Bursts of High-Amplitude Distortion
There is one single problem with digital audio that eclipses every other known problem. Nevertheless, this problem completely escapes the scrutiny of traditional audio measurements. John Siau shows how bursts of high-amplitude distortion are often produced by the digital playback equipment when playing ordinary CD tracks. These bursts of distortion can occur thousands of times during a single track, and the distortion often exceeds 10% during each burst. At 10%, this distortion is just 20 dB lower than the level of the music. It is also louder than the distortion produced by typical loudspeakers. This distortion is caused by a lack of digital headroom in DSP devices, sample rate converters, and oversampled D/A converters. These devices don't clip when playing traditional test tones, but they do clip when playing music!
Caused by a Lack of Headroom
A very simple test can be used to determine if a device has sufficient headroom to accurately render the peaks that occur between successive samples. An analysis of thousands of CD tracks shows that our digital playback equipment must have as much as 3 dB of headroom above the value of any individual samples in the recording. When digital samples reach the maximum digital codes at 0 dBFS, the peaks between these samples can reach +3 dBFS.
Many Manufacturers are Ignoring the Problem
Headroom can easily be added to digital audio devices, and Benchmark has been doing this since 2012, but most of the industry is still ignoring this problem. They blame the "digital sound" on oversampled DACs or on the types of filters used in these DACs. They even blame PCM in general and attempt to solve the problem by turning to DSD.
The Cure is Simple, but We Must Recognize that the Problem is Real!
Once we recognize to root cause, the cure is simple. We just need to add digital headroom. Some manufacturers have been able to add headroom with software updates. You may find that your music playback software has a 'DSP headroom' feature that can add the required headroom. Sometimes the clipping can be eliminated by turning down a digital volume control, but not always!
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