However, for anyone wanting at least an approach to the richness of sound provided by a real pipe organ, a major snag is the use of dividers to provide the various octave pitches. Also, squarewaves contain no even harmonics, so some combining of different octave outputs for each note is necessary if a convincing variety of pipe-like sounds (especially open diapasons) is to be achieved, adding to the complexity. It is difficult to obtain sufficient attenuation when notes are not supposed to be sounding, leading to a residual background noise aptly described by the term ‘beehive’. With all 12 semitones of seven or more octaves available all the time, each individual note has to be passed when the corresponding key is pressed, or else blocked, by its own keying circuit. Advantages of this approach include cheapness and simplicity (though top octave generators are not as easy to obtain as they once were) and an organ which is always in tune, but there are a number of snags as well. Each semitone output is applied to a binary divider such as the seven stage CD4024 to provide the lower octaves. The former use a digital ‘top octave generator’ to produce the 12 semitones of the equal tempered scale, all the intervals being, if not exact, at least very close, and of course ‘set in concrete’. The main varieties are divider organs and free phase organs. One of these applications is as tone sources in an electronic organ, or rather in one class of electronic organs, for there are a number of distinct approaches to design of these, each with its own advantages and disadvantages. These are well illustrated by the subject of this article – keyed tone generators, such as might be used in the two-tone alarm generator of an HF radio telephone or a hundred other applications. Practical analog circuit design is fraught with snags, compromises and difficulties at every turn. Ian Hickman, in Analog Circuits Cookbook (Second Edition), 1999 Introduction
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