Hi Mark,
For the closest approach to Hi-Fi, what ever the data source, be it CD, USB or SPDIF inputs it’s the basic requirement that the Audio “Data” is recovered “Bit Accurate”.
There’s a whole load of talk about different CD mechanisms, or USB cables, or audio reply software – but if any item of software or hardware restrict the transfer of “Bit Accurate” audio, then you can simply NEVER attain Hi-Fi reproduction – if the Data is not Bit accurate, then you cannot reproduce the Audio signal in High Fidelity, its as simple as that.
While Bit Accuracy is the first requirement, second order effects such as Jitter, RF Interference and Ground-loops become the next items that require attention.
There’s simply no reason why a CD servo section should be allow to effect the DAC conversion process if design correctly – a CD “transport” is one of the easiest items to design to remove “Second” order effects.
Its gets more complex when computers / networks are brought into the equation – computers generate large amounts of RF noise and nasty Earth leakage currents with there associated noise loops.
Async USB goes a long way to removing the first order Jitter effects, the DAC / PC interface is controlled via the DAC’s high performance Clock. However RF noise and Mains Earth related leakage currents will have there hand in effecting the conversion process via second order effects that can “Jitter” the DAC’s clock, and / or inject RF into the Analogue sections etc…
As a designer, CD easy to get closer to the “perfect” source (working with the available Data), while USB and other “Multimedia” devices require much greater care.
That said, with care, PC’s and Multimedia devices can and do deliver HiFi results – but it just requires designer's who understand the complexities involved.
As a simple example, there’s simply no reason why an iPod, iPhone or iPad could not deliver the same results TODAY as the Best CD players (replaying lossless files) – it only requires attention to BASIC details – the issue has been to convince Apple that HiFi manufactures deserve the chance to develop such products – and Apple to open a simple avenue for “small” companies to access and license their technology – while still protecting Apple and the Music industry’s IP.
John