Patent GB-713424-A (GB)
Title: Improvements in or relating to sound recording and reproduction
IPC: G10H1/00
Filed: 1950-05-16
Abstract: 713,424. Sound recording systems. KENDAL, O. May 7, 1951 [May 16, 1950], No. 12216/50. Class 40 (4) [Also in Groups XXXVIII and XL (c)] A method of recording synthetic musical sounds comprises the steps of producing individual sustained notes by means of an audiooscillator, introducing harmonic content to simulate desired instrumental tones, providing means to produce a progression of instant amplitudes of the tones to represent the characteristic rise and decay of the instrument simulated, prerecording a control track with pulses at intervals corresponding to the advent times of a particular note occurring in the musical score to be recorded, and utilizing said pulse record to initiate the recording of the particular note in correct time sequence on a record for subsequent reproduction. In the embodiment described with reference to Figs. 1 and 2 a control desk 1 is provided with. an audiooscillator 3, a gain control 43, a polyharmonic generator 4, a master switchboard 5 and an auditioning loudspeaker 8 by which means note' of desired pitch and tone is experimentally obtained. The production of an individually played note is then obtained by means for producing a progression of instant amplitudes of the sound comprising a cathode ray tube 28 bearing a supersonic trace 21 from an oscillator 27, a translucent screen 29 having an experimental rise and decay trace 20 drawn thereon by a grease pencil, whereby a rapid series of pulses of varying time interval are picked up by a photo-cell 22 in a hinged lightexcluding cover 23 to control the instant volumes of sound by means of a variable transmission element 37, from which the signal is controlled, amplified and recorded by mixer 44, amplifier 45 and head 11 respectively. The supersonic signal image is biased off the screen until a single sweep is initiated by the single sweep adjustment of the oscilloscope. An auditioning push button 35 initiates the sweep action for single horizontal scans, the time of the scan depending on the time-constant selected in accordance with the duration time of the note required. The variable transmission unit 37 is designed in two parts; a variable gain amplifier and a trigger circuit which may be connected to control said amplifier. The combination is arranged to integrate the pulse and be conductive in the rectangular display region below the decay contour 20 and switch on deleting or blanking potentials whenever the spot leaves this region and is above the envelope contour 20, and thereby serves to control the strength of the audio-signal accordingly. When suitable tone and decay characteristics have been obtained, initiating pulses are recorded by a head 34 on a control track and spaced in accordance with the time intervals at which the note appears in the score. This control track may be on a separate record tape 16, Fig. 1, or may be on a part of the medium 12 on which the sound is to be recorded. The switch 32, Fig. 2, is moved to the left hand position during a recording operation so that the pulses picked up by head 34 initiate single sweeps on the oscilloscope and effect the recording of the note at correct time intervals on record tape 12. Other frequencies of the same "touch" and character may be produced by simple adjustment of the audio-oscillator 3. When recording further notes, the first recorded sounds are picked up by head 9 and mixed in mixer 44 with the new signals to be recorded and recorded together at head 11, the original signals being erased at head 10 energized by supersonic oscillator 27. Instead of using the oscilloscope for the "shaping" of the signal, an envelope contour obtained from an actual sound source 40 by an audio envelope detector 38 may be injected into the variable gain part of the variable transmission element 37. To avoid undue loss from noise by too many rerecordings, the number of notes of different pitch which may be prepared by the above mode will require to be limited. A modification of the above mode may therefore be employed wherein all non-current notes which are to have the same harmonic structure and the same envelope are grouped by each of the pitches prescribed in the score. The required advent times are pencil marked on the tape and followed by the recording by head 34 of all sweep-firing signals. The magnetic tape is positioned to the advent marks to include an acceleration footage. The audio oscillator signal at each pitch that is required is recorded for a time that is longer than the intended lengths of the final notes. On playback from head 9, these various sustained recorded tones are switched by switch 52 to feed the polyharmonic generator 4 through amplifier 41, switch 42 and gain control 43 and enclosed in the prepared sound envelope contour and re-recorded by head 11. The time interval signals initiate the whole series of envelope decay periods of one type used in the score thereby. re-recording notes of different frequencies on the same track in one operation.