Real time pitch detection

Question

I'm trying to do real time pitch detection of a users singing, but I'm running into alot of problems. I've tried lots of methods, including FFT (FFT Problem (Returns random results)) and autocorrelation (Autocorrelation pitch detection returns random results with mic input), but I can't seem to get any methods to give a good result. Can anyone suggest a method for real-time pitch tracking or how to improve on a method I already have? I can't seem to find any good C / C++ methods for real time pitch detection.

Thanks,

Niall.

Edit: Just to note, i've checked that the mic input data is correct, and that when using a sine wave the results are more or less the correct pitch.

Edit: Sorry this is late, but at the moment, im visualizing the autocolleration by taking the values out of the results array, and each index, and plotting the index on the X axis and the value on the Y axis (both are divided by 100000 or something, and im using OpenGL), plugging the data into a VST host and using VST plugins isn't an option to me. At the moment, it just looks like some random dots. Am i doing it correctly, or can you please point me torwards some code for doing it or help me understand how to visualize the raw audio data and autocorrelation data.

Solution 1

Taking a step back... To get this working you MUST figure out a way to plot intermediate steps of this process. What you're trying to do is not particularly hard, but it is error prone and fiddly. Clipping, windowing, bad wiring, aliasing, DC offsets, reading the wrong channels, the weird FFT frequency axis, impedance mismatches, frame size errors... who knows. But if you can plot the raw data, and then plot the FFT, all will become clear.

Solution 2

I found several open source implementations of real-time pitch tracking

• dywapitchtrack uses a wavelet-based algorithm

• "Realtime C# Pitch Tracker" uses a modified autocorrelation approach now removed from Codeplex - try searching on GitHub

• aubio (mentioned by piem; several algorithms are available)

There are also some pitch trackers out there which might not be designed for real-time, but may be usable that way for all I know, and could also be useful as a reference to compare your real-time tracker to:

• Praat is an open source package sometimes used for pitch extraction by linguists and you can find the algorithm documented at http://www.fon.hum.uva.nl/paul/praat.html

• Snack and WaveSurfer also contain a pitch extractor

Solution 3

I know this answer isn't going to make everyone happy but here goes.

This stuff is hard, very hard. Firstly go read as many tutorials as you can find on FFT, Autocorrelation, Wavelets. Although I'm still struggling with DSP I did get some insights from the following.

https://www.coursera.org/course/audio the course isn't running at the moment but the videos are still available.

http://miracle.otago.ac.nz/tartini/papers/Philip_McLeod_PhD.pdf thesis about the development of a pitch recognition algorithm.

http://dsp.stackexchange.com a whole site dedicated to digital signal processing.

If like me you didn't do enough maths to completely follow the tutorials don't give up as some of the diagrams and examples still helped me to understand what was going on.

Next is test data and testing. Write yourself a library that generates test files to use in checking your algorithm/s.

1) A super simple pure sine wave generator. So say you are looking at writing YAT(Yet Another Tuner) then use your sine generator to create a series of files around 440Hz say from 420-460Hz in varying increments and see how sensitive and accurate your code is. Can it resolve to within 5Hz, 1Hz, finer still?

2) Then upgrade your sine wave generator so that it adds a series of weaker harmonics to the signal.

3) Next are real world variations on harmonics. So whilst for most stringed instruments you'll see a series of harmonics as simple multiples of the fundamental frequency F0, for instruments like clarinets and flutes because of the way the air behaves in the chamber the even harmonics will be missing or very weak. And for some instruments F0 is missing but can be determined from the distribution of the other harmonics. F0 being what the human ear perceives as pitch.

4) Throw in some deliberate distortion by shifting the harmonic peak frequencies up and down in an irregular manner

The point being that if you are creating files with known results then its easier to verify that what you are building actually works, bugs aside of course.

There are also a number of "libraries" out there containing sound samples. https://freesound.org from the Coursera series mentioned above. http://theremin.music.uiowa.edu/MIS.html

Next be aware that your microphone is not perfect and unless you have spent thousands of dollars on it will have a fairly variable frequency response range. In particular if you are working with low notes then cheaper microphones, read the inbuilt ones in your PC or Phone, have significant rolloff starting at around 80-100Hz. For reasonably good external ones you might get down to 30-40Hz. Go find the data on your microphone.

You can also check what happens by playing the tone through speakers and then recording with you favourite microphone. But of course now we are talking about 2 sets of frequency response curves.

When it comes to performance there are a number of freely available libraries out there although do be aware of the various licensing models.

Above all don't give up after your first couple of tries. Best of luck.

Solution 4

Here's the C++ source code for an unusual two-stage algorithm that I devised which can do Realtime Pitch Detection on polyphonic MP3 files while being played on Windows. This free application (PitchScope Player, available on web) is frequently used to detect the notes of a guitar or saxophone solo upon a MP3 recording. The algorithm is designed to detect the most dominant pitch (a musical note) at any given moment in time within a MP3 music file. Note onsets are accurately inferred by a significant change in the most dominant pitch (a musical note) at any given moment during the MP3 recording.

When a single key is pressed upon a piano, what we hear is not just one frequency of sound vibration, but a composite of multiple sound vibrations occurring at different mathematically related frequencies. The elements of this composite of vibrations at differing frequencies are referred to as harmonics or partials. For instance, if we press the Middle C key on the piano, the individual frequencies of the composite's harmonics will start at 261.6 Hz as the fundamental frequency, 523 Hz would be the 2nd Harmonic, 785 Hz would be the 3rd Harmonic, 1046 Hz would be the 4th Harmonic, etc. The later harmonics are integer multiples of the fundamental frequency, 261.6 Hz ( ex: 2 x 261.6 = 523, 3 x 261.6 = 785, 4 x 261.6 = 1046 ). Linked at the bottom, is a snapshot of the actual harmonics which occur during a polyphonic MP3 recording of a guitar solo.

Instead of a FFT, I use a modified DFT transform, with logarithmic frequency spacing, to first detect these possible harmonics by looking for frequencies with peak levels (see diagram below). Because of the way that I gather data for my modified Log DFT, I do NOT have to apply a Windowing Function to the signal, nor do add and overlap. And I have created the DFT so its frequency channels are logarithmically located in order to directly align with the frequencies where harmonics are created by the notes on a guitar, saxophone, etc.

Now being retired, I have decided to release the source code for my pitch detection engine within a free demonstration app called PitchScope Player. PitchScope Player is available on the web, and you could download the executable for Windows to see my algorithm at work on a mp3 file of your choosing. The below link to GitHub.com will lead you to my full source code where you can view how I detect the harmonics with a custom Logarithmic DFT transform, and then look for partials (harmonics) whose frequencies satisfy the correct integer relationship which defines a 'pitch'.

My Pitch Detection Algorithm is actually a two-stage process: a) First the ScalePitch is detected ('ScalePitch' has 12 possible pitch values: {E, F, F#, G, G#, A, A#, B, C, C#, D, D#} ) b) and after ScalePitch is determined, then the Octave is calculated by examining all the harmonics for the 4 possible Octave-Candidate notes. The algorithm is designed to detect the most dominant pitch (a musical note) at any given moment in time within a polyphonic MP3 file. That usually corresponds to the notes of an instrumental solo. Those interested in the C++ source code for my Two-Stage Pitch Detection algorithm might want to start at the Estimate_ScalePitch() function within the SPitchCalc.cpp file at GitHub.com. https://github.com/CreativeDetectors/PitchScope_Player

Below is the image of a Logarithmic DFT (created by my C++ software) for 3 seconds of a guitar solo on a polyphonic mp3 recording. It shows how the harmonics appear for individual notes on a guitar, while playing a solo. For each note on this Logarithmic DFT we can see its multiple harmonics extending vertically, because each harmonic will have the same time-width. After the Octave of the note is determined, then we know the frequency of the Fundamental.

Solution 5

I had a similar problem with microphone input on a project I did a few years back - turned out to be due to a DC offset.

Make sure you remove any bias before attempting FFT or whatever other method you are using.

It is also possible that you are running into headroom or clipping problems.

Graphs are the best way to diagnose most problems with audio.

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