Sound quality is subjective. And on top of that, perception of reproduced sound quality is also a learned response. So one may initially prefer a result that matches the learned expectation of "good sound" as opposed to the more real or natural sound.
We do not recommend the standard filter, because in our experience it sound less natural, less like real music and more artificial. But we also recognise not everyone agrees. Hence the inclusion of the filter switch, to give the choice to an owner, rather than to force our preferred choice.
To discuss the reasoning behind this, any filter is a trade-off between impulse response and frequency response. Specifically for CD and DSD64 sample rate... the problems are severe. CD should use at least an 11th order low pass filter at 20kHz with 0dB attenuation at 20kHz. And SACD/DSD theoretically needs a 7th order filter at 20kHz. Those are very severe slope filters right at the upper edge of the audible range. Neither filter is an easy proposition as analogue filter, so digital (FIR) filters are often employed.
Further filters come in multiple "flavours". Namely we have infinite impulse response - IIR (ONLY analogue filters offer true IIR) and finite impulse response - FIR (only digital filters have this). Further, digital FIR filters may be designed with either symmetrical impulse response (the filter has equal pre- and post-ringing when presented with an impulse) or asymmetrical impulse response (the filter has more or less pre-ringing that post-ringing).
This ringing "must" be added if such steep slope filters are employed. Even analogue filters only will show ringing. Now let us be 100% crystal clear. Adding ringing to an impulse is adding distortion. And a LOT of distortion. Most traditional digital filters add >10% distortion for a significant time after an impulse event.
At AMR we produced a short paper focusing on filter options in the DP-777 DAC, however a read of this may also be beneficial here:
- http://www.amr-audio.co.uk/large_image/Tech%20Paper%203%20-%20Ringing.pdf
Different manufacturers have different views and promote different solutions. Again, we have no desire to comment on specifics. Our preferred view however is that less added distortion is better in all domains, amplitude, frequency and time domain, if we desire a realistic, natural sound.
Previously Head-Fi'er Earphonia measured the square wave response of the iDSD micro and posted it here. Comparing the response of the different filters is likely instructive:
- http://www.head-fi.org/t/728236/ifi-idsd-micro-dsd512-pcm768-dac-and-headphone-amp-impressions-reviews-and-comments/3045#post_11387805
Stereophile USA often posts Impulse responses of DAC's. As example we shall refer to the AMR DP-777, as it offers many different filters. It showcases most possible filter responses:
- http://www.stereophile.com/content/abbingdon-music-research-dp-777-da-processor-measurements
Many other DAC's can be found and compared.
The audibility of these filter differences is a fairly hotly debated topic, something that often starts flame wars - which we have no desire to get involved with. But we hope that the above helps to understand a little better why there are choices and what they are.
In the end the proof is in the listening and one should always use the filter setting one finds most enjoyable. If this is the "Standard / Measure" filter, well all the more power to you.