Network Audio Players/Servers

The technical tale of Bluesound’s Node Icon is, in part, informed by its MQA-derived ‘QRONO d2a’ digital filter. The ES9039Q2M DAC – two are used here – includes a choice of eight different digital filters, but the chip interface will accept pre-processed (upsampled, etc) audio data. This is Bluesound’s chosen route. In practice, the ‘QRONO d2a’ filter is a slow roll-off minimum phase type, with post- but no pre-ringing, and where the coefficients are truncated with increasing sample rate. So, with 44.1/48kHz data there is maximum post-ringing, albeit still much reduced compared with a standard or fast roll-off minimum phase filter, traded against a mere 21dB stopband rejection and a response that’s flat to within ±0.2dB from 20Hz-20kHz. Bass/midrange distortion is vanishing low at 0.00004% (the merest hint of 3rd harmonic) even at the Node Icon’s maximum 3.7V/0dBFs output.

However, there is evidence of digital overload at higher frequencies where distortion increases to a full 0.72%/20kHz [blue trace, see Graph 1]. This effect is reduced at higher sample rates. With 96kHz data the filter ringing is curtailed, as is the stopband rejection at just 17dB, but overload is better managed and THD (at the same 20kHz/0dBFs) falls to 0.035% [pink trace, see Graph 1]. The response, meanwhile, reaches out to –0.1dB/20kHz and –10.4dB/45kHz. With 192kHz data the impulse response is almost NOS-like (no ringing), the filter offering a negligible 5dB stopband rejection while the response extends from –0.5dB/20kHz to –2.7dB/45kHz and –11.3dB/90kHz. But distortion at 20kHz/0dBFs falls still further to 0.0001%. In all cases the A-wtd S/N ratio is a wide 114.8dB and correlated jitter is <5psec (all sample rates), though some phase noise is evident [see Graph 2]. PM