How Does Up-Sampling Work?

[brightonjel]

The main benefit of up-sampling is that it simplifies the D/A process. Let's start at the output. The DAC will always have to convert a stream of digital data into an analog signal at the end of the conversion chain. Put simply, the more bits that analogue output stage (i.e. filter) has to work with, the easier it is to have that happen, and hence to produce a high-quality result. With your background you will easily see that reconstructing an analogue signal via a filter from a 44.1/16 data stream is more challenging to design and implement than doing the same from a 192/24 stream, for example. Given that it's much easier and cheaper to do things in the digital domain (especially inside a single IC at the design stage), it naturally follows that DAC designers will want to up-sample on the input in order ease the life of the analogue filter output stage. In short, those output samples will be "filled-in" anyway by the analogue filter, and these days helping this process by adding more samples in the digital domain is cheaper and easier than in the analogue one; part counts are lower, filter design is easier, and the impacts of the filter on the signal's phase and frequency response easier to manage.

 

Now, there are of course many ways to achieve that up-sampling. The simplest is just to double the input rate and stick the average of two successive input sample in between the source ones to create the intermediate value. However, that's also not a very good solution, because it takes no account of the rate of change of the signal pre- or post-sample. Consequently, a range of other digital techniques have evolved, all striving to do a better job of curve-fitting, if you will, the digital samples to what the original analogue input signal must have looked like before being digitized.

 

Hope this helps?

[brightonjel]

I'm not familiar with that DAC so don't know to what degree it's chip set over-samples. Try it both ways and see which you prefer, because I'm guessing that whether you over-sample in Audirvana or allow the DAC to do it all, will change the sound signature. Personally, I use HQPlayer and up-sample/convert from Red Book to 2xDSD because I prefer the resulting sound in my system to just feeding the DAC 44.1/16 PCM. Software players these days have some very good up-sampling algorithms, and doing it there, all other things being equal, can produce great results.