I would keep EVERYTHING, if I was you
There is no "rule of thumb," per se, but there are a couple of things to keep im mind...
A lot of films from the 90s OR earlier (especially films from the 70s and 80s) have only had remixes/upmixes/lossy audio on their DVD/BD counterparts, while the LDs have the original, lossless audio track, usually the theatrical mix. There are tons of films where the only place to get the original mono/stereo audio is on LDs. However, there are certain films, where even the lossy 5.1 ac3/dts sounds better than the lossless counterpart on BD. Jurassic Park is a great example of this. The lossy 5.1 DTS track blows the lossless BD audio out the water in terms of dynamic range, clarity, and clean bass response.
For numerous films that only contain upmixes on BD, the only place for a lossless, original track is LD. I always thought this is stupid... An original stereo track will be available lossless on LD, lossy on older DVD releases, but the BD will only contain an upmix (and the ones that piss me off the most is when they DO include an original stereo track on BD, but only in lossy 192kbps ac3, as if storage is an issue on a BD - lol)
Based on my experience, a LOT of audio tracks that I compared between LD and BD end up being superior on LD. Funniest is when the BD does actually contain an original audio mix and the LD still sounds better, even though they should both be about the same quality. The reason for this is because now-a-days nearly everything is over processed before its released, (hence why "we," the fanres community, are here) while most of material on LDs is just a "straight transfer" with little to no filtering/alterations done to it. In most cases, its better to work with an unfiltered, "dirty" track than trying to recover fidelity from an over scrubbed track. There are exceptions though, where (for example, the film "National Lampoons Christmas Vacation") the original track on LD is "dirty," slightly distorted, and generally of inferior quality to the point where my VHS copy sounded better. This was one that caught me off-guard and the BD was a pleasant surprise, because they cleaned it up without going overboard and they didn't remix/upmix the audio which gave us a nice, original theatrical stereo track.
So far, there is no "centralized" database that compares LD audio to BD audio. Based on what I've found over the years, people sometimes mention the superior LD audio only in reference to one or two particular films. Even here, we dont have a comprehensive list (although I have been toying with the idea, as soon as I figure out a proper, easy to use template for it)
When it comes to most films and the LD audio, (that haven't been mentioned by users in any part of this forum) you have a couple of options:
- try using google with search terms such as; "title of film" and "laserdisc" (using LD won't always be enough) and "audio" or "PCM"/"WAV" and, sometimes, you might get lucky enough to find a discussion on another forum mentioning the differences
- rip the audio for a film off the LD and BD, load them into Audacity or Reaper (a fantastic, free DAW) and compare them yourself. (in my experience, this is the BEST way to go about it because you can't rely on most people's audio comparison reviews)
I realize u were looking for an easy answer that involves less work on your part, but it is what it is
