In most cases, the phase increment necessary to produce a specified output frequency is not an integer value.
When this happens, we have several options for determining appropriate phase and/or ouput values, including truncation, rounding, and interpolation.
If a given phase (or table index) value at a certain instant was 56.7, for example, the truncation approach would choose the table value at index 56. The rounding approach would choose the table value at index 57.
Interpolation involves computing the table output at a fractional index in terms of a weighted combination of table values in the vicinity of that index. Several interpolation methods are common, including linear interpolation and the use of cubic splines.
The method used to determine the wavetable output can have significant affect on the quality of the resulting signal, as demonstrated in Figs. 2 and 3.
Figure 2:
The magnitude spectrum of a rounding wavetable sinusoidal oscillator implementation.
Figure 3:
The magnitude spectrum of a linearly-interpolating wavetable sinusoidal oscillator implementation.
As seen in the figures above, linear interpolation provides significant improvement in quality over rounding..
Another important factor in wavetable synthesis quality is the size of the table data. In general, larger tables provide more accurate results.