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What is Jitter or FM Distortion?

Jitter, Wow, Flutter or Frequency Modulation or FM distortion is a form of signal processing error that causes input tones that have a steady frequency to become tones that have a frequency that is not steady. FM distortion is distortion of the shape of a signal in the time domain.  FM distortion thus affects unsteady input tones by changing how unsteady they are.  In addition, much jitter is due to signals that are not part of the input signal, so it is also a form of noise. 

Audio signals have only two dimensions, time and amplitude. Therefore, a signal can only be distorted in the time domain or the amplitude domain or both domains. Amplitude domain distortion is the other kind of nonlinear distortion and is called AM distortion. Distortion that affects the size or timing of a signal without changing its shape is called linear distortion.

What FM Distortion Is

 

First, a little background information. Music is composed of a collection of enveloped (i.e., amplitude modulated) tones, some on the usual  12-tone musical scale, some not, some harmonically related (frequencies related by being multiplied or divided by integers), some not related by integer ratios  (also called "aharmonic" or "inharmonic").

To simplify things, we need only look at what FM distortion does to a single  tone. As long as the distortion is relatively small, interactions between multiple tones will be minimal. Therefore, for reference purposes, we need only consider a single pure tone such as the one shown in Figure 1.

FM distortion is nonlinear distortion, so it adds tones to the music, such as is shown in Figures 2-4. Some of the added tones will be at the same frequency as  pre-existing tones in the music. The tones that appear at the same frequencies will add or subtract depending on their relative phase. Some of the tones created by FM distortion will be at new frequencies that did not appear in the original music..

FM distortion is distinguished  from AM distortion by means of the inverted polarity of the lower sidebands created by FM distortion. Sidebands created by AM distortion have the same polarity as the carrier or original pure tone.

Figure 1. Spectral analysis of a pure, undistorted 11 KHz tone. When analyzing modulation distortion, we call the original pure tone the "carrier". Notice that there is only a single tone at 11 KHz and everything else that shows up in the spectral analysis is random noise.

Figure 2. Spectral analysis of a 11 KHz primary tone ("carrier")  with "-20 dB" or about 10% added 60 Hz  FM distortion. Severe FM distortion adds multiple tones or "sidebands". Thus there is an primary pair of sidebands at an amplitude of -20 dB, and additional pairs of sidebands that are further away from the 11.00 KHz carrier. The additional pairs of sidebands have a lower amplitude than the primary pair, but sidebands that are further out are not necessarily at lower amplitudes. Notice that the sidebands are uniformly spaced at 60 Hz intervals. 60 Hz is the frequency of the signal that has FM modulated the 11 KHz carrier. Analysis of just the amplitudes of sidebands cannot distinguish between AM and FM distortion. FM distortion distinguishes itself from AM distortion by reversing the polarity of the lower sidebands. Complex sideband collections like this can also occur when the modulating frequency is not a pure sine wave.

Figure 3. Spectral analysis of a 11 KHz tone with "-40 dB" or about 1% added FM distortion. The additional sidebands beyond the main pair at -40 dB are minimal. The sidebands are spaced at 60 Hz intervals because the modulating frequency remains 60 Hz.

Figure 4. A 11 KHz tone with "-80 dB" or about 0.10% added FM distortion. FM disortion that is this small is difficult or impossible to hear.

What  FM Distortion Sounds Like

FM distortion is heard two different ways, depending on the circumstances.

 

(1) FM distortion is often heard as a *roughening* of the music. "Roughness" is a sort of distinct muddying effect. The opposite effect which is "smoothness" is characteristic of a pure tone.  FM distortion is heard this way when the modulating frequency is relatively low. Usually, modulating frequencies below 125-150 Hz are considered to be "Low" Generally, the lower the modulating frequency in this mode, the more audible the FM distortion is.

 

(2) FM distortion can also be heard as additional, mostly aharmonic tones related to the music, that are at new frequencies or added to or subtracted from the tones that are already there. The tones at new frequencies may  "muddy" the music, and the tones at existing frequencies may change the timbre of the music. FM distortion is heard this way when the modulating frequency is relatively high. 

Generally, the higher the modulating frequency in this mode, the more audible the FM distortion is. However, if the fundamental tone or modulating frequency is high enough, the sideband can move up into the ultrasonic range and not be heard.

 

Causes of FM Distortion

FM Distoriton is caused by changes in the timing of important events. For example, the usual causes of flutter and wow during the playback of or LP records or analog tape  is timing changes caused by variations in the rotation of a the turntable or tape recorder capstan. 

Jitter in digital audio equipment is usually caused by changes in the timing of bits and bytes in the digital data stream. 

FM distortion in loudspeakers comes about when the motion of the cone due to low frequencies shifts the tone of high frequencies much as the motion of a train can change the tone of a train's whistle.

You can listen to what FM distortion sounds like by listening to files that you can download from  http://www.pcabx.com/technical/jitter_power .