...than most of the DVD - and few BD, as I read also in these forum. I don't want to talk about MUSE laserdisc audio now, because I'm not expert in that branch, but whoever wants to talk about it, is welcome!

OK, let's start!

NTSC laserdiscs could contain a different combination of soundtracks:

two mono analog
one stereo analog
two mono analog + two mono PCM
two mono analog + one stereo PCM
one stereo analog + two mono PCM
one stereo analog + one stereo PCM
AC3 + one mono analog + one stereo PCM
DTS + one stereo analog

I'm pretty sure there is no AC3 laserdisc with two mono digital soundtracks, as PCM is used for stereo (surround) soundtrack, and almost sure there is no DTS laserdisc with two mono analog soundtracks, as analog is used for stereo (surround) soundtrack, but I'm sure there is NO laserdisc that contain both DTS and AC3 soundtracks!

Things are easier for PAL laserdiscs:

two mono analog (on laservision)
one stereo analog (on laservision)
two mono digital
one stereo digital
DTS (well, only one titles ever released!)

The PAL laservision is practically another standard, as PAL laserdisc video could be watched on old laservision player, but digital soundtracks could not be heard! The contrary is usually not true, as the most part of the PAL laserdisc players could also play analog soundtracks. There is NO AC3 PAL laserdisc!

Now, let's take a closer look at the different soundtrack types.

ANALOG

Albeit analog soundtrack on laserdisc is the worst of the possible ones that could be found on it, the quality is pretty good - here you are some numbers:

Frequency response: 20-20000hz (±3dB)
Signal-to-noise ratio (CX off): >50dB (up to 58dB)
Signal-to-noise ratio (CX on): >62dB (up to 74dB)
Channel separation: >50dB

Not that bad, uh? Better than vynil, audio cassette, second only to VHS HiFi Stereo. It could contain also surround sound; some concerts could be found only on analog laserdiscs, as they are never released with digital audio; the most part of japanese bilingual laserdiscs have the japanese language recorded onto the analog soundtrack... why? Maybe because it sounds better? The answer is yours...

PCM (Pulse Code Modulation)

The first (and most used) digital soundtrack that appeared on the laserdisc format was PCM, stereo or dual mono, 16bit 44.1KHz 1441kbps - it has the same technical specifications and quality of the CD-audio, but it was available years before it!

Frequency response: 4-20000hz (±0.1dB)
Signal-to-noise ratio: 96dB - up to 117db (EIAJ)
Dynamic range: 96dB - up to 99dB (EIAJ)
Channel separation: 80dB - up to 90dB (EIAJ)
Wow & Flutter: <0.001% (EIAJ)

All the PAL laserdiscs after the end of 1980's have digital audio; NTSC continued to have both analog and digital soundtrack, as the standard allowed it; several surround types could be found on laserdisc, not only the famous Dolby Surround, but also DTS Surround and CHACE surround.

AC3 (Dolby Digital)

The first AC3 laserdisc was "Clear and Present Danger" and was released in 1995; the AC3 soundtrack is stored in the right analog channel, and is RF modulated; to be decoded, a laserdisc player with the AC3 RF output is needed, and must be connected to an RF demodulator and a Dolby Digital decoder, or to an amplifier with built-in RF demodulator and DD decoder.

The AC3 soundtrack has always the 384kbps bitrate at 48KHz, almost always 20bit 5.1 channels, but in some (rare) cases the number of channels could vary - usually during extra material like making of, documentary etc.

I found no proof (until now), but there are evidences that the theatrical mixes are used for AC3 laserdisc soundtracks "as is"; infact, many argue that laserdisc Dolby Digital sounds better than the DVD counterpart, also if the latter has an higher bitrate; infact, most DVD DD soundtracks are mixed taking in account home users, and they should sound good with any kind of audio configuration, and hence it's a sort of compromise, while the laserdisc has that "in your face" sound typical of theaters.

Someone could think that is not possible, as the theatrical AC3 soundtrack is 320kbps, while laserdisc has 384kbps... think that the LD AC3 is stored on analog form, and surely the signal contains some sort of stronger error correction, due to the fact that analog reading is not perfect; also, it is possible that the signal is simply padded from 320kbps to 384kbps, as it was more economic to take the theatrical mix and copy to laserdisc than remake a home version...

DTS (Digital Theater System)

The first DTS laserdisc was "Jurassic Park" and was released in 1997; DTS soundtrack takes the place of the PCM soundtrack, leaving free two analog tracks, (almost) always used for the movie soundtrack, allowing the owner of an old analog-only player, or who has not a DTS decoder, to listen to it. To be decoded, a laserdisc player with digital output is needed, and a DTS decoder, or an amplifier with built-in DTS decoder.

As the DTS soundtrack are in place of the PCM one, it has the same technical data, 16bit 44.1KHz 1441kbps but, at the contrary of the PCM two channels, it has 5.1 discrete channels.

The laserdisc DTS soundtrack IS NOT the same of the theatrical one: infact, in theaters, DTS uses a different codec, APT-X, encoded as ADPCM at 882kbps and recorded on CD-ROMs, with a compression of 4:1, while DTS on laserdisc uses Coherent Acoustics perceptual coding compression scheme, encoded at 20bit 44.1KHz with a bitrate of 1235kbps (padded to 1441kbps to be the same of PCM soundtracks) and a compression of 3:1. Hence, the DTS laserdiscs should be better than the theatrical DTS, as the home codec is newer and better, and bitrate higher with less compression.

As almost all DTS DVD have the so-called "half bitrate" soundtracks (754kbps padded to 768kbps) instead of full bitrate (1509kbps padded to 1536kbps), the laserdisc DTS soundtracks is always better than "half bitrate" DTS DVD; in some cases, could be preferable to full-bitrate DTS DVD due to different mixes used.

CONCLUSIONS

Using a laserdisc soundtrack for preservation purposes is often a good choice; when the DVD or BD soundtracks are of low quality, or technically inferior, or "improved"; when the DVD or BD soundtracks have different formats; when the DVD or BD have no soundtrack in a certain language; or, simply, when there are no DVD or BD of a certain title or version at all!

There is only a price to pay: capturing and converting laserdisc soundtracks is an HARD task... but really rewarding! - just finished JP2 AC3 and DTS, I know what I'm talking about... (^^,)