Outboard DACs

Not only does the Nu-Vista DAC employ an XMOS micro and Altera CPLD to process DSD/MQA streams via USB, with reclocking, there's plenty of spare overhead to accommodate a 'bypass' upsampling mode in addition to a custom 'Optimal Transient' filter algorithm. The latter [red traces, inset Graph] was first seen in the Audiolab M-DAC [HFN Sep '12] courtesy of roving designer John Westlake and joins seven other filter choices that come 'pre-packaged' with the ES9038 DAC. 'Optimal Transient' is very close to a 'NOS' filter, trading zero pre/post ringing (no time domain distortion) for little or no stopband rejection (just 1.6dB) and a mid-treble roll-off of –0.9dB/10kHz to –3.5dB/20kHz with CD sources. The advantages of this filter are best realised with higher – 88.2kHz upwards – sample rate files/streams. The 'Upsampling' mode bypasses the named digital filters but is still a filter in its own right with the properties of a medium-tap minimum phase design [green traces]. It offers a limited 12dB stopband rejection and response that reaches out to –0.6dB/20kHz, –11.3dB/45kHz and –13.2dB/90kHz with 48kHz, 96kHz and 192kHz media, respectively.

The remaining (ESS) filters include two linear phase types [the black traces are Linear Phase Fast] that offer a constant group delay, yielding a time-coherent output, and two minimum phase options that free the impulse response of any unnatural, acausal pre-ringing but increase post-ringing and incur a non-linear group delay. Both 'Fast' versions offer a 75-83dB stopband rejection and similar –0.4dB/20kHz, –2.2dB/45kHz and –5.6dB/90kHz responses and are ideal for low sample rate (CD/48kHz) file types. The Apodising and Brickwall options are linear phase with their own 'twists' in the time/frequency domains while 'Corrected Phase' [blue traces] is equivalent to the 'Hybrid' option seen on competing outboard DACs with ESS converters. PM