North Suburban HAMMOND ORGAN Society

Stereo Tremolo

Stereo tremolo does not involve a direct pitch change, however, once again, after the sound leaves the speakers, a true vibrato of an interesting type results.

Stereo tremolo consists of splitting an audio signal into two identical channels and then applying a tremolo or amplitude variation to each one in such a way that as one channel becomes louder, the other gets quieter. The result that a listener hears becomes an interesting combination of both tremolo and a subtle but unusual pitch shift as well. The pitch shift results because the two audio channels come from different locations.

two sinewaves being tremmed in opposition

Figure 6. Stereo tremolo animation shows how the two otherwise identical audio channels are gated gradually on and almost fully off alternately. In order for the stereo tremolo to be effective, it is necessary to trem the individual audio channels virtually fully off on the low side of the volume exciursion.

The sound waves combine vectorially in the air and the sound appears to move back and forth between the two sound sources (speakers). This results in a perceived signal that is continually changing phase and the listener hears a subtle pitch vibrato. Recall the magic rule of vibrato which is that whenever the phase of a sound wave (or AC audio signal for that matter) changes, the result is an apparent frequency change which lasts as long as the phase of the audio signal is changing.

This effect is occasionally used in modern electronic music, Its effect is quite similar to a heterodyne vibrato, but it is easier to produce as far as circuitry goes because there is no need to shift the frequency of the audio signals. This effect was used in certain early Conn organs, which had two speakers placed at 90° to each other in the console.

The effect is subtly different and a little more complex-sounding than a heterodyne vibrato, and as you move about a room where such a signal exists, the effect appears to change. The effect is best if you are equidistant from both speakers. If you are very close to either speaker, the result is mostly tremolo. The stereo tremolo, like the heterodyne vibrato, is useful for applying a realistic vibrato to an electronic simulation of a vibraharp and is much more pleasing and interesting than just a straight tremolo.

Digital signal processing can easily create a stereo tremolo. It is, however, absolutely necessary to have the two signals coming from separate speakers. If for any reason, the stereo tremolo signals are mixed electrically first, they will effectively neutralize each other and the effect will be just a steady tone with no tremolo or vibrato at all. The next type of vibrato that we'll look at is the electronic equivalent of the stereo tremolo as used in the Wurlitzer electrostatic organs, and also used as the bass channel vibrato for the Hammond X66. For lack of a better name, we'll call it a

Constant Pitch Shift Vibrato

This vibrato is quite similar in both production and in sound to the stereo tremolo, but it is a mono or single channel vibrato. Like the stereo tremolo, the constant pitch shift vibrato requires two separate signals to be gated or tremoloed alternately. The most significant difference is that the phase of one channel or signal is shifted electrically first by a constant amount. Then when the two out of phase signals are alternately tremmed on and off, the resulting signal has a continuous phase shift and thus has a pitch vibrato.

If you have not already done so, go back and read the article about the Wurlitzer 4600 series organ vibrato to see exactly how this is done. This type of vibrato shifts the pitch of all frequencies in its operating range by the same number of cycles. This is quite different from a normal vibrato which is a ratio shift and shifts the higher frequencies by the same percentage which means that the number of cycles of pitch shift is proportionately greater for higher pitches than lower ones.

Because the pitch shift is constant, it follows that as you decrease the frequency of the tones fed into a constant pitch shift vibrato system, the vibrato effect becomes more pronounced. For example, if the constant pitch shift vibrato alters the pitch of all tones by 3 Hz, then 3 Hz is a much larger percentage of 50 Hz than it is of 2000 Hz.

This type of vibrato is very good for simulating vibraharp vibrato, but it is not so useful for general playing if it is the only type of vibrato available. The late Richard Dorf, who among other things designed and marketed the Schober organ kits that were popular 45 years ago, extolled the engineering "elegance" of this type of vibrato, but musicians didn't like it all that much because it is not really natural sounding. [Possibly one of the reasons that Wurlitzer electrostatic instruments never achieved as much popularity as some other makes.] However, if it is confined to the lower end of the audio spectrum, it can be quite useful, and in the Hammond X66 instruments, this is the type of vibrato used for all pitches below (and including) Middle G. However, for the higher frequencies, Hammond used a polyphase scanner vibrato which is much better, and this leads us of course to consider as a topic by itself the well-known

Hammond Scanner Vibrato

The Hammond scanner vibrato sounds very good and in my opinion needs no enhancement by Leslie speakers or any other methods, which is not true of some of the other types. The basic theories and operation of the Hammond scanner vibrato are detailed in our Hammond Article so it's not really necessary to repeat all of that here. The Hammond vibrato in theory should be a regular, simple vibrato, but as produced it actually ends up being quite complex even before it leaves the speakers, the reason for this being the presence of some low amplitude "reflected waves" that form in the vibrato line box and which then as the line box is scanned make the final result a complex polyphase vibrato which under some conditions can sound remarkably like Leslie tremolo. After we have examined Leslie Tremolo, we can return to Hammond scanner vibrato to see that there are some interesting parallels between the two effects. In my opinion, the Hammond scanner vibrato is the best sounding electronic vibrato ever used in any analog electronic organs. Now, let us look at the famous

Leslie Tremolo

Leslie tremolo as such is a misnomer, because the Leslie effect, although it certainly involves amplitude or loudness modulation also includes an infinitely complex array of pitch and phase changes. The Leslie Speaker, which is detailed in this article, actually applies physical motion to the sound sources by rotating either a baffle, horn or in some cases the speakers themselves at between 360 to 400 RPM.


Previous Page   Page 4.    Next page