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Synthesizers, Music & Television © T. Yahaya Abdullah

FM Spectrum Graphs

Bessel Graph

The graph plots the Amplitudes for each Order of Sideband (calculated using Bessel functions significant to the nearest percent). For FM, this is only "one side" of the spectrum because FM will have Sidebands on each side of the Carrier (Carrier = Order Zero). Also, any reflected Sidebands (phase inverted) need to be taken into account.

Bessel Spectrum Graph Note - Negative amplitudes are also shown as positive dotted lines (This gives a better picture of the harmonic strengths generated). Negative amplitudes denote phase inversion.

The graph is scaled for a DX-7 range. The DX-7 Modulator Output is shown in the upper-right corner. The equivalent Modulation Index is shown in the bottom right-corner. The slider-indicator on the right is scaled for a DX-7 Modulator Operator (just another tool to help visualise the process).

FM Spectrums

If we disregard amplitudes, there are basically 2 types of Series; (a) with coincidental reflected sidebands, and (b) with non-coincidental reflected sidebands (reflected sidebands are phase inverted). If we examine the amplitudes, the effect of the reflected sidebands depends on the modulation amount (ie how much the sidebands spread out) as well as the Carrier frequency "C" (as "C" is the centre point of the modulation). The modulation frequency "M" determines the separation of the sidebands.

The spectrums displayed are M:C=1:16, M:C=1:8, M:C=1:1, M:C=2:1 and M:C=3:1 (from left to right). The first four graphs have coincident reflected sidebands while the last does not. Inverted phase frequencies are shown as dotted lines. Output values are for DX-7 series of synths.

FM Spectrum Graphs for various M:C combinations
For M:C = 4:1 or 5:1 or 6:1 or 7:1 etc, the amplitudes would be the same as M:C=3:1 but the separation of the sidebands would be different. In general, series with non-coincident reflected sidebands are easier to predict.

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