How is LaserDisc "analog"?

LD vs. DVD: A Fundamental Difference

Despite Similarities, LD Technology Is Not Digital
Page Edition: 1999.07.03

This article is presented here as it appeared, in slightly updated form, in the June 1999 edition of Replication News, which had apparently vanished when this site was migrated in 2002.

by Bob Niland

Editor's note: Nearly everyone is in agreement that DVD is quickly replacing the laserdisc as the videophile format of choice. Despite both technologies utilizing a laser that reads pits, they are decidedly different beyond the obvious distinction (i.e., the size of the disc, 12-inch laserdisc vs. 5-inch DVD). The author explains why the video data on a laserdisc is not digital.

Despite what some legacy advertising might have led you to believe, laserdisc is not digital video. It's not always digital audio, either. This is not surprising considering that the original "LaserVision" was basically a 1972 technology. LD video is pulse FM.

There is a single track consisting of a continuous spiral of pits and "lands" (non-pits). Encoded on that track are the composite video (luminance, chrominance, sync, info), one or two analog audio channels and a RedBook-style stereo digital PCM audio or DTS digital audio data stream. Dolby AC-3 audio may also be present, and replaces one of the analog audio carriers. Side, track, time, frame and other control signals are encoded, during pre-mastering, in both the Red Book subcodes (if present) and in non-displayed scan lines in the vertical blanking interval of the composite video signal.

The pre-mastered base band composite video signal (about 4.2 MHz wide for NTSC) is used to modulate an FM carrier signal with a nominal frequency of 8.5 MHz. The amplitude of the NTSC signal deviates the FM carrier over the range 7.6 MHz (video sync tip) to 9.3 MHz (video white level).
The two analog audio channels each modulate a pre-emphasized FM audio subcarrier (2.3Mhz 1/left, 2.8MHz 2/right).
The Red Book-style digital audio bits are pre-emphasized and used directly. They represent a signal envelope with a bandwidth under 1.4 MHz.
The audio content is all attenuated 26dB.
The three FM carriers (video, analog 1/L, 2/R) and the digital audio are all summed, then clipped.

The resulting clipped signal looks like square waves, and the low level, low frequency components (analog and digital audio) cause a variation in +/- duty cycle. This signal is then more-or-less directly recorded as pits and lands.

In over-simple terms, the video signal is represented by the pit-to-pit spacing and the audio is represented in the difference between pit/land length.

Bob Niland, a laserdisc enthusiast, maintains a website at
www.access-one.com/rjn/