First of all, a quick lesson in *TV. It takes one or two parameters. The first is the vertical shift offset from the "norm" which is 0. Numbers starting from 1 and increasing move the picture up by one line at a time; numbers starting from 255 and working down move the picture down the screen by one line at a time.

*TV 255 is the most likely setting you might come across, if it is being used at all, because a lot of 1980s tellies cut off the very top line of the screen, so the display needed to be moved down a line. In fact, some commercial software uses a TV 255 setting as a matter of course.

The second parameter is what we're interested in for this picture set, though, which is the interlace setting.

A PAL video frame is made up of 625 horizontal lines. The frame is divided into two fields, one containing the even numbered lines, the other containing the odd numbered lines. Interlacing is the technique of alternately displaying each of the two fields to a video frame as the way of generating and updating a video picture. Without interlacing, all the horizontal lines are displayed progressively which can result in a less smooth picture.

The Domesday System requires the use of video interlacing to genlock the BBC picture to the laserdisc picture, and a check for interlacing is built into VFS.

In the crazy and cockeyed world of 8-bit computing, a *TV interlace setting of 0 actually turns the interlace ON, and a *TV interlace setting of 1 switches it OFF.

As we can see in this picture, my Domesday Machine is configured with no vertical shift and with interlace OFF.

Click here to return to Interlace