Lesson 6: Processors 1.

Effects Processors can be divided into 3 categories.

Time Processors

The first is Time Processors.

The simplest time processor is a Delay. These can be helpful in large systems where additional speakers are required some distance from the stage. Because electricity and radio travel at practically the speed of light, and sound travels much slower the electronic signal needs to be delayed or the sound coming from the stage will sound like an echo. The only user control necessary here is delay time.

By taking the output of the delay and feeding it back to the input we have the second time processor called an Echo unit. Adding 2 more user controls we can now have control over the amount of decay and the balance between original sound and delayed sound.

The third time processor is the Reverb. It is essentially the same as the echo except that the amount of delay is considerably shorter. Reverb starts with a delay around 5 milliseconds whereas Echo is only percievable above 80 milliseconds.

In fact, in a large auditorium or cathedral there are naturally occuring elements of all of the 3 types listed above. Modern computerised digital effects processors (see below) take that into consideration and create programs that are simply called "Cathedral", "Large Hall", "Small Room", etc.



The second type of processors are the Amplitude Processors.

The simplest of these is the Noise Gate. The purpose of these is to turn the output off when the signal gets below a set threshhold, thus eliminating hisses, hums, buzzes etc that would otherwise be heard when there is no signal present. Early noisegates would simply short circuit the output when there was no input above the threshhold, but the more modern ones are a vaiation of the expander which is described below.

The second amplitude processor is the Compressor. This processor is concerned with the "dynamic range" (loud and soft) of the signal. Again there is a threshhold involved (often called the knee point) but this time the signal above the threshhold is reduced in amplitude but the signal below the threshhold is increased, so the loud is not so loud, and the soft is not so soft. The next control is the "ratio" and this controls how much the signal is compressed. This unfortunately introduces a form of distortion as only the top of the waves changes shape.

Compressed Signal Modern computerised compressors are able to analyse the whole wave, decide how much compression is required and compress the entire wave from the zero crossing thus eliminating the distortion.

In fact compressors were introduced to get more sound on recorders like vinyl discs and tape recorders where physical space and magnetic capabilities was a very real consideration, but they were not designed as a stand alone unit. They were designed to be used inconjunction with the next processor, but before we look at that there are two more common functions for which compressors are commonly used. Firstly, to prevent transient peaks overloading speakers we can set the compression ratio to infinity (which means no signal goes higher than the threshhold), and it becomes whats called a "Limiter"

Finally, compressors are used by guitarists who want to create long smooth notes instead of short, twangy notes. The initial loud twang as a string is plucked is heavily compressed, but as the note starts to fade out the compressor increases the volume, making a longer sweeter note that can be bent, vibratoed, hammered, etc.

The Expander was originally designed to be used on a compressed signal with the characteristics (threshhold and compression/expansion ratio) matched, therefore the introduced shape distortion would be corrected. More recently it has been adapted to do the function of a noise gate as well. As the signal gets less the expander turns it down more.

This section would not be complete without mentioning the grandfather of all amplitude processors, the Tremlo unit. This was very popular in the late 50s and 60s because, apart from the tape echo, there were very few other effect units available. The principle is very simple; the signal goes through an amplifier where the gain is voltage controlled. A low frequency (about 1 to 10 Hz) controlling voltage in the shape of a sine, square, triangle, or saw tooth wave then turns the amplifier up and down giving a "trembling" effect to the instrument or voice passing through it.



We'll soon be ready to move on to Processors 2. When its ready you will be able to click here to go to the lesson about Timbre Processors and Digital effects processors, or click here to return to the Stage & Studio Equipment index page.