Method and apparatus for determining line frequency and detecting

Abstract

A line frequency is determined by interleaved sampling of ambient light to obtain analog voltage samples at predetermined sampling rates associated with two or more line frequencies. Where there are two line frequencies, one method separates the first samples by a time period at least equal to an analog-to-digital conversion time, and less than the earliest time when a sample associated with one line frequency would coincide with a sample from the other. There are n samples per flicker frequency period and Q samples for each line frequency are taken. In a second method, a first common sample is taken for all line frequencies, and a condition characterized by simultaneous samples for different line frequency sample groups triggers a common sample for all line frequencies. For both methods, digital sample values one through Q-1 for each group are operated on to obtain a result, and then two through Q are similarly operated on to obtain a second result. The result for samples one through Q-1 for a given line frequency is compared to the result for samples two through Q. If the results for a given line frequency are within a predetermined limit, the presence of that line frequency is indicated. The known line frequency can be used to set the light sampling rate for exposure setting, as well as imprinting light source information onto photographic film.