How does digital audio capture sound?
A sound wave is one thing changing over time: air pressure. A microphone turns it into a voltage, and a converter measures that voltage thousands of times a second.
The sampling theorem (Nyquist–Shannon) says that if you sample at more than twice the highest frequency in a signal, you can rebuild the original wave exactly. Human hearing tops out around 20 kHz, and CD audio samples at 44.1 kHz. Each sample's precision, its bit depth, sets the noise floor: about 6 dB per bit, so roughly 96 dB of range at 16-bit and about 144 dB in theory at 24-bit.
| Format | What it covers |
|---|---|
| Human hearing | about 20 Hz to 20 kHz |
| CD: 44.1 kHz sample rate | up to about 22 kHz |
| 16-bit depth | about 96 dB of range |
| 24-bit depth | about 144 dB, in theory |
The ~6 dB per bit figure is the standard approximation (6.02 dB per bit). Real converters fall short of 24-bit's theoretical range.
Can people hear the difference in blind tests?
In the best-known test, published in the Journal of the Audio Engineering Society in 2007, Brad Meyer and David Moran passed high-resolution recordings through a CD-quality (16-bit, 44.1 kHz) conversion and back. Listeners couldn't detect the CD-quality loop at normal-to-loud levels. Its noise was audible only at very high volume.
The study is still argued about. Critics say some of the "high-resolution" recordings weren't truly high-resolution to begin with. But it's the reason most engineers treat CD quality as enough for listening, and higher rates as useful headroom for recording and processing.
So what are people hearing when they say analog sounds better?
The gear. Tape, tubes, and transformers change the signal on the way through:
- SaturationTape and tubes round off peaks gradually instead of clipping hard, which adds harmonics and squeezes loud moments.
- Frequency shapingTape machines add a low-end lift known as head bump, and roll off some high end.
- MovementSmall speed wobbles in a tape transport (wow and flutter) make pitch drift slightly.
- NoiseTape hiss and vinyl surface noise sit under everything.
None of that is detail digital fails to capture. It's color the gear adds. A digital recorder captures it perfectly if you record the output of analog gear.
Can that character be recreated digitally?
Much of it, yes. Universal Audio's Studer A800 plug-in, released in 2010, was modeled with magnetic recording expert Jay McKnight over 12 months to emulate the full signal path of a two-inch tape machine. Other companies sample the behavior of hardware instead of its circuits, and newer neural amp modeling trains a small model on recordings of real gear.
Stuart's take
The warmth of analog gear in music is the same thing as the depth in film photography. That's what people are actually hearing, and seeing.
Which raises the obvious question. If digital audio is this good, and can even recreate analog warmth, why does a digital photo still not look like film?
Frequently asked questions
Does digital audio lose detail compared with analog?
Not within the range of human hearing. Sampling at more than twice the highest frequency allows the wave to be rebuilt exactly, and CD audio at 44.1 kHz covers hearing up to about 20 kHz.
Can people hear the difference between CD quality and high-resolution audio?
In a 2007 double-blind study published in the Journal of the Audio Engineering Society, listeners could not detect a CD-quality loop at normal-to-loud levels. The study is still debated.
Why does analog sound warm?
Tape, tubes, and transformers add saturation, low-end lift, slight pitch movement, and noise. It is color the gear adds, and digital can record it or model it.