “eye-squared-see” nomenclature we’ve come to love from I2C. Brought to life in 1982, this predated I2S by four years which explains the somewhat strange abbreviation for “Inter-Integrated Circuit Sound”.
The protocol has stuck around because it’s very handy for dealing with the firehose of serial data associated with high-quality digital audio. It’s so handy that you’ve likely heard of it being used for other purposes than audio, which I’ll get to in a little bit. But first, what does I2S actually do?
In a world where standards come and go with alarming speed, there’s something comforting about VGA. It’s the least common denominator of video standards, and seeing that chunky DB15 connector on the back of a computer means that no matter what, you’ll be able to get something from it, if you can just find a VGA cable in your junk bin.
But that’s the PC world; what about microcontrollers? Can you coax VGA video from them? Yes, you can, with an ESP32, a handful of resistors, and a little bit of clever programming. At least that’s what [bitluni] has managed to do in his continuing quest to push the ESP32 to output all the signals. For this project, [bitluni] needed to generate three separate signals – red, green, and blue – but with only two DACs on board, he had to try something else. He built external DACs the old way using R/2R voltage divider networks and addressed them with the I2S bus in LCD mode. He needed to make some compromises to fit the three color signals and the horizontal and vertical sync pulses into the 24 available bits, and there were a few false starts, but the video below shows that he was able to produce a 320×240 signal, and eventually goosed that up to a non-native 460×480.
Composite video from a single-board computer? Big deal — every generation of Raspberry Pi has had some way of getting composite signals out and onto the retro monitor of your choice. But composite video from an ESP32? That’s a thing now too.
There are some limitations, of course, not least of which is finding a monitor that can accept a composite input, but since [bitluni]’s hack uses zero additional components, we can overlook those. It really is as simple as hooking the monitor up to pin 25 and ground because, like his recent ESP32 AM radio station, the magic is entirely in software. For video, [bitluni] again uses his I²S tweaks to push a lot of data into the DAC really fast, reproducing the sync and image signals in the 0-1 volt range of the PAL composite standard. His code also supports the NTSC standard, but alas because of frequency limitations in the hardware it’s monochrome only for both standards, at least for now. He’s also got a neat trick to improve performance by running the video signal generation and the 3D-rendering on separate cores in the ESP32. Check out the results in the video below.
[curcuz]’s BoomBeastic mini is a Raspberry Pi based smart connected speaker. But don’t dis it as just another media center kind of project. His blog post is more of a How-To guide on setting up container software, enabling OTA updates and such, and can be a good learning project for some. Besides, the design is quite elegant and nice.
The hardware is simple. There’s the Raspberry-Pi — he’s got instructions on making it work with the Pi2, Pi2+, Pi3 or the Pi0. Since the Pi’s have limited audio capabilities, he’s using a DAC, the Adafruit I2S 3W Class D Amplifier Breakout for the MAX98357A, to drive the Speaker. The I2S used by that part is Inter-IC Sound — a 3 wire peer to peer audio bus — and not to be confused with I2C. For some basic visual feedback, he’s added an 8×8 LED matrix with I2C interface. A Speaker rounds out the BoM. The enclosure is inspired by the Pimoroni PiBow which is a stack of laser cut MDF sheets. The case design went through four iterations, but the final result looks very polished.
On the software side, the project uses Mopidy — a Python application that runs in a terminal or in the background on devices that have network connectivity and audio output. Out of the box, it is an MPD and HTTP server. Additional front-ends for controlling Mopidy can be installed from extensions, enabling Spotify, Soundcloud and Google Music support, for example. To allow over-the-air programming, [curcuz] is using resin.io which helps streamline management of devices that are hard to reach physically. The whole thing is containerized using Docker. Additional instructions on setting up all of the software and libraries are posted on his blog post, and the code is hosted on GitHub.
There’s a couple of “To-Do’s” on his list which would make this even more interesting. Synced audio being one: in a multi-device environment, have the possibility to sync them and reproduce the same audio. The other would be to add an Emoji and Equalizer display mode for the LED matrix. Let [curcuz] know if you have any suggestions.
What’s coolest about this project is the way that [lhartmann] does it. The tiny ESP8266 has nowhere near the required number of GPIO pins, the primary SPI is connected to the onboard flash memory, and the secondary SPI is poorly documented and almost nobody uses it. So, [lhartmann] chose to use the I2S outputs.
I2S is most often an audio protocol, so this might at first seem like a strange choice. Although I2S sounds like I2C, it’s really essentially an SPI protocol with a fourth wire that alternates to designate the right or left channel. It’s actually just perfect for sending 16×2 bits of data at high data rates.
[lhartmann] takes these 32 bits and feeds them into four shift registers, producing 32 outputs from just the four I2S data lines. That’s more than enough signals to run the stepper motors. And since it updates at 192 kHz sample rate, it’s plenty fast enough to drive them.
The other side benefit of this technique is that it can work on single-board computers with just a little bit of software. Programming very complicated stepper movements then becomes just a matter of generating the right “audio” file and playing it out. [lhartmann] demonstrated this earlier with an Orange Pi. That’s pretty cool, too.
The code for turning the ESP8266 and a short handful of 74HC595s into a 3D printer controller are up on GitHub, so go check it out.