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Open Music Labs Mixtape Alpha Synthesizer

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Description
Mixtape Alpha is the smallest synthesizer we could make without a prescription. It has a stylophone style input for continuous note generation, and 6 buttons for discrete notes. With 4 voices, 4 effects, and 5 note polyphony there is quite a range of expression. But, the best part is, you can record the songs you make, and trade mixtapes with your friends! Perhaps even better, it’s based on the ATmega328p, and can be hacked to make even crazier sounds than we came up with.

Mixtape Alpha was done in collaboration with Jie Qi from the High-Low Tech group at the MIT Media Lab. It’s an attempt to break down the barriers between people and electronics: To get them comfortable touching PCBs, and change expectations about how electronics should look. To this end, there are resistive touch pads, and all the trace routing took aesthetics into consideration as well. These are ideas very familiar to circiut benders, and we’re hoping they catch on with other folks as well.

Mixtape Alpha is the first in a series of super-scaled-down synthesizers and sound generators. We started with a raw chunk of electronics, and removed anything that wasn't a synth. What we were left with was a microcontroller, battery, and some LEDs.

Mixtape Alpha is an attempt to break down the barriers between people and electronics: To get them comfortable touching PCBs, and change expectations about how electronics should look. To this end, there are resistive touch pads, and all the trace routing took aesthetics into consideration as well. This concept was the brainchild of Jie Qi from the Hi-Low Tech group at the MIT Media Lab.
Limitations of the design

The mixtape concept relies upon low-cost, low-power electronics. It was essential to the design that it could run off a coin cell battery. To this end, a number of tradeoffs were made, some of which led to reduced performance. Some of these can be undone by the determined hacker.

Headphone Output - The headphone output puts the left and right speakers in series, which produces a louder sound at a greatly reduced power consumption. The effect, though, is to make the left and right channels inverted from one another. So, if you plug your Mixtape Alpha into a mono system, you won't hear any sound. Also, this means the ground ring of the 1/8" jack is not actually circuit ground. To avoid any problems with this, use an 1/8" mono plug when possible.

Pattern Storage - Writing to internal FLASH or EEPROM consumes 4mA and takes forever, whereas writing to internal SRAM consumes no extra power and is very fast. Unfortunately, this means that there is much less pattern storage space. As it is currently implemented, 64 step patterns can be recorded. Longer patterns can be recorded, but these will become corrupt if the "Echo" (Effect "B") is enabled, as the pattern memory shares the same space as the echo memory. We highly reccomend writing patterns longer than 64 steps and turning on "Echo", it makes some crazy sounds.

Wavetable Length - The longer a wavetable, the more precise of a tone a fixed integer step size can give. But, this also reduces the total number of wavetables that can be stored, and the highest frequency possible. We used 2048 sample wavetables, as there is plenty of storage space, and not enough processing time for interpolation.

Processor Speed - Perhaps the biggest limitation of them all. We decided to use the internal 8MHz processor because it was cheaper than adding a crystal, and it consumed a lot less power. But, this limits the total processing time between samples, and forced a number of other decisions, like the next one on the list.

Assembly Code - Sorry about this one, but it had to be done. At 8MHz, there just isn't time to mess around with C. The code is well commented, and a lot of the interesting variables are easy to change, and clustered together (e.g. attack, decay, note tables, wave tables, effects).

Resistive Sensing - The resistive sensing works pretty well when the device is connected to headphones. But, if it gets connected to earth ground in any way, mains hum makes the touch pads flakey. At 60Hz there is about a 1s beat frequency that can be interesting, but is mostly annoying. We looked into capacitive sensing, but it required more processing time, which we don't really have. To make the resistive sensing work better: 1. Don't touch any of the exposed metal bits on the back while holding the Mixtape, and 2. Press harder if it doesn't seem to be detecting your presses.
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Technical Specifications
Type: Digital
Synthesis: Additive, Frequency Modulation, Oscillator(s), Virtual Analog, Wave Table
Oscillators
Oscillators: *
Osc Modulation: Input, Ribbon
Oscillator Notes:
+ Loads PD patches.
Envelopes
Envelopes: *
Envelope Notes::
+ Pure Data user defined.
Filters
Filters: *
Filter Notes:
+ Pure Data user defined.
LFO
LFO: *
LFO Notes::
+ Pure Data user defined.
Polyphony & Tuning
Polyphony: 5
Timbrality: 1
Tuning: Atonal, Standard
Modes: Polyphonic, Split
Patches
Patches RAM: 4
Storage: Internal, USB
Editing: USB
Effects
+ 4 effects
Sequencer
+ 64 step patterns.
+ Looping recorder.
+ Endless overdubs.
Chips and Operating System
Based on ATmega328p processor
Case
Case: Keyboard
Case Details: 6 key tone wheel
Keyboard: Touch Plates, Metal
Controls: Buttons, Mod - Ribbon
Display Type: LED
Connections
Audio Output Connections: 1/8" Phone Jack, Stereo Main
Audio Output Count: 2
DAC Bits: 8
Power: CR2025 clip battery
Production
Year Released: 2012
Units Made: 500+
Design Notes:

Designed by OpenMusicLabs in collaboration with Jie Qi from the High-Low Tech group at the MIT Media Lab.
Manuals & Documents

Product Links
Company Product Sites:
[+] www.openmusiclabs.com
Repairs and Support
[+] wiki.openmusiclabs.com
Pricing
MSRP List Price: $42 - convert
Retail Street Price: $55 - convert
Used Price: $100 - $200 - convert
Shopping
References & Sources

  Report Synthesizer
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