Minimum FFT sample time - guitar frequencies and harmonics

Question

I am working on a project whereby I am trying to determine the exact note played on an electric guitar with a microcontroller (as quickly as possible after the string is plucked) and to use this information (ie digital value of note) to manipulate an audio output via DAC.

I am using an STM32F4, sampling the DC offset and active fitered guitar signal with a 12-bit ADC (sample rates pretty much as fast as I need with timer triggered ADC DMA) and the highest number of F32 FFT points I can achieve with the RAM available is 4096 (ie 8192 samples).

The guitar frequencies I need to measure are between 82Hz and about 831Hz, with a resolution at the low end of 2Hz and about 22Hz at the top.

• Am I correct in saying minimum sample time to achieve this is about 0.5s, driven by the minimum frequency and resolution requirement?
• And at higher frequencies I can sample for shorter (with a higher sample rate)?
• Or is there (hopefully) something I'm missing?
• Can I measure the lower frequencies with fewer FFT points (and thus samples), saving time?

Answer 1

Your highest frequency is 831 Hz; since undersampling to ignore the sub-82 Hz would only save you 10% sampling rate, let's ignore that possibility.

So, Nyquist says your sampling rate should be at least 1.662 kHz; now, your problem is, as @fibonatic mentioned, that a the freq. resolution of any DFT is alwas

$$\Delta f = \frac{f_\text{Nyquist}}{N_\text{DFT}}\\ \implies\\ N_\text{DFT} = \frac{f_\text{Nyquist}}{\Delta f}$$

and the time taken by capturing of samples you'll need for that DFT is

$$T = \frac{N_\text{DFT}}{f_\text{sample}} = \frac{N_\text{DFT}}{2f_\text{Nyquist}} = \frac{\frac{f_\text{Nyquist}}{\Delta f}}{2f_\text{Nyquist}}= \frac{1}{2\Delta f}$$

So no matter at which rate you'd sample, there's nothing oyu can do about the time period needed for a full-resolution DFT.

You could just pad a shorter observation with zeros, and that will increase your resolution. Point is that you trade resolution for quality, and the variance is going to grow significantly.

The DFT is simply not the right choice here; if you need 2Hz resolution at the lower frequency, you get the same at the higher frequencies without any need.

Honestly, think about it: you're planning to use a DFT that gives you 4096 frequency bins, where you'd need to distinguish maybe 100 tones, if that many.

I'm pretty certain that the cheap guitar tuners you get don't work that way. I'd assume they simply employ dedicated hardware (not firmware like you'd need to write) to implement a simple filterbank – possibly even a IIR filterbank, and if I had a guess, at very low sample bit width, with simple biquads. Or, even simpler, they just have couple of analog filter bandpasses, and just undersample the resulting sample streams and do the same digital filters on all the octaves they preselect that way.

If I had to do this, I'd atually do the same on your microcontroller (by the way, 8192-FFT sounds gigantic for a Cortex-M4, but you know your device better than I do), but with different architecture. Do a rough calculation: If you keep the sampling rate low, how many FIR filters of the different lengths you'll need can you let run over the data? Does implementing your filterbank as a PFB filterbank help? Does making a hierarchic decision (e.g. just first an "octave selector", then depending on the result of that, throwing away half, $\frac38$,$\frac78$,… of the samples and then doing a tone-detection filterbank.

Answer 2

Use edge triggered interrupts and the processor clock counter to time-stamp zero crossings. Maybe take ADC samples between zero-crossings and look for an autocorrelation maxima or AMDF lag minima using only zero-crossing lag candidates (the time between every Nth zero-crossing for some reasonable number of N). This might give you a reasonable pitch candidate in as few as 2 or 3 pitch periods, far quicker than a half second lag or more from waiting for enough samples for a long FFT window.