The HAMMOND ORGAN

North Suburban HAMMOND ORGAN Society

In this diagram, imagine that the buffer rotates clockwise, and the pickup also rotates clockwise, and at the same rate. IF these conditions are true, the frequency of the output signal will be zero.

In this diagram, imagine that the buffer rotates clockwise, and the pickup also rotates clockwise, and at the same rate. IF these conditions are true, the frequency of the output signal will be zero.

In this diagram, imagine that the buffer rotates clockwise, and the pickup also rotates clockwise, and at the same rate. IF these conditions are true, the frequency of the output signal will be zero.

If we arrange it so that the pickup point rotates in the same direction and at the same speed as the circular buffer, the result is a signal of zero frequency. Obviously, we would not do this, because it would effectively give us no output at all. Again it is worth repeating that the above is simply a concept-illustrating diagram. What is important to realize is that if we rotate the pickup point in a direction opposite to that of the buffer, we'll get an increase in frequency, and if we rotate it in the same direction, we'll get a decrease in frequency as long as we keep the rotation at some speed that is LESS THAN the buffer speed. Keeping the pickup at standstill results in no frequency change, and running in the same direction at the same speed results in zero frequency or no signal at all.

If you think about the above, you will soon realize that if we slightly rotate the pickup point a little faster and then a little slower than the buffer speed, we'll end up with a signal whose frequency will slightly decrease and slightly increase. This of course results in vibrato. All of the above is done with the signal in digital form and involves no actual physical rotation of anything. Because this is easily done in the digital domain, it's possible to have several vibratos going on simultaneously, some going lower in pitch while others are rising. This can result in a very complex and full sounding vibrato effect similar to that of a Leslie rotating speaker, thus digital signal processors can quite effectively simulate a Leslie speaker.

Although all of the above involves sometimes millions of calculations per second in processing chips, because of the small size of chips and the basic simplicity of the surrounding circuitry, digital vibrato and Leslie speaker simulation are very easily accomplished. One way that digital signal processors can actually change frequency of signals is to change the sampling rate, and then resample the resulting signal at the original rate.

Digital frequency changing is not only useful for making simple or complex vibratos. Another very useful effect is to change the tempo of music while maintaining its original pitch. If you were to speed up a record-player turntable or a regular tape, for example, you would get not only an increase in the tempo of the music, but a corresponding frequency or pitch rise as well. Here, digital signal processing can be very effective, because it will let you change the tempo of the music and simultaneously shift the pitch up or down by an equal opposite amount so that the pitch or key of the music remains unchanged. This can be very useful if you are playing old phonograph records that were perhaps recorded with slight speed errors in the studio, as the very early record mastering lathes did not have the accurate speed controls of later units.

Another useful aspect of doing this is if you have a recording of a piano that is somewhat out of tune and not up to standard pitch. You can then raise the pitch of the recording so that you can play an electronic organ or keyboard along with it. I have in my collection, a recording of a well-known duo piano team playing along with an orchestra. The record was created long before digital signal processing was developed. The piano track was obviously added at a later time, and there is a most disturbing pitch discrepancy of about ten cents between the orchestra and the pianos. If the studio engineers had modern DSP, they could have easily corrected this problem.

On the next page, we'll look at a few pictures of actual digital signal processors to see not only what the control panel and displays look like, but we'll also look "under the hood" to see what they look like inside.

 

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