I try to explain myself in more details:
After graduation in 1982 Summer in Austin, TX. Then I started selling computers, first IBM PC DOS period packed with my macros for lotus and softwares using dBase II & III then Windows as the platform (when the OS became more stable from 1987). And since then, I had had assembled and sold Desktop / notebook computers of about 300 a year from 1985 to 2000, they were IBM, Dell, HP, Sony, Epson, Fujitsu, Toshiba, Lenovo & Self-Assembles . Then I handled Local Networking & Web -based software systems as project manager. And my only hobbies are AV, Music & DIY audio gears All the people in the computer field whom I know do believe the main unit of a PC is noisy (Noisy fan, video interference) for audio ( CAS) . Also, a lot of threads saying so for many years in knowledge bases . Once , I recalled , back a few years ago, I was looking for the Onkyo PCI sound card, Wavio SE-300PCIE US$200+, a reviewer said he would prefer external over internal sound card more or less repeated what I had said . Also, The shut down of the video circuit in pure audio mode in AVR is for the lesser distortion -not only PS for more headroom but also because of HF interference of the video circuits. The manufacturers ( as what I recall, one is Onkyo) say so for many years til now in their catalog !!!
Bryston says so:
Furthermore , the air turbulence and vibrations all add to loss in ANALOG stage of amplification in the computer .
There may be some multi-tank missile proof chassis with absolute vibration-free mounts , plus absolute silent fan but ..
I am using Dell Dimension Desktop at Home. I watch and hear you-tube with a External DAC ( USB ) ( 1.5" x 1") . Very clean and clear and with amplified sound much much better than that Dell onboard sound card . It was for US$4.5 in 2011!!!
My DAC is next to my keyboard while the computer is under Desk . The copper shielding of Onkyo PCI sound card may or may not do its work but my CDP is noisy with the variable RCA output though the IR remote chip is shielded by 3M copper film tape. Same circuit external / Internal differ only by the USB interface+ PS + chassis - not much lol!
Anyway, I just want to emphasis: digital signals processing, e.g. computer mixing , studio editing .. ,of course , is all digital,,, is lossless and faultless but the output of a DAC is not audible unless it is amplified to line level .. etc and that analog circuit is subjected to RF frequency , variable/rotating magnetic field interaction, mechanical vibration all add to distortion / Loss . By using USB / coaxial to do DAC outside the computer box is my stand.
p.s. I do see CAS is the trend for future audio. I am DIYing a AK 4396 USB Kit (US$40 + case($6.) +PS( $6.) which is 1 box for all my sources (CD, SACD, DVD Audio, Digital Concerts, Computer Files, Internet Music.. for serious music listening in my system 1. to preamp ...