Digital audio data need to be converted to analog before we listen to it because digital signal is actually high freq noise to human ears. After D/A process, there will be a lot of digital signals, which is actually high freq noise, staying in the analog signal. In audio industry, engineers use digital filter and analog filter to solve this problem. Digital filters are like scissors, which can easily shave the freq curve, but it need help from analog signal to kick out high freq noise completely. Most consumer grade mp3 player designs are to set up 20- 20K flat in digital filter (normally the digital filter, d/a converter and cpu controller are build in one $5 to $10 mp3 player chip, such as sigma-tel 3770), and do not build the analog filter. Analog low pass filter actually is the key to eluminate most digital noise. This is the important reason why normal mp3 player are "digital sound", "lean, sharp, dry, ...," : they do not have a real, well designed analog filter. Setup a flat line between 20 to 20k in digital filter is deceiving oneself as well as others because too much digital signal will pass through D/A convertor and stay with analog signal, end with "digital sound". Most hifi grade DAC have high freq roll-off because they do have a well-designed low pass filter. If you carefully check hifi player or DAC's freq curve, you will find that they normally are one of the types in the following four filter types: butterworth, chebyshev or Elliptic".
For a digital player or DAC, filtering digital high freq noise is more important task than make 20-20k flat because the bottle neck of not to be flat actually is in speaker/headphones. It is well known that the final sigal getting to human ears from headphone/speakers is far from flat. However, digital high freq noise is the killer to sound quality because it will make sound signature "lean, dry, not natural".
We are using a special butterworth filter in our players because such filter can keep high-freq digital noise at a lower level. Benefiting from this, our products have very analogue and natural sound signature. For anybody who is really interested in what's going on about rolloff, pls go to sound science and discuss the following link:
Edited by Nankai - 11/23/10 at 6:16am