Sound Design Stack Exchange is a question and answer site for sound engineers, producers, editors, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
In an Optical medium, The Dolby Digital print has a 5776 dots each representing a bit of data in one perforation. Therefore 4 perforations per frame and 96 perforations in one second. So in one second dolby is accommodating 554496 bits of data. However the data in an uncompressed wav in second is 6912000 bits of data is stored.
What kind of compression does my audio go through when the AC3 codec is used ? How come there is no drop in quality and while there is a huge, drastic drop in data ?
It's going through a perceptual (think mp3/aac) and matrix encoding process. And there is a drop in quality, but most people aren't listening on speakers where they would be able to clearly hear the difference (at home anyways). That's how it is with any of the flavors of Dolby or DTS encoding formats. Though it's interesting to note that Blu-Ray have started shipping with unencoded, discrete, multi-channel audio on them.
The way To clearly hear the destruction is to feed strong complex signals to the fronts and some nice sounding orchestra music at lower levels to the rears. Mute the fronts and only listen to the rears... Make sure to mute AFTER the DMU and that you monitor through the ac3 process.
Sometimes bass heavy sounds gets seriously messed up as well. It used to be a big issue, but Definetly have become better the last few years.
The perceptual coding and data reduction for film prints is crazy. There's way more data on a DVD or bluray compared to what's available on a 35mm print. Otoh it's quite a feat getting it to sound as decent as it does with the limited data rate available.
Actually there's a great drop in quality! If you ever listened to a mix while making the Dolby optical disk (so, in the same room where you mixed) you clearly hear drop of quality, definition, dynamics. One of the great things of DCP projections (or Blu-Ray discs), is the uncompressed audio format. I think that abandonment of AC3 for uncompressed audio and better calibrate theaters are the most prominent improvement we could have in the next few years in sound.
I too think the loss of both details, definition, and transients is humongous in Dolby Digital, though it is by all means a good format considering what it's made for. Personally I don't like it anymore though, but I do respect it.
Most people doesn't think much of it as they don't really know what it could have been, but they react to it in a very physical way as the human conception of reality is very much based on small sometimes barely noticeable details and imperfections which helps us distinguish between different sound-sources in a more realistic way. Dolby Digital, like MPEG Layer 3 and such for example, is losing most if not all that small pieces of air in the process of reducing the bitstream. That doesn't mean the mix gets totally bollocks, it just means the audience doesn't get as into the illusion as they would had had the soundstream been richer.
For me, DTS is da shit! It's still lossy, but it is MUCH more transparent to the material due to both lower compression ratio and frankly a better algorithm.
It's difficult for me to explain exactly how it works, but it is not based upon samples but in frequency response. By eliminating stuff the codec doesn't think is perceptible, or at least unimportant, the codec can keep the soundfile small. One of the things lost in this kind of compression is the extreme highs, and in at least MP3, the extreme lows. I'm not sure whether or not the fullrange speakers in AC-3 have much sub-bass. I do know the LFE does (well duh! ;-), but I actually hasn't checked a Dolby stream in an analyzer yet as I has with MP3 :-) A more direct effect is that the sound gets more dull and compact, with more transient sounds risking to drown in the mix, at least on lower settings. Some people like this effect though!
