Patent GB-1048281-A (GB)
Title: Signal bandwidth compression apparatus
Inventor: SEKEY ANDREW ROBERT
IPC: H04B1/66
Filed: 1963-04-05
Abstract: 1,048,281. Bandwidth compression systems; televising speech spectrographs. NATIONAL RESEARCH DEVELOPMENT CORPORATION. April 2, 1964 [April 5, 1963], No. 13703/63. Headings H4F and H4R. In a bandwidth compression system the speech signals are displayed as a time-frequency spectrogram and this spectrogram is scanned, e.g. by a television camera, at a rate slower than the spectrogram is produced except at times when the total energy in the speech is below a predetermined minimum when the scanning is speeded up and a signal transmitted indicating the duration of the speeding up. Fig. 1 shows a transmitter for the system in which the speech input signal is fed at 1 to the inputs of a number of band-pass filters as in a conventional channel vocoder. The outputs of the filter are multiplexed on to one line, quantized into a number of discrete levels in quantizer 5 and applied as brightness modulation to the cathode-ray tube 4. The vertical line scan on the face of the tube 4 is synchronized with the switching frequency of the multiplexer 3 so that a particular vertical position on the screen of the tube corresponds to a particular frequency band in the speech input, while the horizontal frame scan is a constant lower frequency scan so that the brightness modulation builds up a spectrogram of the speech on the face of the c.r.t. In addition the filter outputs are connected to a unit 6 which detects time periods when the sum of the filter outputs is below a predetermined value and for such periods causes the scanning spot of the c.r.t. 4 to be brightened at the end of each line scan to produce a bright line at the top of the spectrogram. A T.V. camera 8 scans the spectrogram so produced at a speed which is lower than that at which the spectrogram is produced, except at the times, indicated by the bright line at the top of the spectrogram, when the energy present is below a certain threshold when the scanning is speeded up to above the rate at which the spectrogram is produced. The resulting television signals may be transmitted via a bandwidth compressor of the type described in Specifications 979,574 and 1,046,779. Since the brightness of the image on the tube decays and the scan of the television camera has a variable delay relative to the scan of the c.r.t., the output from the camera has to be compensated in unit 11 to a degree determined by unit 9 which compares the c.r.t. scan with the camera scan. In addition if the scan of the camera lags the c.r.t. scan by an amount approaching one complete scan a unit 9 causes the camera scan to be advanced by a small amount to omit a small portion of the c.r.t. picture. At the receiver, Fig. 2, the information from the camera is displayed on a c.r.t. 21 the scan of which is made similar to that of the camera 8 at the transmitter so that the picture on the face of the tube 21 duplicates the picture on the o.r.t. 4 at the transmitter. A T.V. camera 22 scans the face of c.r.t. 21 with linear scans and the output from the camera is compensated for brightness changes due to the camera scan lagging the c.r.t. scan by the same method as at the transmitter, using an a.g.c. circuit 23 controlled by the scan comparing device 24. The compensated output is fed via a demultiplexer 25 to a bank of modulators and filters as in a conventional vocoder system. The excitation signal at both the transmitter and receiver may be a baseband portion of the speech or a conventional vocoder excitation function system.