The LaserDisc Technical Page

By Espen Braathen - Home Cinema Editor Audio Video magazine

NTSC Format

NTSC format LaserDisc

Video signal is frequency-modulated on a carrier of 8.1 MHz (black level).
Sync-tip = 7.6 MHz.
White peak = 9.3 MHz.
J0 = main component.
J1 = first-order lower sideband (chroma band), situated 3.58 MHz from the J0 main component.
J2 = second-order lower sideband.
The inter-carrier gap between J2 and J0 is 3.58 + 3.58 = 7.16 MHz.

Audio FM carrier frequency = 2.3 MHz (CH1) and 2.8 MHz (CH2). Frequency deviation of the analog audio carriers is +/- 100 kHz.

The bandwith of NTSC LaserDisc is according to this figure 9.3 MHz - 4.4 MHz = 4.9 MHz. The figure is already way past the 4.2 MHz broadcast NTSC limit. However, the frequency spectrum shows an empty area between J1 and the two audio carriers. It is therefore possible to modulate the FM video carrier with luma information above the chroma information. The 425 TVL horizontal resolution figure given in the LaserDisc technical specs implies that the bandwith is close to 425/78.5 = 5.4 MHz. A visual check with the resolution test pictures on Video Essentials confirms this fact. Thus, the luma side-band extends down to at least 4 MHz.


PAL Format

PAL format LaserDisc

Video signal is frequency-modulated on a carrier of 7.1 MHz (black level).
Sync-tip = 6.8 MHz.
White peak = 7.9 MHz.
J0 = main component.
J1 = first-order lower sideband (chroma band), situated 4.43 MHz from the J0 main component.
J2 = second-order lower sideband.
The inter-carrier gap between J2 and J0 is 4.43 + 4.43 = 8.86 MHz (the second-order lower sideband is mirror inverted relative to the frequency zero point, because 7.9 MHz - 8.86 MHz results in a negative value).

Audio FM carrier frequency = 0.683 MHz (CH1) and 1.066 MHz (CH2).
(The gap between J1 and J2 is smaller than for NTSC, so the audio FM carriers had to be placed below J2.)

The bandwith of PAL LaserDiscs is 7.9 MHz - 2.4 MHz = 5.5 MHz.


NTSC Format LD with Digital Sound

NTSC format LaserDisc with digital sound

The digital audio sound signal was added to LaserDisc by using the low-end frequency range up to 1.75 MHz. The interference caused by J2 must be removed. This is done by a compensation method where the J2 component is cancelled out in the FM modulator.

Dolby AC-3

After the introduction of digital sound (and some years later when most analogue sound only players had been replaced) the analogue audio carriers was no longer needed as the carrier of the main soundtrack. This led to the use of the analogue tracks for secondary audio programes (MultiAudio) with directors comments and/or isolated music tracks.

However, a new and more interessting use for this part of the LD's frequency spectrum was found in 1995: Dolby Digital. By replacing the right analogue carrier with a 500 kb/s QPSK-modulated (Quadrature Phase Shift Keying) carrier containing the 384 kb/s AC-3 (Audio Code #3) bitstream, the sound quality from NTSC LaserDisc was upgraded for the third time: multichannel 5.1 audio. (The 500 kb/s bitstream includes error correction data. The bandwith of the QPSK-signal is about 250 kHz).


PAL Format LD with Digital Sound

PAL format LaserDisc with digital sound

In contrast to NTSC LaserDiscs, the FM audio carriers on PAL LaserDiscs must be removed because they exists in the same frequency range as used by the digital sound signal. J2 is also removed with the compensation method.


Resources:

Multi-Disc players - Principles of Operation and Circuit Descriptions by Jan Maes/Sony Service Senter Europe.
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HTML editing og © Copyright 1996-97 Espen Braathen.
Updated Jan 2, 1997.