公开(公告)号：US3921204A

公开(公告)日：1975-11-18

申请号：US28180772

申请日：1972-08-18

Applicant: POST OFFICE

Inventor： THOMPSON JOHN EDWARD

IPC: H03M3/02 ,  H03M3/04 ,  H04N7/32 ,  H04N11/04 ,  H04N5/40 ,  H04N9/40

CPC classification number: H04N11/046 ,  H03M3/04

Abstract: Methods of encoding in digital form an analogue signal including a modulated sub-carrier wave, such as a colour television video signal, are described in which the rate of sampling of the analogue signal is a simple factor, for example 3, times the frequency of the sub-carrier wave. The methods of encoding include comparing the instantaneous value of the analogue signal with a previously occurring value, which conveniently is spaced by one or more cycles of the sub-carrier from the instantaneous value, and then encoding the difference between the two values. From an N.T.S.C. signal the spacing between the two values may be one cycle of the sub-carrier wave, or about one line of the scan, (the actual spacing being an integral number of cycles of the sub-carrier wave, or both differences can be combined to produce a diagonal difference signal. The same spacings can be used for a PAL signal if a PAL modifier is used, otherwise a vertical spacing of two lines is necessary because of the change of phase of the sub-carrier in alternate lines. The differences from several spacings can be combined trigonometrically to synthesise a difference corresponding to a desired spacing, such as one cycle of the sub-carrier wave.