TriXarp: Triple Xarp-61

The Xarp-61 was originally designed for the Spartan-3E 500K gate device. The triple version is implemented on a Spartan-3A DSP 1800 FPGA which is about 4 times larger.

Each Xarp-61 unit is individually tunable with a patch editor. This allows for slight detuning of each of the Xarps that manifests a nice piano like beating.

The synth is stereo - panned by string number calculated from the note number - and has a stereo delay effect (up to about 180 mSec).

Sample rate is 100.160 kHz.

Currently, the patch editor calculates the tuning ROM values using the standard

frequency = a * k * 2^(n/12)

where a is nominally 1.0 for concert tuning and can be changed to flatten or sharpen the tuning.

However, any sort of tuning can be accomplished at the patch editor source code level, stretched, microtonal, etc.

The design contains three Xarp-61 units with stereo outputs panned by note number. The outputs will than pass through a delay effect of up to approximately 180 mSec per channel.

The patch editor controls tuning by calculating the string lengths and then sending them to the synth. There are 3 tuning factors (representing a in formula above), one for each harp, their settings tune the selected harp. The user can use this to detune one or more harps from the others for a beating effect. The reflection filter has per string control for Fc which affects timbre and decay rate. Each string has a linear attenuation parameter. Excitation pulse width is set as a proportion of string length (currently, just 1/2, 1/4, 1/8, 1/16). With these controls, the user has good control over the timbre.