Time base correctors and the Sony BVT-810

A videotape machine mechanically scans the tape using a rotating tape head. This function introduces errors in the signal, known as time base errors. These are recorded on the tape and are prevalent in small formats like 1/2" open reel and U-matic.

To correct these errors, a digital device called a time base corrector or TBC was developed in the mid-1970s. The output of a videotape machine is sent to the input of the TBC. The TBC writes the unstable signal into a memory buffer and reads it out, stabilizing it with its own crystal-stable clock. It is like capturing a frame of video, or a portion of a frame, stabilizing it and instantaneously sending that frame to the video out. The TBC then does it all over and over again faster, than 30 frames per second. Older TBC would handle 16 lines at a time. Today most external TBCs are also frame buffers, and are known as full frame TBCs. Because these devices have a frame buffer built in, manufacturers usually add image processing features like freeze frame and strobe.

While correcting time base errors is the primary purpose of TBCs, they also perform another vital function. TBCs provide controls for changing the brightness, contrast, hue and color saturation of a video signal. On the control panel, these functions are usually identified as Luma (brightness), Setup (contrast), Hue (color) and Chroma (saturation).

BAVC uses a Sony BVT-810, a 16 line TBC originally used with BVU-800 U-matic tape machines. A TBC that is not full frame requires that the source tape machine be fed an "Advanced Sync" signal from the TBC into a "sync in" of the tape machine. The BVU-800 U-matic has a "sync in" BNC connector. The "video in" of the AV-8650' doubles as a "Sync in." The CV-2100 does not have a "Sync in" and so it cannot be directly used with the BVT-810.

The BVT 810 is an older model TBC. This offers an advantage because full frame TBCs have an inherent flaw when processing weak and unstable video signals. If the full frame TBC cannot process the video correctly, it strobes the image. Since the BVT-810 is only processing 16 lines at a time, it is more likely to be able to address image problems which a full frame TBC cannot.

The DPS TBC is a full frame TBC that is designed to either be mounted in an ISA expansion slot of a PC or Amiga and controlled through software or in a DPS control box with external control switches and buttons.

BAVC chose the DPS TBC because of its functions and price. Early attempts by BAVC to use a much more expensive TBC (Prime Image 600+) with 1/2" open reel had been unsuccessful. The DPS TBC has worked well as an alternative to the BVT-810.

Because it is a full frame TBC, the DPS requires only a video input connection to the source video tape machine. Unlike the BVT-810, this TBC will work correctly with a video signal coming from the CV-2100.

BAVC's DPS TBC is mounted in a DPS box along with a DPS Vscope, a card that provides vectorscope and waveform monitoring.