Audio I43

(appropriate because they do not occur There being now little or no voltage Synchronized oscillators in the scale itself but are used for a across the lamp and capacitor, the lamp Neon -lamp oscillators can be used in sort of embellishment). Each accidental deionizes and the capacitor once more a complete organ if they are synchron- may be identified in two ways : it is the starts to charge. The cycle is repeated ized by a stable source of frequency. sharp of the white key below it and the indefinitely, its time per cycle (and thus flat of the white key above it. Thus, its frequency) depending on the battery note No. 41 may be called either C- voltage, the resistance, the capacitor, sharp or D -flat. The interval between and the striking and extinction voltages any key and its next adjacent one, black of the lamp. Output may be taken across or white, is a half -tone. Since two halves the lamp or across any part or all of make a whole, the interval between two the resistor. The output wave is a saw - notes with one key between them is a tooth like that shown in Fig. 2. whole tone. Example: C to D is a whole The frequency of a neon -lamp relaxa- tone; E to F is a half -tone. tion oscillator is not stable. Even if all Other terms that will be used in these components and voltages are held con- may always articles also refer to intervals. We al- stant, the discharge not Fig. 4 -One method bf synchronizing a ready have the half -tone and the whole take place between the same points set of octavely- related neon generators. tone. Next is the third. That is the in- along the two electrodes in the lamp. terval between one note of the scale and, The gas is somewhat temperature- For a full scale, this requires 12 such not the next, but the second higher one sources, one for each of the notes in - between, for instance, C and E in the the top octave, which may be vacuum - C -major scale, or between F and B. SAWTOOTH OUT tube oscillators, tuning forks, or any I other stable devices. Each source syn- There are also fourths, fifths, sixths, c- NEON and sevenths. In each case, the number CON chronizes one note in each lower octave. is an ordinal, not a fraction, and refers PUISE A typical way of doing this appears in to the third, fourth, etc., note from the RI - R2 a patent issued to Nicholas Langer, starting point, counting the starting Fig. 3 -A simple monophonic instrument who is probably the most prolific work- note as the first. These numbers refer requiring only an amplifier and loud- er with gas -tube oscillators for music only to notes in the scale, not to acci- speaker to complete it. Contactor is use. The idea is shown in Fig. 4. dentals. Thus, a seventh, with C as the pressed by finger to determine pitch. Two neon oscillators are shown, one bottom note, has B as the top one in tuned roughly to the desired frequency sensitive, too, and replacement prob- the key of C, but B -flat is the seventh the and the other to about twice that fre- lem is important, since no two "iden- of C in the key of F, since, as we illu- quency or an octave higher. There lamps same B -flat is part of the F -major tical" ever oscillate at the would be one oscillator in this string strated, frequency. scale, and B- natural is not. for each note and one for each of its octaves which appear in the organ; for Neon -lamp oscillators The Trautonium example, six if the note were C, on a Probably the most truly electronic A single neon lamp has been used in 61 -note instrument. The neon lamps are method of tone generation is the use of a solo instrument with continuously fixed -tuned by resistors Rl and capaci- space-discharge tubes -hard and soft variable frequency by (among others) tors C. Across the common resistor R2 electron tubes -in which nothing moves the German inventor, Friedrich Traut- appears a small part of the variations except electrons and perhaps ions. Of wein, in his Trautonium and similar in current caused by each oscillator. these, the simplest types are gas -tube instruments. The basic idea appears in This is a synchronizing voltage, which relaxation oscillators. Fig. 3. The circuit is standard except locks all the oscillators together in that R2 has been added as an output octave relationship. R2, which should load. Its value is a fraction of that of have a maximum value of about 10,000 OUTPUT the main tuning resistor Rl. A saw - ohms when 1- megohm resistors Rl are tooth wave is obtained from across the used to tune the oscillators, is varied so

NEON lamp and a pulsed output, the result of that locking is obtained with a mini- 11111-I the periodic neon -lamp discharges, from mum of resistance in the circuit. across R2. R3 is across the secondary of a trans- OUTPUT Rl may be a long coil of resistance former which is fed by a primary sync wire or a long composition element like source -a tuning fork or vacuum -tube Fig. 2. -The basic relaxation oscilla- those used in potentiometers. The con- oscillator of high stability with a fre- tor and sawtooth waveform it produces. tactor is a strip of flexible conducting quency equal to or one octave above the material suspended slightly above Rl highest note wanted. By adjusting R3, The basic circuit is shown in Fig. 2, and topped with a nonconducting (in- enough primary sync signal is brought using a neon lamp. When the switch is sulating) layer. The player presses his in to lock all the oscillators to it. closed, the capacitor starts charging. finger on the contactor at any point to Output is taken through the trans- The lamp cannot ionize or fire until short out part of R1 and change the formers associated with each oscillator. the voltage across the capacitor is frequency. The primaries are in series with the above its breakdown point. The rush of Typical values for Rl and C with a lamps and the secondaries are in series electrons from one capacitor plate to 1/25 -watt neon lamp experimented with with each other. Each secondary is nor- the other passes through the resistor, by the writer are 1 megohm and .005 mally shorted. When a key is pressed, the voltage drop across which reduces µf. R2 may be anywhere from 1,000 to the short is removed and the desired the voltage available to charge the capa- about 100,000 ohms. The lower values tone goes through to the output. The citor. As the capacitor continues to are safer because the following circuits output transformers should have fairly charge, however, the electron flow di- will affect the tuning less, but of course low- impedance primaries to avoid ef- minishes and the drop across the resis- more amplification is necessary. fects on oscillator frequency and some- tor becomes smaller until finally the The circuit to which the sawtooth what higher- impedance secondaries to capacitor is fully charged to the voltage output goes must have a very high im- minimize disturbances fed back from of the battery. pedance or a high resistor (and usually other circuits. Ordinary 3 -to -1 audios In practice, the capacitor is never a blocking capacitor) must be inserted do the job very well. allowed to charge fully. When the in series with the output. Another system for syncing neon charge reaches the breakdown voltage Feeding the two outputs to different lamps will be found in the article "Sim- of the neon lamp, the neon ionizes sud- amplifiers and mixing the results gives ple Electronic Organ" in the January, denly and the lamp becomes a fairly various tone qualities. The actual 1947, issue of this magazine. Additional low resistance. This near -short across Trautonium circuit is more complex gas -tube tone generators will be dis- the capacitor quickly discharges it. and will appear in another article. cussed in the next article. AUGUST, 1950