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Chapter 5: The Transformation of Sound by Computer

Section 5.4: Introduction to Spectral Manipulation

There are two different approaches to manipulating the frequency content of sounds: filtering, and a combination of spectral analysis and resynthesis. Filtering techniques, at least classically (before the FFT became commonly used by most computer musicians), attempted to describe spectral change by designing time-domain operations. More recently, a great deal of work in filter design has taken place directly in the spectral domain.

Spectral techniques allow us to represent and manipulate signals directly in the frequency domain, often providing a much more intuitive and user-friendly way to work with sound. Fourier analysis (especially the FFT) is the key to many current spectral manipulation techniques.

Phase Vocoder

Soundfile 5.9
Unmodified speech

Soundfile 5.10
Speech made half as long with a phase vocoder

Soundfile 5.11
Speech made twice as long with a phase vocoder

Soundfile 5.12
Speech transposed up an octave

Soundfile 5.13
Speech transposed down an octave

Perhaps the most commonly used implementation of Fourier analysis in computer music is a technique called the phase vocoder. What is called the phase vocoder actually comprises a number of techniques for taking a time-domain signal, representing it as a series of amplitudes, phases, and frequencies, and manipulating this information and returning it to the time domain. (Remember, Fourier analysis is the process of turning the list of samples of our music function into a list of Fourier coefficients, which are complex numbers that have phase and amplitude, and each corresponds to a frequency.)

Two of the most important ways that musicians have used the phase vocoder technique are to use a sound’s Fourier representation to manipulate its length without changing its pitch and, conversely, to change its pitch without affecting its length. This is called time stretching and pitch shifting.

Why should this even be difficult? Well, consider trying it in the time domain: play back, say, a 33 1/3 RPM record at 45 RPMs. What happens? You play the record faster, the needle moves through the grooves at a higher rate, and the sound is higher pitched (often called the "chipmunk" effect, possibly after the famous 1960s novelty records featuring Alvin and his friends). The sound is also much shorter: in this case, pitch is directly related to frequency—they’re both controlled by the same mechanism. A creative and virtuosic use of this technique is scratching as practiced by hip-hop, rap, and dance DJs.

Soundfile 5.14
Speeding up a sound

Soundfile 5.15
Slowing down a sound

An example of time-domain pitch shifting/speed changing. In this case, pitch and time transformations are related. The faster the sound is played, the higher the pitch becomes, as heard in Soundfile 5.14. In Soundfile 5.15, the opposite effect is heard: the slower file sounds lower in pitch.


The Pitch/Speed Relationship in the Digital World

Now think of altering the speed of a digital signal. To play it back faster, you might raise the sampling rate, reading through the samples for playback more quickly. Remember that sometimes we refer to the sampling rate as the rate at which we stored (sampled) the sounds, but it also can refer to the kind of internal clock that the computer uses with reference to a sound (for playback and other calculations). We can vary that rate, for example playing back a sound sampled at 22.05 kHz at 44.1 kHz. With more samples (read) per second, the sound gets shorter. Since frequency is closely related to sampling rate, the sound also changes pitch.

Xtra bit 5.1
"Slow Motion Sound"

Even with the basic pitch/speed problem, manipulating the speed of sound has always attracted creative experiment. Consider an idea proposed by composer Steve Reich in 1967, thought to be a kind of impossible dream for electronic music: to slow down a sound without changing its pitch (and vice versa).

Soundfile 5.16

A SoundHack varispeed of some standard speech. Note how the speech’s speed is changed over time.

Varispeed is a general term for "fooling around" with the sampling rate of a sound file.


Using the Phase Vocoder

Using the phase vocoder, we can realize Steve Reich’s piece (see Xtra bit 5.1), and a great many others. The phase vocoder allows us independent control over the time and the pitch of a sound.

How does this work? Actually, in two different ways: by changing the speed and changing the pitch.

To change the speed, or length, of a sound without changing its pitch, we need to know something about what is called windowing. Remember that when doing an FFT on a sound, we use what are called frames—time-delimited segments of sound. Over each frame we impose a window: an amplitude envelope that allows us to cross-fade one frame into another, avoiding problems that occur at the boundaries of the two frames.

What are these problems? Well, remember that when we take an FFT of some portion of the sound, that FFT, by definition, assumes that we’re analyzing a periodic, infinitely repeating signal. Otherwise, it wouldn’t be Fourier analyzable. But if we just chop up the sound into FFT-frames, the points at which we do the chopping will be hard-edged, and we’ll in effect be assuming that our periodic signal has nasty edges on both ends (which will typically show up as strong high frequencies). So to get around this, we attenuate the beginning and ending of our frame with a window, smoothing out the assumed periodical signal. Typically, these windows overlap at a certain rate (1/8, 1/4, 1/2 overlap), creating even smoother transitions between one FFT frame and another.


Figure 5.12  Why do we window FFT frames? The image on the left shows the waveform that our FFT would analyze without windowing—notice the sharp edges where the frame begins and ends. The image in the middle is our window. The image on the right shows the windowed waveform. By imposing a smoothing window on the time domain signal and doing an FFT of the windowed signal, we de-emphasize the high-frequency artifacts created by these sharp vertical drops at the beginning and end of the frame.

Thanks to Jarno Seppänen <Jarno.Seppanen@nokia.com> Nokia Research Center, Tampere, Finland, for these images.

Figure 5.13   After we window a signal for the FFT, we overlap those windowed signals so that the original signal is reconstructed without the sharp edges.

By changing the length of the overlap when we resynthesize the signal, we can change the speed of the sound without affecting its frequency content (that is, the FFT information will remain the same, it’ll just be resynthesized at a "larger" frame size). That’s how the phase vocoder typically changes the length of a sound.

What about changing the pitch? Well, it’s easy to see that with an FFT we get a set of amplitudes that correspond to a given set of frequencies. But it’s clear that if, for example, we have very strong amplitudes at 100 Hz, 200 Hz, 300 Hz, 400 Hz, and so on, we will perceive a strong pitch at 100 Hz. What if we just take the amplitudes at all frequencies and move them "up" (or down) to frequencies twice as high (or as low)? What we’ve done then is re-create the frequency/amplitude relationships starting at a higher frequency—changing the perceived pitch without changing the frequency.

Figure 5.14  Two columns of FFT bins. These bins divide the Nyquist frequency evenly. In other words, if we were sampling at 10 kHz and we had 100 FFT bins (both these numbers are rather silly, but they’re arithmetically simple), our Nyquist frequency would be 5 kHz, and the bin width would be 50 Hz.

Each of these frequency bins has its own amplitude, which is the strength or energy of the spectra at that frequency (or more precisely, the average energy in that frequency range). To implement a pitch shift in a phase vocoder, the amplitudes in the left column are shifted up to higher frequencies in the right column. This operation shifts 1 spectra in the left bin to a higher frequency in the right bin.

The phase vocoder technique actually works just fine, though for radical pitch/time deformations we get some problems (usually called "phasiness"). These techniques work better for slowly changing harmonic sounds and for simpler pitch/time relationships (integer multiples). Still, the phase vocoder works well enough, in general, for it to be a widely used technique in both the commercial and the artistic sound worlds.

Soundfile 5.17
"Study: Anna"

Larry Polansky’s 1-minute piece, "Study: Anna, the long and the short of it."

All the sounds are created using phase vocoder pitch and time shifts of a recording of a very short cry (with introductory inhale) of the composer’s daughter when she was 6 months old.


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