Sound must be played with accurate timing, even when stepping forward or back by single frames. A few seconds are necessary so the user can tell where he is, but a keystroke or mouse click must immediately stop the sound and potentially start it at a new location. It is common in searching for a cut point to repetitively step forward one frame at a time to find the correct audio spot. The small, monochrome pictures and telephone sound on disk are only used to let the editor make decisions. Given the final edit decision list, the computer shuttles the VCRs around to copy the desired sections of tape, VCR to VCR, thus maintaining maximum picture and sound quality.
Professional systems incorporate a digital time code telling the position of each video frame.
Consumer camcorders not only lack that, but vacation videos frequently have totally blank sections of tape so simple frame counting will not work. A one-time calibration step lets the computer determine the mechanical delays and timings of the VCRs. Other, proprietary techniques are also used. Ada's encapsulation mechanisms nicely handle the plethora of different hardware. There are multiple bodies corresponding to different supported hardware.