I am puzzled-I have a multi disc CD player in my car, and there seems to be big differences in the audio level on them. One disc is fine at one volume setting, the next is too loud or too soft. Since audio CDs are digitally encoded, how is this possible? Shouldn’t they all perform alike? They are not an analog system (like cassette tapes), where you have a noticable volume difference in the recorded signal.
The difference is in the mastering, not the medium. Note that it is currently common practice for engineers to master CDs with massive amounts of compression. That basically means that all the sounds are amplified to the maximum level the medium will accept. This makes the music sound “hotter” and more exciting, and also makes the music easier to hear in a noisy environment like a car, but it sacrifices the original dynamics of the music (the difference between louder and softer passages) and tends to be fatiguing to listen to over time. The CDs that you perceive as too quiet may actually seem that way only because they were mastered without a lot of compression, and so approach the volume limit only at peak moments, and not all the time like a compressed CD.
As mentioned above, new music is often afflicted by the loudness war. How’s this for a waveform? Yikes.
I did this image ( dynamics | Blown levels and clipping in Metallica's Death Ma… | Flickr ) when Metallica’s Death Magnetic came out. It compares the waveform of a song on their new album with a (just as heavy) song on an album from a different era.
The Master of Puppets waveform is pretty healthy, and pleasant to listen to (if you like that kind of music). It sounds loud if you crank the volume knob. The Death Magnetic waveform sounds loud no matter what, and makes me a little ill if I try to listen to the whole thing at once. Not only does the jacked-up waveform make it sound louder than other albums, the real travesty is that each of those peaks that gets cuts off creates a little bit of static. A lot of people claim not to notice, but to me and plenty of others (and I don’t claim to be an audiophile snob AT ALL) it sounds absolutely terrible. Like my speaker is punctured terrible. Not all albums are mastered that badly, but that is why some sound louder than others.
Nitpick: it means that the difference between loud and soft are made smaller. You can have something that’s very compressed and yet very soft, though in the current “loudness war”, it is indeed used to bring the average level as high as possible.
I don’t follow. Both soft sounds and louder sounds can be digitally encoded. You can encode a CD full of whispers and a CD full of screams. So it stands to reason that you could encode the screams at full volume on one CD and at whisper volume on another CD, doesn’t it? As was noted, it’s how the CD was mastered. Some are loud and some are not as loud. There’s no standard.
All true. I was consciously oversimplifying.