Non-coherent frequency shift keying detection scheme

A non-coherent frequency shift keying detection scheme. The invention has been applied to the special case of capturing caller ID information when the voice-band modem is in the low power mode. The technique provides the capability for capturing and decoding enough of the caller ID information transmitted by the telephone company while the computer is in the sleep or low power mode, and upon awakening the computer can use this captured information to decode the actual caller ID information. Additionally, this invention automatically accommodates both the Bell 202 or V.23 transmission standards by utilizing a threshold concept in which the time that has elapsed between the zero crossings of an FSK tone half cycle are measured. The elapsed times may be used as an indication of which FSK tone has been transmitted thus allowing capture of caller ID information. The bit output of the caller ID capture circuit is output for storage by RAM thus allowing use by the computer when it wakes up.