Digital Audio Basics: Sample Rate And Bit Depth

The most important practical effect of bit depth is that it determines the dynamic range of the signal.

In theory, 24-bit digital audio has a maximum dynamic range of 144 dB, compared to 96 dB for 16-bit but today’s digital audio converter technology cannot come close to that upper limit.

As of this writing, the 24-bit Burr-Brown converters in StudioLive AI-series digital consoles, StudioLive RM-series rackmount mixers, and Studio 192-series audio interfaces offer a dynamic range of 118 dB, which is close to the best dynamic range attainable with current technology.

The Going Rate

As noted earlier, in the digital conversion process, the converters record and play samples at specified sample rates.

The Nyquist-Shannon sampling theorem states that in order to accurately reconstruct a signal of a specified bandwidth (that is, a definable frequency range, such as 20 Hz to 20 kHz), the sampling frequency must be greater than twice the highest frequency of the signal being sampled. If lower sampling rates are used, the original signal’s information may not be completely recoverable from the sampled signal (see Fig. 2).

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FIG. 2: Increasing the sample rate but not the bit depth (a) improves the accuracy of the representation because the converter is taking “snapshots” of the signal more frequently. However, increasing both the sample rate and the bit depth (b) produces much more accurate results.

If the sampling frequency is too low, aliasing distortion can result. Aliasing is a major concern when using analog-to-digital conversion. Improper sampling of the analog signal will cause high-frequency components of the signal to be aliased with genuine lower-frequency components. If this happens, the digital-to-analog conversion will create an incorrectly reconstructed signal.

Which rate you choose depends at least in part on the product you need to deliver. For example, audio CDs and MP3s are delivered at 44.1 kHz, so sampling at 88.2 kHz makes the converter’s calculations relatively simple. Digital broadcast uses 48 kHz, so a 96 kHz sampling rate is an obvious choice. That said, some engineers believe that today’s sample-rate conversion is good enough that it’s not necessary to choose a rate based on keeping the math simple. For these engineers, the higher rate is generally considered better.

The High-Resolution Frontier

Finally, one often encounters the term “high-resolution audio” but it is rarely defined. That’s because there is no agreed-upon definition. For many years, “resolution” referred to bit depth, but in recent years, the term has been used more broadly to refer to both sample rate and bit depth. And “high resolution,” in particular, is a relative term. When 8-bit audio was in common use, 16-bit was “high resolution.” Today, 24-bit, 96 kHz audio is considered “high resolution.” In the future, it might be 32-bit, 192 kHz and beyond.

Wesley Smith is a musician, studio rat, live-sound nerd, and part-time detective novelist. She uses all of these skills and more as a product manager for PreSonus Audio Electronics. The original article can be read here.