Wozamp Turns Apple II Into Music Player

April 10, 2024 by Bryan Cockfield 9 Comments

Besides obvious technological advancements, early computers built by Apple differed in a major way from their modern analogs. Rather than relying on planned obsolescence as a business model, computers like the Apple II were designed to be upgradable and long-term devices users would own for a substantially longer time than an iPhone or Macbook. With the right hardware they can even be used in the modern era as this project demonstrates by turning one into a music player.

The requirements for this build are fairly short; an Apple II with a serial card and a piece of software called surl-server which is a proxy that allows older computers to communicate over modern networks. In this case it handles transcoding and resampling with the help of a Raspberry Pi 3. With that all set up, the media player can play audio files in an FTP network share or an online web radio station. It can also display album art on the Apple II monitor and includes a VU meter that is active during playback.

Although the 11.52 kHz sampling rate and 5-bit DAC may not meet the stringent requirements of audiophile critics, it’s an impressive build for a machine of this era. In fact, the Apple II has a vibrant community still active in the retrocomputing world, with plenty of projects built for it including others related to its unique audio capabilities. And if you don’t have an original Apple II you can always get by with an FPGA instead.

Melodio Self Mate

February 12, 2024 by Navarre Bartz 25 Comments

While the proliferation of the smartphone has caused the personal music player (PMP) market to mostly evaporate, there are still those who prefer a standalone device for their music. The Melodio Self-Mate is one such spiritual successor to the iPod.

Music-only devices really benefit from the wheel interface pioneered by Apple, so we still see it in many of the new Open Source PMPs including this one and the Tangara. The Melodio uses the ubiquitous ESP32 for its brains coupled with a TI PCM5102A DAC and TI TPA6130A2 headphone amp for audio. A slider on the side of the device allows you to switch it between mass storage mode and programming mode for the ESP32.

Since this device packs a little more horsepower and connectivity than the original iPods, things like listening to Spotify are doable once assembled, instead of having to completely rebuild the device. Speaking of building, there are only renders on the GitHub, so we’re not sure if this project has made the jump IRL yet. With more people concerned about the distractions of smartphones, maybe this renaissance of open PMPs will lead to a new golden age of music on the go?

Miss the halcyon days of the iPod? They’re easier to hack now than ever, and if you really want to go old school, how about a podcast on a floppy?

No DAC? Try PDM

January 6, 2024 by Al Williams 16 Comments

Ever notice that the ESP32-S3 doesn’t have a digital-to-analog converter? [Chris] did and asserts that he doesn’t care because he can just use the PDM system to get the same result. PDM — pulse density modulation — is similar to PWM and, like PWM, requires a filter that could range from a simple RC network to an active filter. You can see the result in the video below.

There are several ways [Chris] could produce the output he wanted. PWM was one choice, and some example code uses a timer to do PDM. However, that is not very efficient. The other alternative is to use the I2S output. However, this does require a few workarounds.

In particular, the I2S output is always stereo and incorporates a clock output that isn’t needed for this application. [Chris] simply output the same value on both channels and routed the clock to some pins that are normally used for startup options. That means they can’t easily be used for your own inputs, but it’s OK to use them for unimportant outputs.

We always enjoy seeing solutions like this because it can give you ideas for use in your own projects. Of course, this won’t apply to every project where you need a DAC, but it still might give you some ideas.

We have looked at PDM before. You could, too, build your own DAC hardware.

Continue reading “No DAC? Try PDM” →

Cockpit Instrument Respectfully Retasked As A Clock

December 5, 2023 by Dan Maloney 3 Comments

How do you convert an old cockpit instrument into a clock? Easy: just build a circuit that convinces it it’s in the air, and the rest will take care of itself.

Now obviously, little about [porkfreezer]’s conversion of King KI 266 DME into a clock was actually easy; working with avionics rarely is. DME stands for “Distance Measuring Equipment,” an instrument that’s part of the radio navigation suite of many aircraft. DME measures the line-of-sight distance of a plane to a ground station by measuring the time it takes for a signal to return after the plane interrogates it. The plane-mounted equipment includes a UHF transceiver and a display for the cockpit instrument panel, which accepts an analog voltage signal from the transceiver and translates it into a readout on the nice Panaplex digital display.

Rather than gutting the thing and just driving the display directly, [porkfreezer] decided to build a circuit to generate the proper signals for the DME. The board uses a PIC16 and an MCP47C dual 10-bit digital-to-analog converter to generate the voltages needed, while a USB-powered DC-DC converter provides the ±15 volt supply the DME display expects.

Everything lives on a PCB that fits right on the back of the instrument. Sadly, the connector needed to mate up to the one on the instrument was outlandishly expensive — again, avionics — so [porkfreezer] had to solder the board directly to the DME’s pins. Otherwise, this would have been a completely reversible hack.

Still, it’s an interesting reuse of an unusual piece of gear, and one that respects the original design as much as possible. That counts as a win in our book.

Guitar Distortion With Diodes In Code, Not Hardware

August 23, 2023 by Dan Maloney 14 Comments

Guitarists will do just about anything to get just the right sound out of their setup, including purposely introducing all manner of distortion into the signal. It seems counter-intuitive, but it works, at least when it’s done right. But what exactly is going on with the signal? And is there a way to simulate it? Of course there is, and all it takes is a little math and some Arduino code.

Now, there are a lot of different techniques for modifying the signal from an electric guitar, but perhaps the simplest is the humble diode clipping circuit. It just uses an op-amp with antiparallel diodes either in series in the feedback loop or shunting the output to ground. The diodes clip the tops and bottoms off of the sine waves, turning them into something closer to a square wave, adding those extra harmonics that really fatten the sound. It’s a simple hack that’s easy to implement in hardware, enough so that distortion pedals galore are commercially available.

In the video below, [Sebastian] explains that this distortion is also pretty easy to reproduce algorithmically. He breaks down the math behind this, which is actually pretty approachable — a step function with a linear part, a quadratic section, and a hard-clipping function. He also derives a second, natural exponent step function from the Schockley diode equation that is less computationally demanding. To implement these models, [Sebastian] chose an Arduino GIGA R1 WiFi, using an ADC to digitize the guitar signal and devoting a DAC to each of the two algorithms. Each distortion effect has its own charms; we prefer the harsher step function over the exponential algorithm, but different strokes.

Kudos to [Sebastian] for this easy-to-understand treatment of what could otherwise be a difficult subject to digest. We didn’t really expect that a guitar distortion pedal would lead down the rabbit hole to diode theory and digital signal processing, but we’re glad it did.

Continue reading “Guitar Distortion With Diodes In Code, Not Hardware” →

Bust Out That Old Analog Scope For Some Velociraster Fun!

April 17, 2023 by Dave Rowntree 6 Comments

[Oli Wright] is back again with another installation of CRT shenanigans. This time, the target is the humble analog oscilloscope, specifically a Farnell DTV12-14 12 MHz dual-channel unit, which features a handy X-Y mode. The result is the Velociraster, a simple (in hardware terms) Raspberry Pi Pico based display driver.

Using a Pico to drive a pair of AD767 12-bit DACs, the outputs of which drive the two ‘scope input channels directly, this breadboard and pile-of-wires hack can produce some seriously impressive results. On the software side of things, the design is a now a familiar show, with core0 running the application’s high-level processing, and core1 acting in parallel as the rendering engine, determining static DAC codes to be pushed out to the DACs using the DMA and the PIO.

Continue reading “Bust Out That Old Analog Scope For Some Velociraster Fun!” →

Audio Playback Toy For DSP Adventures

February 2, 2023 by Bryan Cockfield 1 Comment

The declining costs of single-board computers has made serious computing power available for even the most trivial of tasks. It’s easy enough to slap a Raspberry Pi onto almost anything for nearly the same cost as a powerful 32-bit microcontroller platform, but this takes some of the fun out of projects for a few of us. Looking to get into the weeds can be a challenge as well, as [Michal Zalewski] demonstrates in this audio playback device he built from a simple 8-bit microcontroller.

The small toy takes audio input from a microphone through an op-amp and feeds this signal to an ADC within the AVR128DA28 microcontroller. The data is then stored on a separate memory chip ready to be played back through another op-amp paired with a speaker. This is where being familiar with the inner workings of the microcontroller comes in handy. By manipulating the interrupt routines in specific ways, the audio stored in memory can be played back at various speeds.

[Michal] intended this build to be a toy for one of his younger relatives, and for the price of a few ICs and buttons it does a pretty good job of turning a regular voice into a chipmunk voice like some commercial children’s toys some of us might remember. If the design aesthetics of this gadget look familiar, you may be thinking of his minimalist gaming device which we recently featured.