production - Why the discrepancy between the sound levels of dialogue and music in older movies?

I've been watching a lot of older films lately (older, in this instance reads "circa 80's and 70's" and mainly in the Sci-Fi and action genres), and have been asking myself the same thing as I watch each one.

Note: I'm watching these on DVD or Blu-Ray, they're not rips or VHS tapes if that makes any difference.

I've noticed that a lot of these films have their soundtrack's mixed in a very particular way. I've noticed that the musical queues and sound effects seem to be mixed at a higher volume than the dialogue. I the versions of the films that I have and watch, at least.

As an example, I've recently watched the UK version of the Blu-Ray release of Aliens, and I found that I had to turn my sound system up quite high to hear the dialogue, but turn it back down again during the action sequences (or scenes with lots of sound effects or musical queues) as I have room-mates and don't wish to disturb them just so that I can hear the dialogue.

I'm convinced that it's not my sound system, as a lot of the films from the same era seem to be mixed the same way. Also, playing them back through my PC, I notice the same thing.

Is this related to a stylistic decision made by the director/sound engineer? A technical limitation of the technology of the time? Is it related to potential problems with converting the (I'm guessing) analogue soundtrack to a digital format and compressing it?

I know (from a very limited knowledge of how it works) that Dolby is an encoding format aimed at reducing hiss, and that one of the ways it can achieve this is by lowering the overall volume of the soundtrack to a point where hiss isn't as easily detectable. So, I'm guessing that this might just be a by-product of that encoding process.

Any ideas or suggestions?

Answer

It's not a by-product of the Dolby encoding process, although Dolby does allow greater dynamic range.

Sound engineers employ a technique called "dynamic range compression" to reduce the difference between the loudest and softest sounds in a recording. Use of compression is much more pronounced and widespread today than it was in the past, which explains your observations about sound level.

Because many viewers are "casual" viewers (in the sense that they're not necessarily looking for the full-blown cinematic experience, but merely something to watch on a Saturday afternoon), modern sound engineers favor a more highly-compressed sound mix that reduces the overall change in sound levels, so that you can still hear the dialog amidst the explosions. This explains why Blu-Ray discs (which presumably cater to an audience looking for the cinematic experience) tend to have less compression than other forms of video media, such as ordinary cable television.

Applying heavy compression to audio tracks on a DVD or Blu-Ray is a flawed technique: it's much easier to apply compression after the fact (i.e. in your receiver electronics) than it is to attempt to recover the dynamics that are lost due to heavy compression.

Further Reading
The many different mixes of Star Wars: A New Hope