A Sound Of Thunder MK II

HM1973Amk2: Circuit bent expander panel for the Tyme Sefari MKII

The Sound of Thunder is the official expander for the Tyme Sefari Mark II. It features a bank of switch and gate controls to enable additional operating modes, as well as additional I/O for stereo operation.

Four new modes are available on this expander, represented by eight toggle switches and gate inputs. To enable each, flip the associated switches up, or insert a 5V gate signal into the input. The new addition of the gate jacks allow full remote control of the Tyme Sefari's recording mode, allowing unique rhythmic effects and controllable corruption of the sampled audio signal.

Audio Format Control

The first two switches control the format of audio that the Tyme Sefari records to its internal memory. Four choices are available, represented by a combination of the two switch settings and explained by a graphical key on the front panel. The four choices are 16-bit linear, 12-bit linear, 12-bit logarithmic, and 8 bit linear. The 16-bit mode is the default under which an unexpanded Tyme Sefari operates. The 12-bit linear is of slightly lower quality. The next two modes double the available recording time due to their decreased consumption of memory. The 12-bit logarithmic mode compresses the signal into a single byte, using a scheme originally designed for 1970s-era digital telephony. The 8-bit mode is the fuzziest of the bunch and the most straightforward, mimicking the behavior of the classic "mark I" Tyme Sefari.

While the subtle differences between each recording format may seem uninteresting, they are made musically useful by making the Tyme Sefari unit ignorant of the format of the data stored in its memory. It will decode audio according to the format switch settings, leading to interesting and often extreme distortions when the playback format does not match the original recording. Since the transport speed also changes with some of the format settings, one may achieve speed doubling or halving as well.

Pitch Shift Mode

This switch enables a crude pitch shift effect. It directs the playback head to run at a different speed than the record head, controllable by the large frequency knob and CV input. This will occasionally cause glitch effects as new data being recorded overruns the playback of old audio. By turning up the feedback, one may achieve bizarre, frequency-staircase "crystallizer" effects, causing the pitch of the delayed audio signal to endlessly travel in one direction. The pitch shift control changes the playback speed of the audio to one octave above or below that of the original recording, with the distortions present in the sampling rate mismatch contributing to the unique character of this effect.

Stereo Mode

Flipping this switch will enable the audio input, output, and attenuator at the bottom of the Sound of Thunder panel. Tyme Sefari will now record and play back in stereo. Since this mode consumes twice as much memory and runs the transport twice as fast, it can also be used to temporarily speed up the sound of a mono recording. In stereo mode, the feedback loop of the Tyme Sefari acts as a summed mono signal.

Circuit Bending

This group of four switches will skew the internal data and address busses of the Tyme Sefari in unpredictable ways. Dependent on the sampling rate, input audio characteristics, and loop settings, the module will crackle, fizz, escape into out-of-bounds memory locations, and irreversibly foul the audio playback. Periodically triggering these four switches with external gates, with live recording and feedback is a sure way to access a world of sounds previously available only to those brave enough to modify old digital keyboards with experimental circuit bends.

Causes Tyme Sefari to draw an additional 5ma of current.

SOUND OF THUNDER EXPANDER

CONNECTION
Remove your Tyme Sefari from the rack and view the two 10-pin headers named "THUNDER" and "THUNDER2". Connect the two ribbon cables from the SOUND OF THUNDER to these headers, making sure the red bands point down. Install both module and expander into the rack.

OPERATION

The SOUND OF THUNDER adds several optional modes to the Tyme Sefari, activated through 8 sets of toggle switches and gate inputs. Like the Tyme Sefari, the parameter is active if the switch OR the gate input is high. The gate inputs are provided to automate control of this module's deep functions, and to present further sound design possibilities. For example, the stereo and format controls change the way that audio data is encoded AND decoded from memory, but the data in memory is not tagged with its proper format description. If you record in one format, but then play back in another, new, extreme distortion effects may be heard depending on the combination of modes used for recording and playback.

FORMAT SWITCHES
The first two switches change the audio recording and playback format. The default setting is 16-bit linear. The other formats are as follows:
- 12-bit linear. This is stored into memory with the bytes swapped, and restored to normal when played back.
- 12-bit logarithmic (µlaw) - This setting doubles the amount of memory available for recording, and slightly increases the maximum sampling rate.
- 8-bit linear - This setting doubles the amount of memory available for recording, and slightly increases the maximum sampling rate.

PITCH SHIFT MODE
This mode changes the behavior of the SAMPLING RATE control. When active, the rate of recording is fixed. The playback rate is variable, with the center position of the large knob meaning that there is no pitch shift. Turning the knob counter-clockwise results in a downward pitch shift, and turning it clockwise results in an upward pitch shift. Try this in "delay" mode with lots of feedback for some interesting harmonic staircase effects.

STEREO MODE
This mode enables the right channel input and output seen on the lower half of the SOUND OF THUNDER expander. When this switch is active, the right channel data will be interleaved in memory with the left channel. The feedback is a monaural sum of the two output signals, applied to both inputs. When engaged, this mode will cut the available recording time in half, and also reduce the maximum sampling rate.

BEND SWITCHES
This group of four switches will interfere with the address and data busses of the TYME SEFARI MARK II. Effects similar to a "circuit bent" toy keyboard will be heard as these switches perform identical functions.

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