Editing Video tape recorder (section)

==Technology==
{{Unreferenced section|date=April 2022}}
[[Image:Video tape scanning diagrams.png|thumb|upright=1.4|Scanning techniques used in video tape recorders. (A) Transverse scanning used in the early [[2-inch quadruplex videotape|quadriplex]] system requires several vertical tracks to record a video frame. (B) [[Helical scan]], by recording in long diagonal tracks, is able to fit a full video field onto each track. The first full-helical system uses one head, requiring tape to wrap fully around the drum. (C) Half-helical system with 2 heads only requires tape to wrap 180° around the drum.]]

In the technique used in all transverse-scan video tape recorders, the recording heads are mounted in a rapidly spinning drum which is pressed against the moving tape, so the heads move across the tape in a transverse or nearly vertical path, recording the video signal in consecutive parallel tracks sideways across the tape. This allows use of the entire width of the tape, storing much more data per inch of tape, compared to the fixed head used in audio tape recording, which records a single track down the tape. The heads move across the tape at the high speed necessary to record the high-bandwidth video signal, but the tape moves at a slower speed through the machine. In addition, three ordinary tracks are recorded along the edge of the tape by stationary recording heads. For correct playback, the motion of the heads has to be precisely synchronized with the motion of the tape through the capstan, so a control track of synchronizing pulses is recorded. The other two tracks are for the audio channel and a cueing track.

The early machines use the Ampex [[quadruplex videotape|2-inch quadruplex]] system in which the drum has 4 heads and rotates at 14,400 RPM perpendicular to the tape, so the recorded tracks are transverse to the tape axis. With 2-inch tape this requires 16 tracks for a single analog NTSC video frame or 20 for a PAL frame.

The helical scan methods use a recording drum with a diagonal axis of rotation. The tape is wrapped longitudinally around the drum by idler wheels, so the tape heads, instead of moving across the tape at almost 90° to the direction of motion as in the quadruplex system, move across the tape at a shallow angle, recording a long diagonal track across the tape. This allows an entire frame to be recorded per track. This simplifies the electronics and timing systems. It also allows the recorder to be paused (freeze-framed) during playback to display a single still frame, by simply stopping the tape transport mechanism, allowing the tape heads to repeatedly pass over the same track.

This recording technique has many potential sources of timing errors. If the mechanism runs at an absolutely constant speed, and never varies from moment to moment, or from the time of recording to the time of playback, then the timing of the playback signal is exactly the same as the input.  However, imperfection being inevitable, the timing of the playback always differs to some extent from the original signal. Longitudinal error (error arising from effects in the long direction of the tape) can be caused by variations in the rotational rate of the [[capstan (tape recorder)|capstan]] drive, stretching of the tape medium, and jamming of tape in the machine. Transverse error (error arising from effects in the cross-tape direction) can be caused by variations in the rotational speed of the scanning drum and differences in the angle between the tape and the scanning heads (usually addressed by video [[video tape tracking|tracking]] controls). Longitudinal errors are similar to the ones that cause [[wow (recording)|wow]] and [[flutter (electronics and communication)|flutter]] in audio recordings. Since these errors are not so subtle and since it is standard video recording practice to record a parallel control track, these errors are detected and [[servomechanism]]s are adjusted accordingly to dramatically reduce this problem.