Method for Recognizing Tonal Frequencies Using AM-PM-VSB Modem
Original Publication Date: 1985-Jan-01
Included in the Prior Art Database: 2005-Feb-18
The present method recognizes frequencies of tonal signals by using AM-PM-VSB modulation logic adapted to the Group Two facsimile communication protocol recommended by CCITT. In the above protocol, tonal signals are used as control signals to establish a link between two facsimile terminals, and image data are modified by the AM-PM-VSB method and transmitted from one facsimile terminal to the other terminal. Each tonal signal has a specific frequency and duration to distinguish itself from the others. In the present method, a certain tonal signal sin 2 ffit is added to a reverse carrier frequency signal sin (2 ffot + f) in the AM-PM-VSB modem to provide the following signal: sin 2ffi(t) + sin (2ffot +f) = 2 sin (fi - fo)ff . cos (fi + fo) ft where fo and fi are frequencies of the carrier and tonal signals, respectively. Fig.