The shape of tonewheel teeth are generally designed to produce a pure sine wave. However, the lowest octave of tonewheels in late model organs do not produce a sine wave. This octave is used only in the pedals. With only two [Drawbars] to control the pedal waveform, the pedal tones in early Hammonds (A, B-1 & B-2, etc.) sounded a bit dull. In the most popular model Hammonds (B-3, C-3, etc.) the tonewheels of this octave are cut with a more complex pattern to improve the sound (See [Complex Tone Wheels|ComplexToneWheels]).
While each output above the first octave of the [Tone Generator|ToneGenerator] should be a pure sine wave, there is some distortion:
-* The [Synchronous Motor|SynchronousMotor] pulses slightly as it rotates. Left unaddressed, this would cause a throbbing quality to the sound. Hammond effectively limited this by the use of a resilient [Coil Spring |CoilSpring] coupling between the synchronous motor and the drive shaft to absorb the pulses.
+* The [Synchronous Motor|SynchronousMotor] pulses or "cogs"
slightly as it rotates. Left unaddressed, this would cause a throbbing quality to the sound. Hammond effectively limited this by the use of a resilient [Coil Spring |CoilSpring] coupling between the synchronous motor and the drive shaft to absorb the pulses.
* Eccentricities in the shape of the tonewheels can create a subtle low frequency amplitude modulation. The frequency of this modulation will be the same as the rotation speed of the tonewheel. Usually this modulation is low enough not to be objectional but if too apparent, the only solution is to replace the offending tonewheel. For a more in-depth discussion of the effect that eccentricities in the shape and mounting of the tonewheels has on the sound, read the [patent|HammondPatents] application for No. 2,314,496. This patent describes a tonewheel generator that apparently was never commercially produced. Start reading about the end of the third page for Hammond's description of how the rotational speed of the tonewheels adds undesirable harmonics and how the "improved" generator disguises these by making them more "vibrato-like".
* Eccentricities in the gear train can create subtle low frequency amplitude and frequency modulation. It is impossible to precisely machine the [Driving Gears|DrivingGear] and the [Driven Gears|DrivenGear]. Out of round gears, irregularities in the teeth, backlash and the clearances between the teeth can cause these modulation problems. [Laurens Hammond|LaurensHammond] describes this problem at some length in the original patent (nearly all of page 6 - See [Hammond Patents|HammondPatents]). This is why each driving gear is isolated from the drive shaft by a resilient coil spring. Eccentricities are dampened but are not completely eliminated by the springs. Like eccentricities in the tonewheel, if the sound is too distorted the only option is to replace the offending gear(s).
