Network Audio Players/Servers

Ringing The Changes

When it comes to digital filters, hi-fi brands typically fall into one of two categories – those that 'voice' their product(s) with one type of filter (a variant of linear or minimum phase, apodising, hybrid, etc, that's either home-grown or baked into an off-the-shelf DAC) and those that open the field with a smorgasbord of coefficients. Depending on your viewpoint, multiple filters are either manna from heaven for hi-fi tweakers, ambivalence on the part of the brand or a crafty attempt to drive me around the twist by exponentially increasing my workload in the lab... To its credit, dCS is, I believe, unique in curating digital filters whose coefficients (and hence the transition band, stopband, response and ringing characteristics) are adapted to each sample rate in turn. So, while Filters 1-4 and 6 are all linear phase types their performance with 44.1/48kHz media changes with 96kHz or 192kHz files.

The permutations are considerable, if you have the time, patience and auditory acuity to explore them all! For example, Filter 1 [black trace, inset Graph], Filter 2 [red] and Filters 3 [cyan] and 4 [orange] retain a ruler-flat response (–0.02dB/20kHz) with 44.1/48kHz media but trade progressively poorer stopband rejection (108dB for Filter 1 to just 16.6dB for Filter 4) for reduced pre/post-ringing. With 192kHz files these same four filters have very different HF responses – a steep cut to –13dB/90kHz, Filter 1; a gentler roll-off to –7dB/90kHz, Filter 2; an early post-20kHz roll-off to –6dB/56kHz, Filter 3; and a gentler post-30kHz roll-off to –18dB/90kHz, Filter 4. Filter 5 [green trace] is a minimum phase type with low sample rates but a NOS/Gaussian type above 176.4kHz [hence the early –6dB/39kHz rollout with 192kHz media on the inset Graph]. Filter 6 [purple] is another linear phase filter, but with a very long tap length, trading extended ringing for exceptional stopband rejection. PM