Analog cameras produce video signals which represent spatial image brightness variations as a time-varying voltage. The output signal of such cameras usually includes timing information to indicate the beginning of each scan line and the beginning of a frame (or field in interlaced cameras). Typically, analog camera waveforms comply with one of the analog video standards such as NTSC or PAL. More general standards are also somtimes supported.
The framegrabber detects the timing signals and then digitizes the voltage waveform to produce a string of digital values for each line and coding to indicate the start of each frame. This data string is then passed to the computer. Analog framegrabbers may also include outputs to provide functions such as scan timing or triggering to those cameras that can accept these signals.