Nerd Alert: Sample Rate

Audio can exist in many different states (just like H20 can be vapor, water and ice). The sounds you hear are acoustic energy waves traveling through the atmosphere. When these acoustic waves are captured by a microphone, a process called electromagnetic induction (induction is when energy is transformed from one state into another - like your toaster, electricity to heat), the audio becomes an analog or electrical signal. This analog signal can be captured by an audio interface, a device that gives your computer additional audio inputs and outputs, which converts it into a digital signal through a process called sampling. This brings us to our topic today.

Sampling

An audio sample is a digital representation of one instantaneous moment in an audio recording. Huh? In other words, your audio interface takes a sample of the audio (like you might with the ice cream flavors at Baskin Robbins), which measures the amplitude of the audio signal at that given instant. Still huh? Think of a sample as a look at the incoming audio. Every time you look at the audio you measure the position of the audio waveform. The more looks you take, the better you can determine how the waveform evolves over time, and the more accurately you can capture the audio. The number of "looks" you take per second is called the sample rate.

Sample Rate

Remember those flip books you would draw as a kid on a pad of post-it notes? A stick figure being chased by a dog and jumping over a fence (was that just me?). Each new post-it note, the stick figure would advance one step further, and when you flipped through it, it came to life. This, of course, is how motion pictures work, and each of your drawings would be called a frame. The number of frames per second is a similar concept to the number of samples per second. Frames for our eyes, samples for our ears. Here's an interesting fact though: Your ears need much, much, MUCH more information than your eyes (or perhaps your eyes are more efficient at processing information). When you go to the movies, you are watching film at a rate of 24 frames per second. When you play a CD (remember those?), you are listening at 44,100 samples per second, a rate with 1,837 1/2 times the information. Sample rate is the number of samples per second, measured in hertz (Hz) or kilohertz (kHz). A rate of 44,100 samples per second is more commonly referred to as 44.1 kHz.

Nyquist-Shannon Sampling Theorem

Why 44.1 kHz? Typical human hearing range is from 20 Hz (referring now to frequency or pitch, not sample rate) to 20 kHz. One audio waveform is one push and one pull of energy. An up and a down. Swedish engineer, Harry Nyquist, and American engineer, Claude Shannon, posited that the highest frequency that could be captured digitally is 1/2 of the sample rate (having to do with needing to sample both a push and a pull of a waveform - a very high frequency waveform could squeak through two samples undetected). At a sample rate of 44.1 kHz, frequencies of up to 20.5 kHz (just beyond typical human hearing range) could be captured, thus rendering a lush representation of the audio, with the full spectrum of frequency.

Typical Sample Rates, and Why 192 kHz?

CDs are 44.1kHz. If you do any work with audio for film, your standard is 48 kHz. With computers getting more powerful, and with hard drive space getting cheaper, higher sample rates have become common. You will likely see 88.2 kHz (double 44.1 kHz), and 96 kHz (double 48 kHz) as sample rate options, and even up to 192 kHz (double 96 kHz). The very highest frequencies humans can hear are 25 - 30 kHz, and a sample rate of 192 kHz would render frequencies up to 96 kHz. So why, then, the need for such a high sample rate? There are arguments for and against, with some saying 192 kHz is just hype, and is useless as a sample rate, while others argue that the higher sample rate provides better fidelity after audio manipulation, such as pitch-shifting an audio recording down several octaves. Still others argue for the higher sample rate as a way to "future proof" their recordings. Not knowing what audio file formats will become standard in the future has driven them to record their audio in the best available quality, so that mix downs at a higher sample rate are possible later.

These audio examples are the same piece of music, bounced out at different samples.