Manchester Codes

Laserdiscs defined special encodings of their own. Line 11 of each field contains what is known as the "white flag", which is a simple binary indicator of whether the current field is the first field of a frame. Laserdisc players look for this white flag to know how to do a still frame that doesn't consist of a split between two film frames. Since it takes two fields to make one frame, and since many film-based laserdiscs are encoded in a 3:2 cadence, it is not straightforward to ensure this without some additional information.

Even more interesting than the white flag are the "Philips codes" that are encoded on lines 16, 17, and 18 (note that all the laserdisc encodings are on lines different from Closed Captioning, so that laserdiscs could be encoded with Closed Captioning data as well). The Philips codes are special 24-bit digital values that are encoded in the VBI area of each field, and which are used to describe the field. In general, there are up to two Philips codes encoded on three lines, with line 17 generally being a redundant copy of line 18 for added resilience against dropouts or other transmission errors.

So how do you encode a 24-bit binary number into a single scanline of an analog video signal? The answer is to use an encoding technique known as Manchester codes. If you treat bright white as a '1' bit, and black as a '0' bit, then this encoding uses two bits to encode each bit of the code. The reason for two bits is to ensure that the decoded value makes sense, and also to establish a clock for the codes. This works because Manchester codes encode a '0' bit as 10 and a '1' bit as 01. Because of this, you are guaranteed that in the middle of each bit there is a transition from black to white or vice-versa, and this can be used to establish the clock. Once you figure out the clock, then the direction of each transition tells you what the actual value of the bit is.

Below is a picture of the VBI region for a CAV laserdisc. The top few rows are an ugly green because those lines of VBI data were not provided by the capture card. The remaining lines show what the white flag and Manchester-encoded Philips codes look like:

Once the Philips codes are extracted, they can be treated as binary data and evaluated. Unfortunately, it seems as though most of the information about what the various Philips codes mean has been kept relatively secret. However, the most important codes are understood:

Identifyer	Usage	Discription
$88FFFF	Lead-in code	indicates the field is located before the official start of the disc
$80EEEE	Lead-out code	Indicates the field is located after the official end of the disc
$FXXXXX	Frame code	specifies the 5-digit frame number (XXXXX) in BCD format
$8XXDDD	Chapter code	specifies the 2-digit chapter number (XX) in BCD format
$82CFFF	Stop code	indicates the player should pause at the current field

So as the player reads data from the disc, it also needs to detect the white flag and Philips codes, and act on them as necessary. By capturing the VBI data along with the normal active video, we are able to preserve this information, and write a laserdisc simulator that operates off of the same information that the original players did.

Credits

Aaron's Almanac Live / Archive

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