Actually, this should be titled "Playing Back", as that's the more difficult part of the operation. Lets begin by imagining that we are all comfortable with recording audio signals on an audio tape.

Early attempts at recording video using audio recording techniques were problematic due to high frequencies involved..... audio ranges upward to 20,000 cycles per second (20 Khz), but Broadcast video requires an upper limit of 4,200,000 cycles per second (4.2 Mhz)!

Increasing the tape speed is the easiest way of recording faster fluctuations. One author calculated that the video recorder would need a reel of tape 14 feet (4.3 meters) in diameter.

To make a long story short, that's basically what happened! To keep the reel size manageable, a wide tape was wrapped around a drum of spinning record/play heads. The effect was to draw many fine "tracks" on the wide tape. The first practical recorder stored 35 Kilometers of tracks on a 15 inch reel of two-inch-wide tape... one hour of video.

Helical Recording

Today, all video tape recorders use a tape helically wrapped around a spinning drum. It's "helical" because the shape is one half of one coil of a helix. The name made a little more sense on the first machines that used a full wrap. In half-wrap machines, at least two heads 180 degrees apart are required on the drum... as one spinning head is leaving the tape, the other is just entering.

There is a problem with the helical recording scheme.... playing the tape back! The head drum records one field per track, so on a half-wrap machine the drum is spinning at one turn per frame. During playback, the video head must land back on the tracks it recorded, so during record, a sensor on the drum creates a signal that represents the rotational position of the drum. This signal is recorded along the edge of the tape, and is called the Control Track. In playback, the control track is used to control the rotation of the drum so that the heads fall on the video tracks. You can think of control track as electronic sprocket holes. The Tracking control allows an adjustment of the tracking system, as the control track heads may be in slightly different positions on different machines.

Stability and Timebase Correctors (TBC's)

Here, a picture is worth a thousand words. Have a look at a helical machine's timing stability. 

The fix for this is the Timebase Corrector, or TBC. Many machines found around a TV studio have the TBC built in to them.

The timebase corrector does its thing with computer ram (random access memory). The video is digitized into computer data, then read into the memory with the sloppy horizontal timing of the playback tape. The digits are read out of the memory using pulses generated by a genlocked sync generator. The digits are converted back to analog video at the output. Advanced Sync There is a nominal delay of one half the TBC's memory length... if the TBC's memory can store 16 lines of video, video will come out the end 8 lines late. (This allows the error to be plus or minus eight lines "big"). For this reason, TBC's supply an advanced sync signal to be fed to the tape player.  Advanced sync makes the player play video earlier in time, so that it comes out of the TBC's delay at the correct time.

Frame Synchronizers

Frame Synchronizers and TBC's are close cousins. Recall the delay through a TBC, and the advanced sync trick. If we tried to time-correct a remote feed with a TBC, it would get retimed horizontally, but might well have a big vertical sync interval in the middle of the picture.... and we can't send advanced sync to the remote location (this used to be done, and it is a timing nightmare).

So... plug enough RAM (memory) into the TBC so that it can store an entire frame. This is the equivalent of pointing a locked camera at a monitor with the un-synchronous feed, except it's a lot prettier. The nominal delay through the frame-sync is one complete frame. If the incoming video is slower or faster than the local sync generator, frames will occasionally be dropped, or repeated as the synchronizer runs out of correction "room". This generally only happens once every few minutes.

Frame synchronizers can also function as timebase correctors, but some models can't handle the rapid timing errors from helical tape machines, and are only for retiming slowly drifting remote feeds.

Frame synchronizers can display freeze-frames, thanks to their full frame of RAM. There is an option for a freeze frame or field.

A freeze FRAME of real video is rarely satisfactory.... any motion that occurred during the two field scans will flicker horribly as the two interlaced fields are repeated over and over. Syncronizers have an option to display a freeze-field, that gives more pleasing results.... only half the vertical resolution, but no motion flicker.

TBC's and Frame Synchronizers timing and picture adjustment controls

Before TBC's and Syncronizers, only "processing amps" had these controls, and they are still referred to as "proc amp" adjustments.

These controls go by slightly different names on different units. They are fully explained below the pictures.

It is extremely important to understand the relationship between HUE and SUBCARRIER PHASE... they seem to do the same thing. The HUE control affects only the picture (the dots in colour bars for example). SUBCARRIER PHASE adjusts the entire machine's phase.... burst, dots, picture, and all. It is used to time the colour burst to the production switcher. Once that is done, the HUE control rotates the picture phase into position if necessary.

• Video, Chroma, and Pedestal are explained with the pictures.
• Hue is the same as "tint" on a TV set. Use it to align the colour bar dots after everything else is timed properly.
• H-Pos (horizontal position) is found on frame syncs. Use it to center the picture if necessary after the horizontal timing (phase) is done ie. the horizontal sync pulse timing of the frame sync to the production switcher is correct.
• H Phase. This is the timing adjustment of the TBC or Framesync to the switcher. See "Timing".
• Subcarrier Coarse. This will be a four position switch used to get the colour burst timing in the ballpark.
• Subcarrier Fine is used to time the colour burst of the unit to the switcher.