Digital Audio Primer: What is Sample Rate?

Welcome to the first in a series of articles where I will help explain the basics of digital audio. Today's topic is sample rate.  In future posts, I will also explain bit depth, the difference between compressed and uncompressed audio, and audio delivery containers.

Before digital audio was invented, recordings were made in analog - and the most common analog medium was magnetic, such as audio tape.  Sound would enter a microphone as sound waves at various pressures, in a continuous fashion (hold your hand 2 inches from your mouth and speak the following into your hand "Peter Piper Picked a Peck of Pickled Peppers"...feel that pressure against your hand?), which in turn converted the pressure into a certain voltage that matched that pressure (via a transducer), which in turn took that voltage and moved a bunch of randomly placed magnetic particles on a tape into a very distinct pattern on the tape (called modulation).  When these very distinct patterns on the tape were played back, it would reverse the process and convert those patterns back into air pressures that we can hear.

Digital audio is really just a modification of the analog process discussed above - and to make it a little more complicated - still uses analog signal!  Whaaat?  Except this time, instead of recording to a magnetic medium such as audio tape, we are now going to record to a digital-based medium such as a hard drive or memory card.  

Here's the process.  Sound still enters the microphone as sound waves at various pressures, except this time those sound waves are converted using what's called an "analog to digital converter (ADC) - and instead of translating into a voltage, it's converted into a series of binary data (you know, zeros and ones).  However, digital audio, by it's nature and limitations (storage comes to mind)  must be defined ahead of time. 

"ADC Symbol" by Tjwikcom - Own work. Licensed under Public Domain via Commons -

This  pre-definition is called the sample rate.

So during the ADC process the sound goes through a process that captures exact moments in time as a "sampling" of that very long and continuous analog signal and breaks that very shapely waveform into small representations, turning the new signal into a very blocky and stair-stepped signal.

The sample rate, noted as a frequency (frequency meaning how often), such as 44.1khz, or 48khz determines the maximum frequencies that are captured during the recording.  So human hearing generally runs the range of 20hz to 20,000hz, and most of us can't hear much over 18,000hz anyway.  The sample rate is defined by it's Nyquist frequency, which is exactly half of the sample rate.  So in the case of a sample rate of 44.1khz, the maximum frequency it can "sample" is 22.050khz (which is half of 44.1), which is just over what we humans can hear!  Perfect!

What we know today as CD quality audio has a sample rate of 44.1khz.  So it represents pretty much the entire range of human hearing, and is a very good sample rate to use for your podcast recording.  In fact, at KNVP Studios, your final audio file is converted to 44.1khz  - BUT if you record your podcast at something lower than 44.1khz, such as 32khz, or 16khz, or 8khz - what happens to the audio quality?  It gets lower and lower because, the maximum frequency range is lowered.  So if you record your interviews at 8khz, we are now limiting the amount we can hear to no more than 4khz!  However, our ears can go all the way up to 20khz!  8khz is the sample rate used for digital phone calls.  So if you are using Skype to record your interview, why not bump up the sample rate to 44.1khz?  It will vastly improve your recording.

So why not set my sample rate higher than 44.1khz to get really great sounding recordings, Tom?  Well, while technically true, by nature of the digital recording, it's really pointless to record any higher than 44.1khz because the human hearing model can't hear anything above it!  AND, the file size of your files will get much bigger too!  This is why, the higher the sample rate, the bigger the file, and the smaller the sample rate, the smaller the file.  See, CD quality audio takes up 10.5 MB of space for every minute.  So in 60 minutes, your file is now over 650MB!  This is why your .wav files are so big - but that's ok - this will be explained in a later blog post.

If you have any questions about digital audio, or how sample rate affects the sound quality of your podcast - or if you are interested in starting your very own podcast, please get in touch with me!  I would be happy to help.  Stay tuned for more digital audio primers here on the KNVP Studios Blog!  Don't forget to subscribe!