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Once we assume an accurate digital bitstream the quality of the music is entirely down to the DAC ?

Yes, in theory, but the challenge is in getting an accurate digital bitstream to the DAC. Getting the right data is no problem (although some PC soundcards with digital outs modify the data), but the data has to arrive at the DAC at exactly the right sampling rate and this is harder to achieve as it depends on the interface between the source and the DAC. The advantage that custom music servers could have is that their designers can control delivery of the data to the DAC without an external interface. How well they achieve that depends on their design parameters and their knowledge and experience.

IMHO if a separate source and DAC solution has an interface that delivers the right data to the DAC at the right rate then a music server might equal it but will not better it in this regard.

Dave

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Thats an interesting point, do DAC's work synchronously or asynchronously ? if its the latter than the source timing would be unimportant.

In most configurations the source will use its internal clock to time the data through its digital out(s). It will embed timing codes in the data stream which the DAC will read to synchronise its own clock to the incoming data stream. It follows that the accuracy of the sample rate at which the data is finally converted will depend on the accuracy of the source clock, the quality of the interface between the source and the DAC, and the DAC's ability to read the incoming timing codes and match its clock to them.

Some of these issues can be overcome by sharing the DAC's clock with the source through a separate wordclock interface. In such an implementation the data being passed through the source's digital out(s) is timed by the DAC's clock so there is no need for the DAC to read the timing codes as the data should be arriving at the sampling rate set by its own clock.

In either implementation the sampling rate could vary from the recorded sample rate due to clock variances. There are varying opinions on how much effect timing errors have on the final sound quality. I operate my music PC sound card slaved to my DAC via its workclock interface but I would not claim to be able to reliably tell the difference between this configuration and having it operate from its internal clock.

Dave

To try to answer this question, I'm first going to take a shot at providing some definitions. Any digital media playback system will have these components:

• Storage Media - This could any be type of physical digital storage media including CD, DAT, DVD, Flash memory, a hard drive platter or even a USB thumb drive.
• Media Transport - A device to read bits from the selected content on the Storage Media and transfer those bits to the Digital to Analog Converter (DAC) via the Data Interface.
• Data Interface - A communications channel to transfer the bits from the Media Transport to the DAC. This interface consists of physical circuitry at each end, the connecting wire or fiber as well as a protocol. Currently available Data Interfaces include; S/PDIF, AES/EBU, USB, TCP/IP, Firewire, ADAT, I2S, PCI and HDMI.
• Word Clock - A digital signal whose transitions define the exact moment when the received data is valid. This clock should be very stable and exhibit minimal jitter.
• Digital to Analog Coverter (DAC) - The DAC receives data from the Data Interface and converts these data to analog at a rate determined by the Word Clock.
• User Interface (UI) - An interface for the user to select content and control its playback. This interface could include some sort of visual display, front panel buttons, a remote control or even web pages presented through a browser.

Based on the number of choices for many of these components, there are a very large number of possible digital audio playback systems each with their own set of technical and user experience challenges and advantages. The CD transport (Media Transport/Word Clock/UI), S/PDIF interface (Data Interface), DAC (Word Clock/DAC/UI) combination has evolved over the many years since the CD was introduced and there are many examples of systems built on this model that can produce exemplary sound. The biggest technical challenge faced by this model is that the Word Clock must be derived from the S/PDIF signal. The S/PDIF interface is unidirectional - there is no means for the DAC to control the rate at which the Media Transport delivers data. A high-quality CD transport can deliver a very stable, low-jitter embedded Word Clock and can, therefore, sound better than a poor implementation. The better DACs with this interface have gone to great lengths, sometimes with great success, to minimize the effects of jitter from this interface.

Instead of "music server", I'd prefer to user the terms Media Server and Media Player. The Media Server provides the Media Storage/Transport functions as well as database and search functions for the User Interface. The Media Player receives data over the Data Interface, renders that data as analog outputs and is controlled by the User Interface. The Media Server/Player model can also take many forms but generally includes a hard drive for the Media Storage/Transport function. The immediate advantage for this media is that storage and transport of vast libraries of content becomes possible from a device that occupies a very small space. Large racks of CDs can be moved to a box the size of a toaster.

Depending on the choice for the Data Interface and its implementation, it is possible to derive a very precise, low-jitter Word Clock with this model. Provided with this pristine Word Clock, the DAC can deliver an analog output that approaches the theoretical limits for the content sample rate and word length of the music content. The Logitech Transporter and the Linn Klimax DS have demonstrated that systems relying on a hard drive for storage and transport of audio media can deliver performance at least equal to the very best systems relying on a CD transport.

A system model that includes a CD transport, of course, is constrained to store and play back only the Red Book CD format. Based on their implementation, systems utilizing other Media Storage/Transport models can also offer high resolution audio formats with word sizes of at least 24-bits and sample rates of at least 88.2 kHz and can also provide storage and transport of other media such as video and pictures.

The strongest advantage, IMHO, of the Media Server/Media Player model lies in the User Interface. This interface can offer the convenience of quickly selecting content for playback based on any whim I might have for artist, composer, title, genre or even mood. If the display supports graphics, I could view album art or even read liner notes as I'm listening. All those great benefits that iPod/iTunes brought to the masses and more can be enjoyed in the audiophile world. I'm never confronted with stacks of recently listened to CDs that must be returned to their correct location on the rack or experiencing the horror of finally finding that particular CD on the rack only to discover that it's jewel box is empty.

I hope my lengthy explanation hasn't added to your confusion.