iPods are the most ubiquitous music-playing devices on the planet, but are they the best sounding? Not exactly. iPods tapped into the boom of the MP3, and with iTunes and its own AAC encoding system, Apple quickly grabbed control of the lossy audio world and never let go. That position is what makes a recent statement from the chairman of the A&M record label so intriguing--he claims that Apple is interested in offering hi-fi 24-bit audio files on the iTunes music store.
That would give iTunes better than CD-quality audio, but the move might not make sense for Apple. Users go to iTunes for quick downloads of compressed lossy files, and 24-bit implies a brave new world of audio that many portable devices can't fully take advantage of.
Deep BitsWhen it comes to judging digital music quality, the discussion usually begins and ends with bitrate. A song encoded at 320 kilobits/second is going to sound a whole lot better than a song with a 128kbps bitrate, right? Well, sure, but it's a bit more complicated than that. Bitrate stems from two different elements: bit depth and sample rate. Here's where we can understand the difference between 16-bit and 24-bit audio.
Bit depth is essentially the number of bits you have to contain a piece of audio--the range from the imperceptible whispers of virtually no sound to the loudest noise a piece of audio gear can crank out. The difference between 16-bit audio and 24-bit audio isn't just a matter of eight bits. As TweakHeadz explains,
sample rate. Sample rate refers to the number of samples or measurements taken each second from a recording. The typical CD sample rate is 44.1kHz, or 44,100 samples per second. High-end audio gear often samples at an even higher rate, and DVD-Audio quality--which employs 24-bit audio--sample at 96kHz or even 192 kHz.
"The easiest way to envision this is as a series of levels, that audio energy can be sliced at any given moment in time. With 16 bit audio, there are 65,536 possible levels. With every bit of greater resolution, the number of levels double. By the time we get to 24 bit, we actually have 16,777,216 levels. Remember we are talking about a slice of audio frozen in a single moment of time."
Without turning to compression formats, those sample rates mean big file sizes. A 16-bit, 44.1kHz song requires a bitrate of 1.35 megabit/second of data, and a single minute of stereo audio takes up about 10 megabytes of space. A 24-bit song with a 96kHz sample rate, by contrast, requires a bitrate of 4.39mbps and requires 33 megabytes of storage for a single minute of stereo audio. Now you can see why MP3 filesizes are so appealing.
But How Does It Sound?24-bit sound is a tricky thing to gauge. Does it provide for a greater resolution of sound? Definitively. It has room for 256 times the data, remember. Are you going to be able to hear that difference? Harder to judge. Human hearing supposedly tops out at 20kHz, but that doesn't make higher sample rates useless. According to the Nyquist rate, to fully capture a wave, it should be sampled at twice its highest frequency. In other words, a higher sample rate, and a greater bit depth, gives your sound more wiggle room, meaning sound peaks are less likely to be truncated and the subtleties of the music are less likely to be drowned out.
Lossy, Lossless and EAC
Lossy audio--your MP3s, AACs and (blech) WMAs--offer far more compression by sacrificing quality. A technique called variable bitrate (VBR) can improve compression quality by doing exactly what the name implies: varying the bitrate of a song. With VBR, more complicated portions of a song are allocated more bits, while simpler segments take up fewer bits.
Even with lossless compression, achieving perfect fidelity isn't easy. Popular CD ripping tool EAC demonstrates exactly how complicated the process of reproducing sound bit-for-bit can be--it will read each sector of a CD several times, perhaps dozens of times, compensate for errors, and present you with a dizzying range of compression options to give you the perfect rip.
More Bits, More Problems
Hopefully at this point you have a handle on what separates all these file formats and types of compression, so let's get back to the original topic: Apple and 24-bit audio. As we've clearly demonstrated, 24-bit sound files are big--something like 100MB for a regular song, though FLAC compression can cut that down to something more manageable. Even if we assume Apple is interested in 24-bit audio, will there be a market for it? After all, their most popular music players have taken a step backwards in terms of capacity, opting to use flash memory over physical disks. You sure couldn't fit many lossless files on an iPod nano.
a company like SanDisk or Samsung. But until the price of flash memory has drastically lowered, 24-bit audio on portable media players will remain a space-hogging luxury.
Thankfully, we don't face those same constraints on our desktops--cheap terabyte harddrives are voluminous enough to hold hundreds of uncompressed albums. If you feel like trying out some awesome lossless FLAC music, check out Archive.org's selection of 24-bit music. It's free! foobar2000 is an excellent (and skinnable) music player that can handle FLAC, and dBpoweramp on the Windows side and Max on the Mac side can help you convert between FLAC and ALAC. It takes a bit more work than downloading a 99 cent AAC song off iTunes, but just remember: 256 times as many bits!