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1

preenfm2 and preenfm3 / Subtle distortion on one channel

« on: November 15, 2017, 01:50:12 PM »

My PreenFM2 seems to have a subtle distortion on one side of the stereo image. It doesn't sound like clipping, but rather that kind of digital distortion you get when the steps in the DAC aren't quite the same size. I've ignored it for a long time as it's very subtle and I rarely use it in stereo but I'd like to diagnose it now if I can.

Can anyone suggest any steps? Swapping the DAC ICs would be an obvious one... see if the distortion follows the DAC or not.

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preenfm2 and preenfm3 / Thoughts about operator tuning

« on: March 22, 2016, 05:15:07 PM »

I noticed that some FM synths allow detuning in cents and Hz. I originally assumed that it was just to allow finer degrees of detuning, but now I understand more about synthesis and sound in general I realise the purpose

 Hz is of course a linear measurement whereas cents are a (very small) musical interval. So a detune in Hz is actually a different interval depending on what note you're playing. "How could that be useful!?" I hear you think. Well anyone who is familiar with detune by cents on a digital (therefore stable) synth may have noticed that the beat frequency is proportional to the note you're playing. A note an octave higher will beat twice as fast.

The beat frequency is the difference in Hz between the two oscillators, so the beating is changing frequency because when you play an octave higher you've doubled the frequency of both oscillators, so the difference between their frequencies has also doubled - doubling the rate of beating.

If you detune by a fixed number in Hz then you get the same beating rate on every note. This is particularly useful in FM where detune between modulators and carriers causes timbral changes and note just a 'chorusing' sound.

This isn't a feature request, just some thoughts.

I would propose it as a third "FTyp" parameter, maybe called KeHz or something. Freq would still be a ratio setting on this mode, but Ftun would be in Hz instead of cents. This would allow for sounds that have a cycling quality that is at a consistent rate across the keyboard. I think 2 decimal places (allowing for 0.01) is good as it allows incredibly slow phasing.

3

preenfm2 and preenfm3 / Newbie questions

« on: July 15, 2015, 05:06:01 PM »

Hi,

Not been using my PreenFM2 (or indeed FM/PM in general) for long and I'm still marvelling at the possibilities. There are a few things I'm wondering about though:

The LFO waveform 'saw' sounds a lot like a triangle and there's a noticeable absence of a triangle waveform in the list for the LFO. It seems like maybe it's just a triangle wave misnamed? It wouldn't make sense to have actual separate ramp and saw waveforms when we have positive or negative amounts in the matrix.

On the topic of operator feedback, I understand from other posts that feedback is very difficult to implement with genuine FM like the PreenFM2 uses as opposed to Yamaha's PM version. Assuming the sine waves use a phase accumulator and a look up table, would it not be possible to use PM to calculate the feedback even if the rest of the engine is FM? Forgive me if that's a very stupid question, I'm not a real programmer.

Also, is the S^2 waveform literally just an octave up sine wave? If so, I'm not sure what the point is (other than effectively increasing the range of the ratio to 32). Is there any other special use I'm missing? If not, I'm wondering if I might make a custom firmware build to replace it with a band limited triangle wave? The triangle with its quickly diminishing harmonics seems like an ideal compromise between the pure sine and the rich saw and square.

One last topic - how many harmonics are the saw and square waves bandlimited to? When played further down, the hard bandlimiting almost sounds like they're being tracked by a resonant filter. I assume it's because the ear finds the top harmonic conspicuous because there's nothing above it. I was experimenting with additively generating a saw with just the first 32 harmonics last night and found that it was more pleasing and less resonant sounding if the last few harmonics rolled off in volume so that the top one didn't stick out. If it's possible to change the S^2 wave to a triangle, I might change the saw and square in this way too.