Pitch

This video just has a plain transcript, not time-aligned to the videoPeriodic signals have a very important perceptual property of pitch.
That means that periodic signals are perceived as having a musical note: a tone.
Here are some signals that are periodic.
They all have a repeating pattern.
And so we predict, just by looking at them, that when we listen to them, there will be a pitch to perceive.
There's the simplified glottal waveform we've seen before.
Well, not very pleasant, but it certainly has a pitch.
Here's a sine wave: that's a very pure, simple sound, again, with a very clearly perceived pitch.
Finally a short clip of a spoken vowel.
I'll play that again.
Again, a clear pitch can be perceived.
Pitch is a perceptual phenomenon.
We need to establish the relationship between the periodicity, a physical signal property of F0 (fundamental frequencyP and this perceptual property of pitch.
Let's do that by listening to some sine wave, some pure tones.
I'll play one at 220 Hz and then I'll play one at 440 Hz.
Hopefully, you have a musical enough ear to here that's an octave.
There's a clear musical relationship between the two.
The second one is perceived as having twice the pitch of the first.
So let's go up another 220 and see what happens.
No, that's definitely not in octave!
You don't need to be a musician to know that.
So let's go up again.
That sounds like it might be an octave above 440.
So let's listen to octaves.
We've discovered something really important: that the relationship between the physical signal property F0 and the perceptual property pitch is not linear.
To perceive the same interval change in pitch - an octave - we don't need to add a fixed amount to the frequency: we need to double the frequency.
So this relationship between F0 on pitch is actually logarithmic.
It's non-linear.
That non-linearity is one aspect of a much more general property of our auditory system as a whole.
It is, in general, non-linear.
We can probably make use of that knowledge later on.
So for speech, where the pitch is varying in interesting ways because it might be carrying part of the message, we would need to measure the local value of F0 and then plot how that changes against time.
Now, because there's a very simple relationship between F0 and pitch, you'll find the two terms actually used interchangeably in our field.
But that's not technically correct!
They are not the same thing.
F0 is a physical property: it's the rate of vibration of the vocal folds.
We could measure that if we had access to the speaker's vocal folds.
Or we could estimate it automatically from a signal.
Here's some software that will do that.
It's called Praat.
Other software can also do the same thing.
It will make that measurement of F0 for you.
In fact, Praat calls it pitch, even though it's estimating at F0!
But it's very important to remember the software does not have access to the speaker's vocal folds.
It can only estimate F0 from the speech signal, using some algorithm.
That's a non-trivial estimation, so you must always be aware that there will be errors in the output of any F0 estimation algorithm.
This is not truth: this is an estimate.
'Nothing's impossible'
The term pitch really then is about the perceptual phenomenon.
It only exists in the mind of a listener, and so to do experiments about pitch would have to involve humans listening to speech.
Experiments about F0 could be done on speech signals analytically.
So speakers can control the fundamental frequency as well as the duration and the amplitude of the speech sounds they produce.
They can use all of those acoustic properties - and others - to convey parts of the message to a listener.
We use the term 'prosody' to refer to collectively the fundamental frequency, the duration, and the amplitude of speech sounds (sometimes also voice quality).
Later, then, when we attempt to generate synthetic speech, we'll have to give it an appropriate prosody if we want it to sound natural.