Ever heard of the above digital audio condition? No? Me neither until I was reading a review of my Panasonic S97 HDMI DVD player by Christine Tham on Audioholics. 0 dBFS+ is illegal (in digital audio). It is a distortion of an audio signal above the absolute digital audio amplification level ceiling of 0 dB. It can hide on your CDs from the original mastering (loudness wars), appear when your digital music files are “normalized” (like iTunes’ and your iPod’s “Sound Check”) and be introduced by an upsampling DAC while converting the signal to analog.
Tham explains further in her article Issues with 0dBFS+ Levels On Digital Audio Playback Systems:
When this recording is played back, a digital to analog converter converts these binary values back to a set of voltages, and a reconstruction filter is applied to convert these voltages back into a continuous signal. It is possible for the reconstructed analog waveform to have voltages that are higher than the highest digital sample recorded. So, if the highest digital sample captured happens to correspond exactly to 0dBFS, the recording, when played back, may result in an analog waveform exceeding 0dBFS. The subsequent analog stage of the playback chain may not be capable of handling a signal greater than 0dBFS (also known as “0DBFS+”), resulting in clipping distortion.
It has all to do with inter-sample peaks. If you record a sine wave at frequencies near integer fractions of fs (where fs = “sampling frequency”), such as fs/4 and fs/2, then (depending on the phase of the sine wave with respect to the sampling times), the digital samples may never actually capture the true peak of the analog waveform. Hence, when this recording is reconstructed back into analog, it will result in analog peaks higher than the highest digital sample captured.
A “raw” recording – captured directly from an analog to digital converter (ADC) with no subsequent digital signal processing, should NEVER contain 0dBFS+ levels. This is because the original analog waveform should always be below 0dBFS, provided the recording gain is set correctly. However, subsequent processing of this recording may create 0dBFS+ inter-sample peaks. For example, “normalization” is a common technique – this multiplies every digital sample by a constant value so that the highest digital sample captured becomes 0dBFS. This will cause any inter-sample peaks that exceed the highest digital sample captured to exceed 0dBFS.
Tham then goes on to test some current DVD players to see how they handle 0dBFS+ signals. Most did not do well. Building headroom into your DAC and its signal path to allow for the illegal signal isn’t on the top of any DVD/CD player manufacturers’ feature list, so we get gross distortions with many different digital recordings.
I know I’m late with this one as the article is more than two years old but it is still relevant. The loudness wars are still raging and loud distortions on CDs are still winning. And we have to wonder why digital recordings can sound so bad.
I’m doing what I can with my personal hi-fi to combat 0 dBFS+. I plan to run my own sine waves through my main digital paths (my Onkyo’s and Pioneer’s DACs) and see how they handle the clipping. I’ve already disabled “Sound Check” on my iPod and iTunes (it never really helped with all the different volume levels my favorite albums have anyway). Lastly, I want to compare my Panasonic S97’s music performance with my Airport Express and Pioneer DV 47ai sources. Fortunately, Tham revealed that the S97 has an audio attenuation mode that lowers the digital signal by 5.4 dB giving headroom to 0 dBFS+ signals. I’ve never even used the Panasonic player as a CD source, I just thought it made pretty DVD pictures over HDMI.
Are 0 dBFS+ digital distortions killing your music? Is this why we can’t relax when listening to CDs? Is anyone else measuring this distortion other than Audioholics?