VizmayaTech

What is VBI?

VBI stands for Vertical Blanking Interval. In analog video display, such as a CRT, a video frame is shown by drawing horizontal lines from the top to the bottom of the screen. Once a frame is completely drawn, the beam has to travel back to the top left, to start drawing the next frame. Since there is nothing to show while the beam retraces, this interval is blanked and hence called the vertical blanking interval.

VBI Data

During vertical blanking time, no video is shown. So the display is not consuming anything from the incoming video signal. This interval effectively leaves open a chunk of bandwidth that can be used for data communication. To keep things simple, the horizontal lines (referred to as just ‘lines’, henceforth) are continued to be transmitted during the vertical blanking interval, but the lines do not contain video. Each VBI line, or group of lines may contain data, encoded as per a variety of standards. The most common used VBI service is the closed caption service, which is used to show text subtitles.

To ensure that VBI data goes wherever the video goes, VBI data is actually encoded as video. This allows VBI data to be transmitted transparently across any carrier media (Over The Air (OTA), cable, satellite, fibre optic cable etc) that is designed to transmit the video signal.

Raw and Sliced data

The hardware that receives VBI data may just sample the VBI lines to collect the data bits, with no knowledge of the type of data it contains. This is called raw data. The raw data may then be processed by software to interpret the contents.

The other possibility is to have the VBI hardware interpret the VBI data, based on the type or standard used on the line(s). The output in this case is extracted information such as closed caption characters, or data packets. This is called sliced data.

Typical VBI data extraction hardware supports both modes. The ’sliced’ mode is much more useful, since a lot of low level functions such as data extraction , error correction are handled by the hardware. Raw data is primarily used to work with non standard VBI data, and for re-encoding unmodified into an output video signal.

A note on Interlaced video

NTSC analog video uses interlaced video. This means that even though the video has 30 pictures (frames) per second, the video is transmitted as 60 fields by splitting each frame into an odd and even field.

How much data can VBI carry?

In NTSC, the VBI consists of 21 lines. Lines 1 to 9 are ‘reserved’ for television timing signaling. Lines 10 to 21 are available for data transmission. Line 21 is widely used for closed captioning, hence the available lines are 10 – 20, a total of 11 lines.

Each VBI line can transmit up to 288 bits. At a rate of 60 fields per second this amounts to a raw line data rate of 17, 280 bits per second. Note that actual useful data rate would be much lower, due to packetization and error correction overheads.