Sawtooth wave

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The sawtooth wave (or saw wave) is a kind of basic non-sinusoidal waveform. It is named a sawtooth based on its resemblance to the teeth on the blade of a saw. A sawtooth wave can be created by an instrument such as an oboe.

The usual convention is that a sawtooth wave ramps upward as time goes by and then sharply drops. However, there are also sawtooth waves in which the wave ramps downward and then sharply rises. The latter type of sawtooth wave is called a 'reverse sawtooth wave' or 'inverse sawtooth wave'. The 2 orientations of sawtooth wave sound identical when other variables are controlled.

A bandlimited sawtooth wave pictured in the time domain (top) and frequency domain (bottom). The fundamental is at 220 Hz (A2).

The piecewise linear function

$x(t) = t - \operatorname{floor}(t)$

based on the floor function of time t, is an example of a sawtooth wave with period 1.

A more general form, in the range −1 to 1, and with period a, is

$x(t) = 2 \left( {t \over a} - \operatorname{floor} \left ( {t \over a} + {1 \over 2} \right ) \right )$

This sawtooth function has the same phase as the sine function.

A sawtooth wave's sound is harsh and clear and its spectrum contains both even and odd harmonics of the fundamental frequency. Because it contains all the integer harmonics, it is one of the best waveforms to use for constructing other sounds, particularly strings, using subtractive synthesis.

A sawtooth can be constructed using additive synthesis. The infinite Fourier series

$x_\mathrm{sawtooth}(t) = \frac {2}{\pi}\sum_{k=1}^{\infin} \frac {\sin (kt)}{k}$

converges to a sawtooth wave. In digital synthesis, the series is only summed over k such that the highest harmonic, N_max, is less than the Nyquist frequency (half the sampling frequency). This summation can generally be more efficiently calculated using the Fast Fourier transform. If the waveform is digitally created directly in the time domain using a non-bandlimited form, such as y = x - floor(x), infinite harmonics are sampled and the resulting tone contains aliasing distortion.

Animation of the additive synthesis of a sawtooth wave with an increasing number of harmonics

An audio demonstration of a sawtooth played at 440 Hz (A4) and 880 Hz (A5) and 1760 Hz (A6) is available below. Both bandlimited (non-aliased) and aliased tones are presented.

Sawtooth aliasing demo (file info) — play in browser (beta)
- Sawtooth waves played bandlimited and aliased at 440 Hz, 880 Hz, and 1760 Hz
- Problems listening to the file? See media help.

[edit] Applications

The sawtooth wave is the form of the vertical and horizontal deflection signals used to generate a raster on CRT-based television or monitor. Oscilloscopes also use a sawtooth wave for their horizontal deflection, though they typically use electrostatic deflection.
- On the wave's "ramp", the magnetic field produced by the deflection yoke drags the electron beam across the face of the CRT, creating a scan line.
- On the wave's "cliff", the magnetic field suddenly collapses, causing the electron beam to return to its resting position as quickly as possible.
- The voltage applied to the deflection yoke is adjusted through various means (transformers, capacitors, center-tapped windings) so that the half-way voltage on the sawtooth's cliff is at the zero mark, meaning that a negative voltage will cause deflection in one direction and a positive voltage will produce deflection in the other direction, allowing the whole screen to be covered by a center-mounted deflection yoke. Frequency is 15.75 kHz on NTSC, 16.75 kHz for PAL and SECAM)
- The vertical deflection system operates the same way as the horizontal, though at a much lower frequency (60 Hz on NTSC, 50 Hz for PAL and SECAM).
- The ramp portion of the wave must be perfectly linear - if it isn't, it's an indication that the voltage isn't increasing linearly, and therefore that the magnetic field produced by the deflection yoke won't be linear. As a result, the electron beam will accelerate during the non-linear portions. On a television picture, this would result in the image being "squished" to the direction of the non-linearity. Extreme cases will show obvious brightness increases, since the electron beam spends more time on that side of the picture.
- Most TV sets used to have manual adjustments for vertical and/or horizonal linearity though they have generally disappeared due to the greater temporal stability of modern electronic components.