Theory of Upsampled Digital Audio

In the last few years, several DACs have appeared on the market and sold as “upsampling” DACs. Upsampling DAC manufacturers claim that their products improve the sound quality of standard CDs as compared to conventional DACs and most listeners agree. What can possibility account for the improved sound quality? After all, the digital data on the CD is the same no matter what DAC is used to convert it to analog form. The most popular theory to explain the superior sound quality of upsampling DACs is the time smearing theory. As will be explained shortly, all upsampling DACs employ slow roll-off digital reconstruction filters as opposed to conventional DACs, which employ sharp roll-off or brick wall reconstruction filters. Since slow roll-off filters show less time smear than brick wall filters when measured using artificial digital test signals, many have concluded that reduced time smearing is responsible for the subjective improvement in sound quality when playing CDs through upsampling DACs. Although this seems logical and is an appealing explanation, as will be shown, it falls apart upon closer examination. The main reason is that the digital audio data residing on a CD is already irreparably time smeared. No amount postprocessing of the digital audio data by the playback system can possibility remove or reduce this time smearing. This paper concludes that the slow roll-off characteristic of upsampling digital filters is indeed responsible for the improved sound quality but that the mechanism has nothing to do with time smearing or the lack of it in the playback system.