Sound could be stored in either analog or digital format and in a variety of surround sound formats; NTSC discs could carry two analog audio tracks, plus two uncompressed PCM digital audio tracks, which were (EFM, CIRC, 16-bit and 44.056 kHz sample rate).[18] PAL discs could carry one pair of audio tracks, either analog or digital and the digital tracks on a PAL disc were 16-bit 44.1 kHz as on a CD; in the UK, the term "LaserVision" is used to refer to discs with analog sound, while "LaserDisc" is used for those with digital audio. The digital sound signal in both formats are EFM-encoded as in CD.[18] Dolby Digital (also called AC-3) and DTS—which are now common on DVD titles—first became available on LaserDisc, and Star Wars Episode I: The Phantom Menace (1999) which was released on LaserDisc in Japan, is among the first home video releases ever to include 6.1 channel Dolby Digital EX Surround.[19] Unlike DVDs, which carry Dolby Digital audio in digital form, LaserDiscs store Dolby Digital in a frequency modulated form within a track normally used for analog audio. Extracting Dolby Digital from a LaserDisc required a player equipped with a special "AC-3 RF" output and an external demodulator in addition to an AC-3 decoder. The demodulator was necessary to convert the 2.88 MHz modulated AC-3 information on the disc into a 384 kbit/s signal that the decoder could handle. DTS audio, when available on a disc, replaced the digital audio tracks; hearing DTS sound required only an S/PDIF compliant digital connection to a DTS decoder.
In the mid to late 1990s many higher-end AV receivers included the demodulator circuit specifically for the LaserDisc players RF modulated Dolby Digital AC-3 signal. By the late 1990s with LaserDisc players and disc sales declining due to DVD's growing popularity the AV receiver manufacturers removed the demodulator circuit. Although DVD players were capable of playing Dolby Digital tracks, the signal out of DVD player were not in a modulated form and not compatible with the inputs designed for LaserDisc AC-3. Outboard demodulators were available for a period that convert the AC-3 signal to standard Dolby Digital signal that was compatible with the standard Dolby Digital/PCM inputs on capable AV receivers. Another type marketed by Onkyo[20] and others converted the RF AC-3 signal to 6-channel analog audio.
The two FM audio channels occupied the disc spectrum at 2.3 and 2.8 MHz on NTSC formatted discs and each channel had a 100 kHz FM deviation. The FM audio carrier frequencies were chosen to minimize their visibility in the video image, so that even with a poorly mastered disc, audio carrier beats in the video will be at least ‑35 dB down, and thus, invisible. Due to the frequencies chosen, the 2.8 MHz audio carrier (Right Channel) and the lower edge of the chroma signal are very close together and if filters are not carefully set during mastering, there can be interference between the two. In addition, high audio levels combined with high chroma levels can cause mutual interference, leading to beats becoming visible in highly saturated areas of the image. To help deal with this, Pioneer decided to implement the CX Noise Reduction System on the analog tracks. By reducing the dynamic range and peak levels of the audio signals stored on the disc, filtering requirements were relaxed and visible beats greatly reduced or eliminated. The CX system gives a total NR effect of 20 dB, but in the interest of better compatibility for non-decoded playback, Pioneer reduced this to only 14 dB of noise reduction (the RCA CED system used the "original" 20 dB CX system). This also relaxed calibration tolerances in players and helped reduce audible pumping if the CX decoder was not calibrated correctly.
At least where the digital audio tracks were concerned, the sound quality was unsurpassed at the time compared to consumer videotape, but the quality of the analog soundtracks varied greatly depending on the disc and, sometimes, the player. Many early and lower-end LD players had poor analog audio components, and many early discs had poorly mastered analog audio tracks, making digital soundtracks in any form most desirable to serious enthusiasts. Early DiscoVision and LaserDisc titles lacked the digital audio option, but many of those movies received digital sound in later re-issues by Universal, and the quality of analog audio tracks generally got far better as time went on. Many discs that had originally carried old analog stereo tracks received new Dolby Stereo and Dolby Surround tracks instead, often in addition to digital tracks, helping boost sound quality. Later analog discs also applied CX Noise Reduction, which improved the signal-noise ratio of their audio.
Both AC-3 and DTS surround audio were clumsily implemented on LaserDiscs, leading to some interesting player- and disc-dependent issues. A disc that included AC-3 audio forfeited the right analog audio channel to the modulated AC-3 RF stream. If the player did not have an AC-3 output available, the next most attractive playback option would be the digital Dolby Surround or stereo audio tracks. The reason for this is the RF signal needs to bypass the audio circuitry in order to be properly processed by the demodulator. If either the player did not support digital audio tracks (common in older players), or the disc did not include digital audio tracks at all (uncommon for a disc which is mastered with an AC-3 track), the only remaining option was to fall back to a monophonic presentation of the left analog audio track. However, many older analog-only players not only failed to output AC-3 streams correctly, but were not even aware of their potential existence. Such a player will happily play the analog audio tracks verbatim, resulting in garbage (static) output in the right channel.
On a DTS disc, digital PCM audio was not available, so if a DTS decoder was also not available, the only option is to fall back to the analog Dolby Surround or stereo audio tracks. In some cases, the analog audio tracks were further made unavailable through replacement with supplementary audio such as isolated scores or audio commentary. This effectively reduced playback of a DTS disc on a non-DTS equipped system to mono audio, or in a handful of cases, no film soundtrack at all.[21]
Only one 5.1 surround sound option existed on a given LaserDisc (either Dolby Digital or DTS), so if surround sound is desired, the disc had to be matched to the capabilities of the playback equipment (LD Player and Receiver/Decoder) by the purchaser. A fully capable LaserDisc playback system included a newer LaserDisc player that is capable of playing digital tracks, had a digital optical output for digital PCM and DTS audio, would be aware of AC-3 audio tracks, and had an AC-3 coaxial output; an external or internal AC-3 RF demodulator and AC-3 decoder; and a DTS decoder. Many 1990s A/V receivers combined the AC-3 decoder and DTS decoder logic, but an integrated AC-3 demodulator was rare both in LaserDisc players and in later A/V receivers.[22]
PAL LaserDiscs had a slightly longer playing time than NTSC discs, but had fewer audio options. PAL discs only have two audio tracks, consisting of either two analog-only tracks on older PAL LDs, or two digital-only tracks on newer discs. In comparison, later NTSC LDs are capable of carrying four tracks (two analog and two digital). On certain releases, one of the analog tracks is used to carry a modulated AC-3 signal for 5.1 channel audio (for decoding and playback by newer LD players with an "AC-3 RF" output). However, older NTSC LDs made before 1984 (such as the original DiscoVision discs) only have two analog audio tracks.