Lesson 6: Processors 2.


The third type of processors are Timbre Processors.

Timbre (pronounced "Tamber") is simply another way of saying "tone", and is directly related to harmonic content. It is the quality that helps us to differentiate between the tone of a violin, flute and trumpet, so when we talk about Timbre Processors, we are talking about devices that change the harmonic content of a sound. We will first look at a couple whose mode of operation could be described as "static".

Unquestionably, the simplest of the static timbre processors are Tone control knobs as found on low to medium quality guitars. All this does is to turn down the volume of the high frequencies. A single capacitor short circuits all high frequencies straight to earth subject to a variable resistor. From the electronics course we can see that capacitors only pass high frequencies, and coils only pass low frequencies, so using an appropriate combinations of capacitors and coils we can break up the audio spectrum into as many bands as we like. The 9 band "Graphic Equalizer" is an example of a more complex static timbre processor, and its very common to find 31 band graphic equalizers in sound systems today.I should point out here that because coils are big and costly it is more common today to find the filter network uses operational amplifiers and capacitor networks to divide the frequency spectrum. Again, look in the electronics course to get more information about this process. 9 Band Graphic

The first animated timbre processor we will look at is the Phaser. As the name suggests it operates by shifting the phase of a signal and combining it with the original signal. By sending the signal through a delay of about 0.1 to 5.0 milliseconds, and then combining it with the original signal, we see that some frequencies are reinforced, whereas others are cancelled. Moving a signal 1 millisecond and recombining it with a wave that is 10 milli seconds long hardly impacts it at all, whereas, a signal that has a wavelength of 1 millisecond will be reinforced because the delayed wave will come out in exactly the same phase as the original, only 1 cycle later. On the other hand, a wave that is 2 milliseconds in length will be 180 degrees out of phase, and will therefore cancel the original wave when the 2 are combined. This will also happen at all multiples of these frequencies. The result is whats called a "comb filter".

Now, if we left it at that all we would get is a single tone with many peaks and notches in the frequency spectrum, but no variation, much the same as if you got a graphic equalizer and set the sliders like the graph on the right. To make it interesting a low frequency triangle wave modulates the delay unit between the minimum and maximum delay parameters, causing a sweep of the affected frequencies. Its not easy to describe sound, but I would say the phaser creates a gentle, swaying effect.

By taking the output of the phaser and feeding it back to the input, a more active timbre processor is created, called a Flanger. Alternatively the same effect could be created by having hundreds of phase shift inverters in a single Integrated Circuit Chip, creating hundreds of notches in the comb filter. Once again the delay is modulated by a low frequency triangle wave, and the resultant sound is best described as a "swirling" of the harmonic content of the sound. One of the best known examples of the flanger is in the opening riff of Heart's 1977 hit song "Barracuda".


The third timbre processor is the "Chorus" pedal. Once again it works by using a part of the signal passing through a very short delay and then recombining with the original signal, but the purpose this time is to continually change the pitch of the delayed wave, which gives the same type of effect as two or more singers or violins. No two singers or violinists can produce exactly the same pitch all the time, and its these slight variations in pitch that give the thicker, softer sound. When used with a string synthesizer it can produce soft thick sounds similar to a big string section. Used with an organ sound it can give a similar effect to a leslie speaker, but used with an electric guitar it gives a "sparkling" or ringing sound like many bells. One of the best known uses for the chorus pedal is on the powerful chords throughout the 1979 song "Walking on the Moon" by the Police.



Digital Effects Processors.

Digital Effects Processor To understand the input and output stages please first read the section on analogue/digital conversion in the Analogue and Digital Audio course. The process is in fact exactly the same for every digital audio processor.

The diagram on the left shows the audio signal travelling along the blue lines, and the control signals travelling along the red lines. The analogue signal enters the A/D converter as a continuous and varying signal, and it is immediately converted to a series of numbers representing the shape of the input wave. These numbers are stored in the memory. Once they are stored, the numbers can be manipulated in many different and unique ways, depending on the skill and creativity of the programmer.

At a point in time determined by the program the numbers are then presented to the D/A converter which creates an analogue output wave whose shape is determined by the manipulated numbers.

Using this method it is possible to program the processor to do the digital computations for anything from a single effect to an infinite number of effects, with the only changes being made in the software. Keep in mind that the more calculations it must do, the longer it will take, so the more effects you put in a processor the faster it must run.



We'll soon be ready to move on to Wireless Systems. When its ready you will be able to click here to go to the lesson about AM and FM transmitters and recievers, or click here to return to the Stage & Studio Equipment index page.