How are these DAC's able to perform as well or better than DACS that use filtering to diminish aliasing effects? I understand that there are some who believe that the best sounding DAC's in the world are NOS/non-filtering. How is is this possible?Robersong, when you talk about aliasing effects, you are usually talking about a filter that comes BEFORE the converter (be it D/A or A/D). Such an anti-aliasing filter is almost always used when we employ A-D conversion. The anti-alias filter band limits the incoming signal between 0Hz & Fs/2 which is the fold-over frequency. Fs is the A-D converter's clock frequency. The purpose of the anti-alias filter is to prevent frequencies between Fs/2-Fs to fold into the A-D conversion band & create distortion/aliasing.
Robertsong
In a D-A converter you might not need to use an anti-alias filter since the incoming signal is a digital bit stream. In many D-A converters there is a PLL type IC used that locks onto the incoming bit stream, extracts the clock from this bit stream & by PLL action cleans up this extracted clock. Cleaning up the clock would mean reducing the jitter (or in analog terms reducing the phase noise of the extracted clock). If the incoming bit stream is sync'd to this clean clock, one can directly feed this bit stream into the D-A converter.
I *think* you mean to write "reconstruction filter" which is the analog filter after the D-A converter. Is that right?
While it is true that many non oversampling (NOS) DACs do not use explicit analog filters, they still filter the DAC signal. There are many tubed output stage NOS DAC where the tube itself acts like an analog low-pass filter that has a -3dB bandwidth of 22-25KHz. In other cases where the output stage is solid-state, many NOS DACs use a SInc filter where the sinc filter has a 0 amplitude at, say, 60KHz. This would mean that lower frequencies (such as 30KHz, 40KHz, 50KHz) are heavily attenuated (but not zero amplitude) such that any clock energy in this frequencies is also heavily attenuated.
So, contrary to what meets the casual eye, there is analog filtering taking place - either using the natural roll-off of the output tube or employing a higher freq sinc filter.
I think one of the key reasons that NOS DACs sound as good as they do is that they do not use up sampling or oversampling which is a digital filter to interpolate the incoming music signal as the incoming signal's data rate is increased from 44.1KHz to something higher. This digital interpolation filter is essentially software devised/created/invented by the manuf & it is a sophisticated algorithm to interpolate while keeping the distortion to a minimum as defined by the manufacturer. If you agree with the manuf then you will like his/her interpolation hence upsampling/oversampling DAC. If you don't agree, you'll be back in the market hunting for another DAC.
When you buy an up/oversampling DAC you are essentially subscribing to the manuf interpretation of guessing at the music signal as the data rate is increased. That's why there are so many different up/oversampling DACs - each manuf has his/her own interpretation of this process & nobody is right or wrong; they are just different. The other thing is that since they are up/oversampling i.e. DSPing the incoming signal, the original music signal has been changed much more than if one were to use a NOS DAC. Any time you meddle with the original signal you add distortion, no matter how little. And, this can detract from the listening experience a little or a lot.
In a NOS DAC if one keeps the electronics to as precision circuits as possible then this sort of DAC does the least required to convert the incoming music signal to an analog equivalent. OTOH, an up/oversampling DAC does the most processing to covert the incoming music signal to an analog equivalent.
Of course, as in all of audio, NOS DACs are not always the best - there are many excellent up/oversampling designs.