We are all familiar with the two audio file descriptors sample rate and bit depth. While these specifications sound routine, I often get questions from producers and mixers about the optimum settings for a given project. This article will cover the basics and best practices for setting sample rates. Don’t worry, though, in another article we have cover bit depth!
Sample Rate Defined
Sample rate tells us how many times per second we take a measurement of an analog audio waveform as it is converted to a digital signal. Since sample rate has a speed, or frequency, the sample rate defines the frequency response of an audio recording. Specifically, the Nyquist Theorem states that the highest frequency we can record is half of the sampling rate. This means a sample rate of 44.1 kHz can record audio signals up to 22.05 kHz. Accordingly, a 96 kHz sample rate allows for 48 kHz of audio bandwidth. If we attempt to record above half the sample rate, or the Nyquist limit, audible artifacts called aliases occur. Analog to digital converters eliminate aliasing by low pass filtering the analog signal at half the sample rate. This low pass filter is referred to as an anti-aliasing filter. In practice, the low pass filter requires a range to operate, so we state 20 kHz as the practical upper limit for 44.1 kHz sample rate.
We know that human hearing covers from about 20Hz to 20 kHz, so why would we need sampling rates above 44.1 kHz? One answer is that many people, including scientists, claim that humans can perceive sounds as high as 50 kHz through bone conduction. That claim may theoretically be correct, but through air humans only hear up to about 20 kHz. The second reason is a more practical one. The low pass anti-aliasing filter is not a perfect filter, so it creates some of its own distortions. There is a design trade-off between how steep a filter can be vs. how little phase distortion the filter produces.
Recommended Sample Rates
Since our hearing is only capable of 20 kHz, can’t we just stick to a sample rate of 44.1 kHz? We just learned that sample rates above 44.1 kHz may sound better simply because the analog filter design in the A-D converter has less impact in the audible range. In other words, 44.1 kHz captures all the audio bandwidth humans can hear, but the low pass filter may adversely affect audio below 20 kHz. For this and other reasons, it is recommended that we produce and mix pop music at 48 kHz. First, 48 kHz allows for better sounding anti-aliasing filters than 44.1. Second, 48 kHz uses only slightly more disk space than 44.1. Third, videos usually require 48 kHz audio and much of our audio will be embedded in a YouTube or other video as part of distribution. If you produce music solely for audio CDs, then 44.1 kHz would be the recommended way to go.
Higher Sampling Rates
For audiophile jazz, classical, world music, and some sound design projects, I would recommend the 96 kHz sample rate. This sample rate all but eliminates audible high frequency aliasing and filter-induced distortions. Furthermore, 96 kHz audio files may provide lower processing latency and allow for excellent sounding downward pitch-shift effects for sound design and game audio. Additionally, a 96 kHz recording can be neatly reduced to 48 kHz when the need arises. If you feel the need to record at sample rates above 96 kHz, you should spend a considerable amount of time testing analog recording chains, converters and DAWs to find a workflow that suits your purpose. Sample rates above 96 kHz may be more susceptible to jitter problems and will certainly tax your CPU, reduce your track count, and provide fewer plugin choices. I would generally recommend against 176 or 192 kHz unless you have truly studied the pros and cons of those high sampling rates. For reference, the Grammy’s Recording Academy Recommendations for Hi-Resolution Music Production document proposes a minimum sample rate of 48 kHz and preferred sample rate of 96 kHz for hi-res audio production and delivery.
Sample Rate Conversion
Sometimes sample rate conversion is unavoidable and many software utilities provide excellent sample rate conversion. From a recent survey of mastering engineers, some of the popular sample rate converter programs are: Voxengo r8brain, Weiss Saracon, Pro Tools SRC (using Tweak Head settings), Izotope Resample, and SoX. Many other programs provide excellent results and DAWs are continually improving their SRC algorithms.
So we see that choosing a sample rate is relatively straight-forward. Here is a cheat sheet to help keep it all organized:
Sample Rate Cheat Sheet
Recommended sample rates for various situations:
Recording: For pop music stick to 48 kHz, but 44.1 kHz is acceptable. For audiophile music or sound design you may prefer 96 kHz.
Mixing: Mix sessions should remain at the sample rate of the recording. You will not improve the sound of a project by upsampling a session to a higher sample rate session. If you are mixing on an analog console, print your mix at either 48 kHz or 96 kHz as described above.
Mastering: Do not upsample during mastering. For in-the-box mastering, master at the same sample rate as the delivered project. For analog mastering, play out the digital file at its native sample rate, process via analog processors and capture the result at the sample rate the client requires, usually 48 kHz or 44.1 kHz. Also capture a 96 kHz file for archiving. Do not sample rate convert the final master unless it is unavoidable.
Distribution: Video producers usually require 48 kHz files while digital music distributors like iTunes will accept any sample rate of at least 44.1 kHz. Do not sample rate convert the final master unless it is unavoidable.