The FV-1 Crystal Oscillator
The Fv-1 has internal circuitry to support an external crystal, preferably a 32768Hz watch crystal. These crystals are extremely inexpensive and widely available. Frequencies other than 32768Hz are available, but not as cheap and not as plentiful. If the system the FV-1 is integrated into has local clocks available, a logic level clock signal can be directed to pin X1 of the FV-1 through a small resistor, on the order of 10K ohms.
Crystal oscillators for these low cost crystals are very different from oscillators designed for megahertz range crystals and resonators. The watch crystal oscillator has very high internal resistances, on the order of 10M ohms. When attempting to observe the crystal signals, the loading of a 10X scope probe can load the crystal terminals to the extent that the oscillator may stop working altogether. Further, leakage across the PCB could cause problems, so be sure to have the product board well cleaned after assembly, if this is not a normal part of your PCB processing. Leakage should only become a problem in the case of very poorly assembled product.
NOTE: Some small 32KHz crystals have been found to operate in 3rd overtone mode during startup. It is suggested that the EC-38 style crystal should work well with the FV-1, but smaller crystals have shown problems. In any case, the addition of a 15pF capacitor from the X2 pin to ground will allow all crystal types to function properly.
The 32768 clock frequency causes the overall response of the FV-1 to be limited to 15KHz. This may seem insufficient for a quality audio product, but in the case of reverb and most effects, response beyond this range is unnecessary. My own hearing is limited to about 13KHz, and has been since I was in my 20s; neither of my children have been able to hear beyond 16KHz since I began testing them at an early age. Most adolescents and adults can only hear signals above 15KHz as faint 'impressions' as opposed to actual sounds.
Our idea of the human hearing response being 20Hz to 20KHz is out of convenience. That '20 to 20' range is easy to remember and speak, but is not true. There really is no lower limit of hearing if one considers earthquakes within our 'hearing' range, as they are most obviously sensed; perhaps the lower limit of our actual hearing is perhaps 50Hz, where most of us are actually hearing the gross distortion products of headphones and low end drivers at these frequencies. If you listen to a really clean subwoofer, you're only feeling the result, and probably will miss the bass harmonics, as we are used to bass having fairly severe distortion! Perhaps we should say our hearing range is 50 to 15K, but then that would be heretical in the Church of Modern Audio, no? So, if your product is targeted toward those that are interested in excellent sound, a watch crystal should suffice. If your product is targeted toward customers that look at specs and only then decide how they like the sound, chose a 48KHz clock frequency.
This circuit can run an inexpensive 6MHz resonator and divide it to 46.875KHz, which will easily allow 20KHz FV-1 signal path operation. Be advised: Precision may be important to you, but when it comes to effect design, there is nothing precise about it. SOUND is more important than any objective measurement you can make!
As an added dimension to effects creation, the FV-1 can be clocked by a simple oscillator, made available as a variable to the user. When allowing an adjustable clock, reverb size can be adjusted continuously, and the unique sound of the sharp filters within the ADC and DAC portions of the FV-1 can be exploited. Here's an example of a simple oscillator for this purpose:
You may wish to have the oscillator frequency 'smoothed' so that slight discontinuities in the potentiometer element do not lead to abrupt changes in clock frequency. This circuit may be useful:
The output goes directly to the X1 pin, and should allow a 25KHz to 50KHz range. The 1uFd cap will smooth out the control from the variable resistor. Component changes can easily tailor the oscillator's range.