Patent GB-645542-A (GB)
Title: Improvements in and relating to the transmission of speech
Inventor: GABOR DENNIS
IPC: G10L21/00
Filed: 1947-02-11
Abstract: 645,542. Recording and reproducing sound. BRITISH THOMSON-HOUSTON CO., Ltd. Feb. 11,- 1947, Nos. 3967 and 31339. [Class 40 (ii)] [Also in Group XXXIX] In a speech transmission system using frequency compression and expansion substantially as described in Specification 616,156, the preferred frequencies of the system are automatically varied so as to follow the pitch of the speech. The preferred frequencies P are the frequencies at which the system gives most faithful reproduction and are related to the compression factor K and to the frequency S at which the record is scanned by openings in the scanning band, Fig. 8, by an expression of the form P(I-K)=n. S where n is an integer. As P varies, the integer n may be changed to confine the frequency S within a limited range. A sound record A, Fig. 8, and a scanning band B are passed between lenses of the optical system at different speeds. An attenuator G between the record and the scanning band is transparent in the centre and gradually becomes opaque towards the ends. The scanning band may be an opaque film punctured with holes produced by a sparking electrode operated at the scanning frequency S. The scanning frequency is derived from the pitch of the original speed by the arrangement shown in Fig. 5. The pitch filter selects a single substantially pure frequency representing the pitch of the speech. The signal generator produces a constant amplitude pulse in every cycle of the pitch frequency providing that it exceeds a minimum amplitude. The counter carries out a frequency division by producing a pulse signal for every fifth pulse counted and the signals control a multivibrator which locks in with the counter signals. In the absence of signals, the multivibrator vibrates at a frequency below the smallest scanning frequency required. If the signal frequency gets too high, the multivibrator locks in with every other signal, thereby confines the scanning frequency to a limited range and in effect changes the integer n already referred to. The multivibrator provides the scanning frequency in the compressor. At the receiving end the process is reversed to expand the frequencies. Scanning- devices and temporary sound records. For continuous transmission, as in telephony, a temporary record is used. A temporary record may be formed on a phosphorescent drum D, Fig. 9, by means of a source of ultra-violet light of variable intensity. An opaque coating of ink on a transparent drum forms the scanning band E and the slits are formed by means of a continuously driven scraper G, Fig. 10, which is moved into the cutting position by pulses from the multivibrator energizing a winding H. In another arrangement (Fig. 11, not shown), the multivibrator signals produce bubbles of gas by electrolysis in a flowing liquid ; the bubbles serve as scanning slits. A temporary sound record may be produced by means of a rotating transparent ring prism I and a slightly adhesive light scattering tape J, Figs. 12, 13. The combination of tape and prism transmits light only when optical contact is produced by the operation of a small hammer L at ultra-sonic frequency on a needle M which vibrates longitudinally in response to the sound signal. The sound record may be as a dark trace on a rotating cathode-ray tube Q, known by the Registered Trade Mark " Skiatron," Fig. 14. For scanning purposes, one or more cathoderay tubes N are fixed in front of the " Skiatron " and images of their screens are projected on to the " Skiatron " screen through an objective O. Light from the combined sound and scanning record is collected through a window R by an annular photo-cell surrounding the objective. The sound may be recorded as a charge picture on the narrow rotating mosaic drum of an iconoscope (Fig. 16, not shown), which may be scanned by one or more cathode-ray beams operated at the scanning frequency. The sound record may be formed as a running wave on a string loaded with light reflecting reeds (Figs. 17 to 20, not shown), and the scanning may be carried out by applying the scanning pulses to a second string carrying masking reeds. The pulses deflect the masking reeds to enable the sound record to be scanned. In a further modification, Fig. 1 (Prov.), an electron gun 15 is used to record sound as a variable width charge record on a rotating annular mosaic 8 of the type used in television cameras. Images of a flash tube 2 are thrown on to a phosphorescent disc 1 in accordance with the scanning frequency and the images are in turn projected on to the charge record through a graduated attenuator (not shown), and through a transparent plate 10. Photoelectrons are emitted all along the scanning line of the charge record but only those emitted from the uncharged portions are collected by a thin transparent conducting layer 11, Fig. 3 (Prov.), applied to the insulating plate 10 and maintained at a potential less positive than the charged portions of the image record. The record is wiped out by means of a wiping cathode 17.