North Suburban HAMMOND ORGAN Society

The Hammond scanner vibrato system works entirely with the electrical audio signal of the instrument, however there are some significant parallels between it and the Leslie tremolo. [We use the term "Leslie tremolo" to mean the effect that a Leslie speaker produces even though as we have just seen it is actually a complex stereo tremolo and vibrato effect. but the term "Leslie tremolo" is so well known that it seems the correct term to use for the effect.] Go back and review the Hammond vibrato system in the Hammond article before proceeding.

In a sense, the Hammond vibrato line box can be thought of as a long, narrow room of sorts in its effect upon the electrical audio signal entering it. I say this because if you have a long, narrow room and put a sound source at one end of it, then as you go along the distance of the room away from the sound source, you will encounter a sound wave (at any particular place along the room) whose phase is retarded relative to the phase of the sound wave right at the source.

Now, if you listen carefully as you traverse the length of this room, you'll hear a slight pitch decrease because you'll encounter a sound wave whose phase is, with respect to your instantaneous position, retarded relative to the phase of the sound wave you encountered when you were closer to the sound source. Needless to say, if you started at the far end of this room and approached the sound source, you'd hear a slight increase in pitch because you were encountering sound waves that were advanced. Obviously the greatest phase retard is at the far end, and the phase is less and less retarded the closer to the source you get as you walk the length of the room and approach the source.

I read one time of a demonstration that was done as an experiment, in which there was a stationary speaker, and a stereo microphone mounted on a turntable around 20 inches in diameter. When the turntable was spun at 360 rpm, the sound picked up by the microphone sounded virtually identical [it was reported by the experimenters] to the sound of a Leslie speaker. It had a similar complex, very Leslie-like vibrato/tremolo effect.

The Doppler effect works both ways. That is, if the sound source moves and the listener remains stationary; or, if the listener moves and the source is stationary, the end result is the same because it is based upon relative motion between the listener and the source. It says nothing about which is moving. It only says that if a listener and a sound source approach, the listener hears a sound whose frequency has been raised and conversely, if the listener and the sound source move away from each other, he will hear a sound whose frequency is decreased.

Needless to say the amount of pitch change depends on how quickly the source and the listener move with respect to each other. So, if you could sit on a turntable and be spun around at 360 RPM while listening to a stationary speaker, you would hear a good Leslie tremolo effect with all of the myriad complexities that you hear from a regular Leslie speaker. If music is playing and you are on certain amusement park rides, you will hear complex pitch, phase and volume effects in the music, although I am not aware of any amusement park ride that goes at 360 RPM. [I have no idea what would happen to a person spinning at that speed, but it probably would not be too good for him.]

So for now we have to rely on the experiment done with a rotating microphone and infer from that experiment that a listener, who hears in stereo would, if he lived through a few minutes at 360 RPM, hear a good Leslie tremolo.

So how does this relate to Hammond scanner vibrato? Seems we digressed significantly here! You can think of the Hammond vibrato line box as being a kind of "room" for AC audio signals. The rotating vibrato scanner pickup then becomes the "microphone" of the turntable experiment above. At each successive tap or take off point on the line box, the signal's phase is retarded slightly when compare with the signal at the previous tap. The scanner picks up signals sequentially from one tap to the next and encounters signals of increasing phase shift.

This then is analogous to our example of a person walking down a long, narrow room and listening to a sound from a stationary source at one end of the room. Actually, in order to get a noticeable phase and frequency shift, the person should run as fast as possible down the room.

The point being however that the rotating pickup of the vibrato scanner gets signals of increasing phase delay as it rotates through the first half revolution. This is like the person running down to the far end of the long, narrow room. During the second half revolution of the scanner pickup, it gets signals which are more advanced in phase, because it starts at the far end of the line box where the phase shift is greatest; and as it approaches the beginning of the line box, it encounters signals that are progressively less retarded until it gets to the starting point where there is no phase shift. In our room analogy, this is when the person has reached the far end of the long narrow room and turns around and runs back.

The microphone rotating on a turntable in front of a stationary sound source is a kind of "reverse" Leslie, but the end result is similar. In that sense, the rotating pickup of the Hammond vibrato scanner working with the line box is the electrical equivalent.

The Hammond vibrato scanner is a capacitive type pickup in that all around the perimeter of the scanner are a series of small metal plates with a little space between them. The pickup is a corresponding set of plates and it meshes with the groups of stationary plates. By the property of a capacitor to allow AC flow, we can state that the signals on the stationary plates pass through to the rotating set of plates and as the rotating set of plates passes by and meshes with each set of stationary plates in turn, whatever signal is present on any set of stationary plates gets transferred to the rotating plates.

Because of the way that the scanner is designed, as the rotating set of plates leaves one set of stationary plates, it enters the next set. Therefore, the signal that it picks up is continuous, gradually transitioning from one set of stationary plates to the next without any interruption. The signals combine vectorially. See the vector diagram in the Wurlitzer section for an example of the smooth transition between two signals that are 90° apart. The phase difference, however, between any set of stationary plates in the Hammond scanner and the next set is less than 90°, but the transition is just as smooth regardless.

For your convenience, here below is the Flash animation that also appears in the Hammond article. It shows how the Hammond vibrato scanner works. Each of the nine connections goes to a tap on the vibrato line box. As you can see. the way that the wiring is arranged, the continuous rotation of the scanner pickup plate(s) meshes with each of the sixteen sets of stationary plates so that the scanner scans the nine accessed points of the vibrato line box in sequence, both forward and reverse.

cartoon animation of Hammond vibrato scanner showing how it
		scans back and forth along the vibrato line with each revolution

Here for your convenience once again is the theory diagram of a typical Hammond vibrato scanner in operation, showing how it scans in succession, nine take-offs, or taps on the vibrato line box both in the forward and then in the reverse direction, making the two direction scans during each revolution.


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