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Everything you need to know about digital audio files

Kirk McElhearn | Jan. 13, 2016
Don’t know the difference between lossy and lossless? What’s the deal with bit rates? Let us explain.

What is high-resolution audio?

High-resolution audio, once a niche format, has gotten a lot of press recently. Neil Young’s beleaguered PonoPlayer raised awareness of this type of digital audio. Strictly speaking, high-resolution audio is distributed in files that are “better” than CD quality. High-resolution audio is defined by certain numbers: the bit depth of files, and their sample rate.

CDs contain 16-bit audio at a sample rate of 44,100 Hz. So high-resolution audio has a bit depth and/or sample rate that exceeds that of the CD specification (known as the Red Book standard). Much high-resolution audio is 24-bit, 96 kHz, often abbreviated as 24/96. Some companies sell files at 24/192 and 24/384. And there are also several types of DSD (direct-stream digital) files, which use a different recording method. DSD is used on SACDs, or Super Audio CDs, a format designed by Sony and Philips that is pretty much deceased.

itunes hi res
iTunes showing information about a high-resolution audio file. You can see that the bit rate is much higher than for a standard lossless file. What iTunes calls the sample size is the bit depth.

When we talk about bits in high-resolution audio, we’re not looking at the bit rate, which I discussed above, but the bit depth. This is the number of bits in each sample, and it mostly affects dynamic range, which is the difference between the softest and loudest parts of the music. (Though, as you can see in the screenshot, the actual bit rate of a high-resolution audio file is much higher than that of a CD or of a file ripped in a lossless format.)

A good example of music with a very broad dynamic range is Mahler’s third symphony. Listen to the final movement, and you’ll hear some very soft sounds as well as an extremely loud crescendos. Or listen to Led Zeppelin’s “Stairway to Heaven;” it starts with a soft acoustic guitar and builds up to a fuzz-box finale.

A higher bit depth allows music to have a wider range of volume from its softest to loudest passages. But with a lot of contemporary music, the volume of the music is “compressed” to make it louder. (This is dynamic range compression, not the compression used to make files smaller.) So you don’t hear much of a difference with that type of audio if the bit depth is higher.

The sample rate is the number of “slices” of audio that are made per second, and are measured in Hz (Hertz). 44,100 Hz means that the music is sampled 44,100 times a second; 96 kHz means it is sampled 96,000 times a second. The sample rate affects the overall fidelity of music, but also the range of frequencies that can be reproduced. Files sampled at 44,100 Hz can reproduce up to about 20 KHz, or the highest frequencies that humans can hear. High-resolution files can reproduce sounds above that frequency, sounds that humans cannot hear at all. (And extremely high sample rates, such as 192 kHz, may even result in distorted sound.)

 

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