JaZZ
Headphoneus Supremus
Getting back to your main question - if I had used a bandwidth limited signal, then an ideal infinite sinc function filter would reproduce that bandwidth limited signal perfectly - with no added ringing at all. The interesting aspect about the theory is that it states that you must use an ideal sinc function filter to perfectly reproduce the original; that means that if you as a designer want a transparent filter that neither adds not subtracts then you must make your interpolation filter get as close to the ideal as possible - and that is simply what I have been trying to do over the past 20 years; and I will continue to do this until I can hear no change in SQ with a doubling of tap length. Every time I double the tap length, the difference between actual and ideal halves.
So theory is very simple - we have one and only one way to get perfection and that is unusual in audio, as often I am faced with many alternative ways of converging upon perfection. What is curious is that sampling theory has no message about the first bandwidth limiting filter before the signal is sampled - the ADC filter. Will pre-ringing be important? Will limiting to 22.05 kHz be audible? What is the best way to be able to do this? These important questions I hope to answer with the Davina project.
Thanks for your response, Rob!
So with «ideal» you're addressing the sonic difference between redbook and high-res, I guess? Do you expect Davina to definitely close the gap – since the requirement of a critical bandwidth limitation in close proximity to the audio band before digitization or during decimation from a high-res recording doesn't come with audible losses? The requirement of a bandwidth limitation for creating a CD-compatible recording is an underestimated issue which definitely compromizes the actual capability of a sophisticated DAC like DAVE with 44.1 kHz in my book. However, there's still a chance that ultrasonics do represent a vital component for an ultimately realistic music reproduction. How is your take on that? I'm thinking of interferences like this...
...between a 14.51 and a 26.1 kHz sine wave. The possibility that the resulting beat be audible can't be excluded – at first glance one would even think it will inevitably create an audible difference compared to a continuous 14.51 kHz tone. So the hypothesis would be that overtones could sound too sleek and polished without any ultrasonic content. Of course such a scenario is only imaginable with test tones like above which don't exist in nature, but after all the same applies to sine waves generally, which nevertheless serve as main signal for judging the fidelity of audio-reproduction devices.
On more question – to an entirely different topic. We both agree that adding a headphone amp to Mojo, Hugo and DAVE is a bad idea. My own experiments to the matter have revealed that the direct connection can't be beaten when it comes to accuracy and transparency, so I'm absolutely firm in my ampless approach myself. However, there's still a theoretical possibility that the addition of an amp would objectively improve a certain aspect of the sound, namely dynamics – by providing a «better» (faster, more powerful...) power supply. As mentioned, that's not in line with my own experiences, but I could imagine this scenario to be valid in some cases. Isn't the better (faster?) power supply in the form of supercapacitors and other measures responsible for the TT's sonic advantage over the Hugo? I'm mainly asking to have a better perspective, also in view of corresponding forum discussions with people swearing by excessive power reserves. On the other hand, according to my experience most low-impedance line outputs drive headphones very well and without compromising dynamics in any way. Moreover you have demonstrated yourself that the DAVE is better at driving high-efficiency speakers than via any amp (if you're ready to limit yourself to neighborhood-friendly loudness levels), and Romaz will confirm it.
That said, during my own audio journey I also experienced that a signal path as direct as it gets can absolutely sound disappointing (read brittle and hard) if something in the chain is not entirely right. Actually every component with a revealing characteristic contributes to a higher dependency of the over-all sound from perfection within the chain. So a smooth sounding headphone (be it due to forgivingness, e.g. created by internal reflections, or a really coherent and even characteristic) is a much better precondition for a signal path as direct as possible to sound pleasant and ear-friendly. Generally an uneven frequency response has to be equalized to make the headphone sound «better» than through a euphonically coloring headphone amplifier introducing the necessary forgivingness.