Spectral Modeling Synthesis

This section reviews elementary *spectral models* for sound
synthesis. Spectral models are well matched to audio perception
because the ear is a kind of spectrum analyzer [293].

For *periodic* sounds, the component sinusoids are all
*harmonics* of a fundamental at frequency
:

where denotes time in seconds, is the th harmonic radian frequency, is the period in seconds, is the amplitude of the th sinusoidal component, is its phase, and is the number of the highest audible harmonic.

*Aperiodic* sounds can similarly be expressed as a *continuous*
sum of sinusoids at potentially *all* frequencies in the range of
human hearing:^{11.6}

where denotes the upper bound of human hearing (nominally kHz).

Sinusoidal models are most appropriate for ``tonal'' sounds such as
spoken or sung vowels, or the sounds of musical instruments in the
string, wind, brass, and ``tonal percussion'' families. Ideally, one
sinusoid suffices to represent each harmonic or overtone.^{11.7} To represent the ``attack'' and ``decay''
of natural tones, sinusoidal components are multiplied by an
*amplitude envelope* that varies over time. That is, the
amplitude
in (10.15) is a slowly varying function of time;
similarly, to allow pitch variations such as vibrato, the phase
may be modulated in various ways.^{11.8} Sums of
amplitude- and/or frequency-enveloped sinusoids are generally called
*additive synthesis* (discussed further in §10.4.1
below).

Sinusoidal models are ``unreasonably effective'' for tonal audio. Perhaps the main reason is that the ear focuses most acutely on peaks in the spectrum of a sound [179,305]. For example, when there is a strong spectral peak at a particular frequency, it tends to mask lower level sound energy at nearby frequencies. As a result, the ear-brain system is, to a first approximation, a ``spectral peak analyzer''. In modern audio coders [16,200] exploiting masking results in an order-of-magnitude data compression, on average, with no loss of quality, according to listening tests [25]. Thus, we may say more specifically that, to first order, the ear-brain system acts like a ``top ten percent spectral peak analyzer''.

For noise-like sounds, such as wind, scraping sounds, unvoiced speech, or breath-noise in a flute, sinusoidal models are relatively expensive, requiring many sinusoids across the audio band to model noise. It is therefore helpful to combine a sinusoidal model with some kind of noise model, such as pseudo-random numbers passed through a filter [249]. The ``Sines + Noise'' (S+N) model was developed to use filtered noise as a replacement for many sinusoids when modeling noise (to be discussed in §10.4.3 below).

Another situation in which sinusoidal models are inefficient is at
sudden time-domain *transients* in a sound, such as percussive
note onsets, ``glitchy'' sounds, or ``attacks'' of instrument tones
more generally. From Fourier theory, we know that transients, too, can
be modeled exactly, but only with large numbers of sinusoids at
exactly the right phases and amplitudes. To obtain a more compact
signal model, it is better to introduce an explicit transient model
which works together with sinusoids and filtered noise to represent
the sound more parsimoniously. Sines + Noise + Transients (S+N+T)
models were developed to separately handle transients (§10.4.4).

A advantage of the explicit transient model in S+N+T models is that
transients can be *preserved* during time-compression or
expansion. That is, when a sound is stretched (without altering its
pitch), it is usually desirable to preserve the transients (*i.e.*, to
keep their local time scales unchanged) and simply translate them to
new times. This topic, known as *Time-Scale Modification* (TSM)
will be considered further in §10.5 below.

In addition to S+N+T components, it is useful to superimpose
*spectral weightings* to implement linear filtering directly in
the frequency domain; for example, the *formants* of the human
voice are conveniently impressed on the spectrum in this way (as
illustrated §10.3 above)
[174].^{11.9} We refer to the general class of such
frequency-domain signal models as *spectral models*, and sound
synthesis in terms of spectral models is often called *spectral
modeling synthesis* (SMS).

The subsections below provide a summary review of selected aspects of spectral modeling, with emphasis on applications in musical sound synthesis and effects.

- Additive Synthesis (Early Sinusoidal Modeling)
- Additive Synthesis Analysis

- Sines + Noise Modeling

- Sines + Noise + Transients Models

- S+N+T Sound Examples

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Center for Computer Research in Music and Acoustics (CCRMA), Stanford University