Network Audio Players/Servers

In common with the earlier Altair G1, this G2.1 variant offers some user-selectable 'sound tuning' in the form of four alternative digital filters – Precise, Dynamic, Balance and Smooth. Seven pre-programmed filter algorithms are baked into the ESS DAC but it will also accept custom versions, typically generated using MATLAB and then downloaded as C code. For the Altair G2.1 the response is flattest with all sample rates via the Dynamic filter [see inset Graph]. The respective time (impulse) and frequency responses with 48kHz media (solid traces), 96kHz (dotted) and 192kHz (dashed) are colour-coded here in black, red, orange and green. These are a mix of linear phase (Precise, Dynamic, Balance) and minimum phase (Smooth) filters that trade response extension and stopband rejection (99dB, 83dB, 14dB and 18dB, respectively) for reduced group delay and pre-ringing.

For example, 'Precise' [black] is a typical high-order linear phase type with long pre/post ringing and moderate group delay but a flat and extended HF response with a steep cut-off and excellent rejection of aliasing images. Time domain performance is traded for excellent frequency domain performance in a filter ideally suited to lower sample rate media. 'Smooth' [green] offers a contrast with its slightly early but gentle HF roll-off and poor alias rejection, but much reduced 'time domain' distortion. The benefits of this type of filter progressively outweigh any 'negatives' at higher sample rates, but let your ears decide! PM