fir is a basic up-sampling and interpolation algorythm, it takes two different samples, and interpolates them to make what is a step wave into something closer to a sine wave. It works exactly like anti aliasing works in games.
imagine a image, with pixels, applying a FIR interpolation you smooth the edges, and image becomes much more smoother. something similar. music is ideally made only of smooth waves, but in digital domain, it is made only of step waves; Every DAC in the world has a FIR filter for interpolation.
a TAP in a FIR filter is the number of times it interpolates two samples. 5 taps, mean that between two steps in a step wave, the filter will put 5 new smaller steps, the wave becomes closer to ideal, which is perfectly smooth, without any step
ideally, the best FIR filter is infinite, situation in which it makes the WAVE perfectly analogue, without any step wave. No DAC can do this, at the moment, no matter the price.
Hugo is the strongest in this aspect, it has around 20.000 taps. imagine that between two samples, two steps in a step wave, it creates based on mathemathics 20.000 new samples. the sound is very very closer to natural and totally analogue.
This is what the sinc function on hq player does, it adds a single TAP digitally, based on poly/sinc function. one, and it makes day/night difference. imagine 20.000.
ideally most DACs already add a few taps, some around 30-50, depending on DAC and implementation, the hq player doubles the number you DAC already did, or adds one tap in DSP, then your dac adds 50 before the new sample and 50 after the new sample, depending on DAC implementation.
on my x5, it was not worth it, x5 alone sounded better. on my on-board laptop soundcard, it made the headphone output of the laptop sound close to my fiio x5... mind opening. Cheap soundcards do not add even one tap, and adding one makes a world of a difference.
best ideea is to not employ this technology in software, but get a DAC which adds taps, you are better off, there is a reason for which this thing is not stabe in computer processor,
chord company build DAC only solutions cheaper than hugo that have the same properties, qute if i remember right, the new one has exactly the hugo sound, but no amp built in, and it is not usable as a portable, but costs half of hugo price.
Do not understand me wrong, all DACs would be capable of outputing a perfect smooth wave, they just cannot create it because it was not recorded like this, and not all DACs nor processors have the power to create FIR taps enough to re-build the analogue smooth wave.
DSD files are trying to reduce the step size, acting like there is a FIR filter around there. it works some times, and some times not, you need very high quality dsd for this to work, it does the exact same thing as hugo, but emplying another technology, instead of adding fir taps, DSD tries to divide a single second in so many small parts that eventually there will be no difference to a smooth analog wave. it does not work well, because it does create lots of divisions from a second, but they are sqare waves, based off 1 or 0. this is not a lot of data, so sometimes, depending on song it sounds better or worse than normal PCM. PCM is pulse code modulation, it divides the second always by the same number and the data is stored for a period of a second. you can always choose your poison, DSD will become better with future upgrades, but PCM technology is also far from touching it's limit.
please tell me if you cannot understand, i will try to explain even in more detail if i will be able to.
