Pre/Power Amplifiers

As illustrated almost every month in our unique ‘HD Music’ section], far too many digital downloads and streams are normalised right up to, or very close to, the 0.0dBFs digital limit. This drive to achieve ‘maximum loudness’ is counter-productive for several reasons – most modern media has a peak-to-RMS range of just a few bits anyway, while running the analogue output at full gas may highlight limitations of its own. More insidious still is the phenomenon of ‘intersample clipping’, particularly with lower sample-rate CD media. Never heard of it? Here’s a brief primer...

During digital recording, music signals are sampled and then quantised to create the binary code that represents the amplitude of each sample in time. However, if the highest digital values are then normalised close to 0.0dBFs, then it’s entirely possible that a DAC’s oversampling filter will reconstruct signal peaks between samples that notionally exceed 0.0dBFs. It’s important to appreciate that the digital data is not ‘clipped’ but that in the process of upsampling this data, signals are realised that lie above this digital glass ceiling, beyond which is a netherworld of catastrophic clipping and momentary bursts of distortion. Of course, if the interpolator is designed with a few dB of headroom in tow – as is the case with MBL’s ‘True Peak Technology’ – then these peaks, known as ‘intersample overs’, are carried through without clipping.

Few commercial DACs have this headroom, or so it seems. To test the efficacy of MBL’s custom upsampler I programmed a digital feed based on integer (and half integer) divisors of the 44.1kHz sample rate with a combined peak value of 0.0dBFs and an RMS value of –6.4dBFs. The Graph here compares the result of this data passing through a well-known converter equipped with a Burr-Brown DAC [red spectrum] with the clipping-free output of MBL’s C41 [black spectrum]. Proof positive I’d say, and something we’ll be keeping an eye on in future DAC reviews. PM