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Most audio oscillations are 'periodic': that is, when displayed on a screen, they exhibit a recognisable shape that repeats and repeats and repeats… The best known examples of periodic waveforms are the sine waves, triangle waves, pulse waves, and sawtooth waves produced by Integrator modules such as the RS90 VCO and RS80 LFO. Not all audio frequency oscillations exhibit this repetitious nature, and the most common of these 'aperiodic' waves is called "white noise". But why "white" and why "noise"? Let's explain each in turn… A signal is perceived as noisy if it is random. Described another way, pure noise contains no discernible frequencies or tones. But some noises may be described as rumbles, hisses, or even shrieks. In each of these cases, the waveform itself is random, but the signal only contains a narrow band of frequencies. Audible noise may, therefore, be "coloured" in exactly the same way that visible light is. If lower frequencies predominate, the noise has a deep, rumbling character, and it is called 'pink' or 'red' depending upon its exact nature. If high frequencies predominate the noise is called 'blue'. But if all frequencies are present in each amplitudes, the noise is 'white'. This is exactly analogous to light, where the presence of all the visible frequencies at equal amplitudes is perceived by the eye as the colour 'white'. White Noise is, therefore, a signal that contains all the audio frequencies in equal amounts, but which manifests no recognisable pitches or tones. Another definition is this: a white noise signal is one in which the probability of a frequency being present is equal to the probability of any other frequency being present. Many natural sounds display noisy characteristics - the crashing of waves and the howl of a strong wind are prime examples of these - and it would be impossible to synthesise such sounds without a noise generator. Many musical instruments also generate noise, although the amplitude of this part of the signal is generally low, and its colour is largely dependent upon the instrument. But if you start with white noise you can filter it to produce all other 'colours' of noise. You can also use resonant filters to accentuate certain frequencies, and modulate the noise amplitude to create such things as chiffs or wind and wave effects IN USE The RS40 incorporates a highly stable S&H circuit that will sustain a constant voltage almost indefinitely. Because it provides external inputs for the signal to be sampled and for the trigger, it can be used with any signals, not just a noise source and a clock. The inputs are the source signal input (EXT SRC IN) and the trigger input (EXT CLK IN). The output is... well, the output. The controls are an output level control, and a manual trigger (ONE SHOT). EXT SRC IN This input accepts signals in the range ±10v. These signals may be CVs or audio signals, and may be generated within the RS Integrator or presented by outside sources such as CDs, microphones, or other musical instruments. If the signal amplitude is too low for effective use as a source, you should use the Pre-Amp sub-module in the RS70 to boost it to an appropriate level. EXT CLK IN This input will generate a trigger when it receives any positive-going waveform with a sharp leading edge. A pulse wave is the traditional signal to use for this, but almost any waveform will suffice - even sine waves above about 3Hz will provide a trigger. Note, however, that not all waves will cause the S&H to trigger in the same fashion. For example, a sawtooth with a trailing edge produces a pronounced 'click' compared to a sawtooth with a leading edge. Furthermore, sawtooth waves will often trigger the S&H more frequently than a pulse wave of the same frequency. It has even been observed that different waveforms presented at the clock input will lead to different ranges of voltages being produced by the S&H circuit itself. You should use the RS80 LFO to experiment with these effects. LEVEL The LEVEL control adjusts the range of voltages produced by the S&H. Think of it as a 'range' control: the source may exhibit variations of ±10V, but the LEVEL allows you to compress this to any range of voltages from 0V (fully anticlockwise, no effect is generated) to the full ±10v (fully clockwise). ONE SHOT If no signal is presented to the EXT CLK IN socket, you can take a sample by pressing the ONE SHOT button. This will cause the S&H circuit to measure the instantaneous amplitude of the signal presented to the EXT SRC IN socket, and hold it until the next time you press the button. S&H OUT Sampled voltages, once scaled by the LEVEL control, are output here. The maximum signal amplitude is ±10v.