Back in the August issue of last year, Jim Lesurf investigated the topic of 'out of range' signals in digital audio systems. We are used to the idea that sample values cannot exceed OdBFS (OdB full-scale) in digital signals but this does not mean that the analogue signal reconstructed from them is so constrained. In fact it can exceed the OdBFS value by a significant amount. A simple illustration of this is shown in Figure 1 where a sine wave at one quarter the sampling frequency (that is, 11.025kHz in the case of CD) is sampled such that the digital representation looks like a full-scale square wave. In this instance the peak analogue output voltage is 3dB above the OdBFS equivalent. As Jim showed, with more contrived waveforms it is possible to generate peak output voltages over 2dB higher than this.
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