Apple to offer 24-bit files? Three terms you need to know
Much has been made of the story (reported by CNN) that Apple may soon start offering high-resolution 24-bit recordings through the iTunes® music store. In the ensuing discussions, some people seem to be confusing some basic digital music terminology. Here's a quick run-down of the three most common terms you need to know to follow this story: bit depth, sample rate, and bit rate. Understand those, and you'll better understand what everyone's excited — or up in arms — about.
Bit-depth: the amount of digital information in a single sample
Bit depth is simply how many bits (the building blocks of digital information) are used to encode a single moment, or sample, of audio. This information is layered in levels, which is why it's referred to as "depth." This is important, as the dynamic range is spread over the number of available levels. So the greater the depth, the greater the dynamic range, and the more natural the sound.
16-bit is the most common depth for digital music track, including CDs. A 16-bit recording has a bit depth of a little over 65,000 possible levels. Every added bit doubles the number of levels, so a recording with 24 bit depth has 16.7 million levels available.
Most studios record at a 24 bit depth because it more accurately captures the sound. When those files are down-converted to 16-bits, a great amount of detail can get lost. That's why the possibility of iTunes offering 24-bit recordings is considered newsworthy in the first place.
(For a great plain-English explanation of bit-depth, read Tweakheadz.com's 16 Bit vs. 24 Bit Audio)
Sample rate: the number of samples per second
Think about a film projector running a movie reel. If the projector's running slow, it's easier to see that the movie is actually made up of individual snapshots. When it's running faster, motion looks smoother and more natural.
The sample rate is sort of the same thing. It describes how many samples per second are required to properly transmit the file. Most digital audio files (including CDs) have a 44.1kHz sample rate. Studio-grade 24-bit recordings usually have a 96kHz sample rate. So not only are you getting more detail per sample, but more samples per second, too. And just as in our analogy, the higher the sampling rate, the smoother and more natural the audio can sound.
Bit rate: where it all comes together
Most of the discussion about digital audio centers around the bit rate. This is the total amount of data sent per second. So the higher the bit rate, the more information is being relayed, and therefore the more detailed and realistic the audio file will sound. The equation's pretty basic:
Bit rate = Bit depth x sample rate x number of channels
A standard CD stereo track has a bit rate of 1.4MB (megabits) per second (16 bit x 44,100 HZ x 2 channels).
By comparison, a studio-grade stereo track would have a significantly higher bit rate, and a correspondingly higher sound quality. Notice how the higher bit rate and sampling rate change the equation: 24 bit x 96,000 Hz x 2 channels = 4.6MB per second. In other words, about three times the detail found in a comparable CD track.
So what does this mean for me?
If Apple is indeed going to carry 24-bit audio files, then that could be good news to music lovers who use iTunes.Because it could mean you can start downloading better-quality audio files through the store.
Now it's important to keep in mind that the only spec being talked about is the bit depth. If there's not a corresponding change in the sample rate, then the upgrade might not be that dramatic.
But if indeed we're talking about 24-bit/96kHz audio, then you might possibly be able to download tracks from iTunes that are closer to the sound of the studio masters. And if you have a high-performance playback system, that can open up all kinds of possibilities.