thanks alot, cornwaller and whell
I have to say both of you are quite right. Now I can see that when you do analog signal processing, there are a million factors affecting the final sound coming out the speakers so it's difficult to judge the DAC based on the limited experiments I have done.
Then what about the my second question? As I understand, the digital output should give the signal which is the digital information recorded on the cd so there is no quality factor involved. it's either right or wrong, isn't it? If so, the digital output of different players should give the same digital signal as long as the disc is the same, right or wrong? Or maybe a good player makes less mistakes when reading a disc. I guess that's why cd/dvd player advertisments alsway say something like "precision optics".
Happy new year!