Re-done right

The Shruti-1 created much demand, and we were delighted to see your interest in small, open-hardware, hybrid, quirky synths. There was one problem though: it was never meant to get bigger, because of its dependance on a rare vintage chip. As soon as a limited number of Shruti-1 kits was sold, we started a new mission: design a synth with the same philosophy… minus the hard to source parts!

Streamlined hardware design…

The Shruthi-1 hardware design bares little resemblance to its predecessor. The main MCU has been upgraded to an ATMega644p with better I/O capabilities, allowing us to get rid of most of the components on the control panel board and make room for the LCD display. All the digital section is on a single board, with no pesky cables. Parts have been carefully chosen, to reduce the number of suppliers and allow us to be more productive when preparing kits. Finally, the CEM3379 has been replaced by a classic OTA-based VCF/VCA design which employs only modern, cheap and widely available components.

... but faithful to the Shruti-1 tradition!

A large part of the Shruthi-1 firmware code is shared with its predecessor. The menu and navigation system is largely unchanged, as well as the overall architecture and waveforms/oscillators. More than that, the Shruthi-1 sticks with the core values that made the success of the Shruti-1: DIY friendly, open-source, inexpensive, musical, without any trace of analog or 8bits fundamentalism.

New features

More advanced synthesis features

The processor upgrade gave us some room to add new features. A bunch of classic wavetables from the PPG or Ensoniq heritage are there, ready to be scanned and morphed. New phase distortion and “self-sync” oscillator modes have been added. More importantly, all the oscillator algorithms are available for both oscillator 1 and 2! The LFOs come with new options (attack time and slave/master synchronization with the envelopes). New entries make their apparition in the modulation matrix: a noise generator sampled at each key press, or the two independent halves of the step sequence — for even more intricate patterns.

Sequencer and new play modes

The greatest addition to the Shruthi-1 is not in the synthesis features, though: it is the integrated sequencer. The tracker-like view makes programming patterns easy… But the real fun is in using the new play modes: use your pattern as a rhythmic template for the arpeggiator, trigger and transpose it from the keyboard, or let it play and override steps by playing extra notes on the keyboard.

Is that polyphony?

It looks like a relatively small addition to the firmware code, but the consequences are massive… The Shruthi-1’s polychaining mode allows several units to be chained together by MIDI, and to respond to notes like a polyphonic synth would do – each unit responsible for a voice. Parameter changes on the master units are forwarded downstream. Or maybe you’ll prefer stack a pair of units in unison mode?

A modular system

A tale of two boards

The main goal of the redesign was to set a different border between the two boards: the Shruthi-1 has a “control” board with the display, controls, MIDI connectors, and MCU ; and an “audio” board with the analog filter, audio outputs, and power supply. It is very easy to exchange the audio board for a different one. For now, only two audio boards are available, but we plan to release more, and we publish Eagle schematics/boards templates to let you create your own boards compatible with the system. Ever dreamt of playing the Shruthi-1 with a TB-303 filter and a tube distortion?

Available audio boards

• The SMR-4 filter board uses an analog OTA-based 4-pole VCF/VCA (a jumper on the board can be used to configure it to a 2-pole configuration). Its sound is rounder, more liquid, less aggressive than the CEM3379.
• The CEM3379 filter board contains the same signal processing and amp section as the Shruti-1. If you have a CEM3379 around or want to upgrade from your Shruti-1, you’ll love it!

(to be continued)