Saturday, June 06, 2009

Audio over USB

The arrivals of the iPod, iPhone, Airport Express and Apple TV have raised the bar of consumer expectations of convenience when it comes accessing their music collection. With hard drives being as cheap as they are and so many new DACs coming equipped with USB inputs putting together a DIY music server seems like a better and better idea every days. However, like many things in high end audio it's not as simple as it first seems. At least two major factors stand between the interested audiophiles and good sound.

  1. Jitter: In essence all of the "1" and "0" are present and in the correct order but the word clock that tells the DAC when to decode them is wrong. We're talking about fractions of a second here but our ears do pick up on it. Think of it this way. The world's best marching band all taking their time from a drummer with great rhythm vs. that same marching band with someone with no rhythmic sensibility. The musicians haven't changed, their instruments haven't changed, and the sheet music is still the same. But the rhythm won't be the same and things will start to sound sloppy and lax. Many of the new digital to analog converters are built specifically to reject incoming jitter are based on the Burr Brown PCM270x chips. That chip set has two major limitations. First it has a strong jitter component at 1kHz, which as misfortune would have it is smack in the audio band AND the frequency at which the human ear is MOST sensitive. Second, the chip is limited to a maximum sampling frequency of 48kHz which excludes it's use to transfer high resolution 88.2 kHz, 96kHz, and 192 kHz content. A better solution is a DAC capable of exploiting USB's asynchronous mode where the DAC requests the data when it is needed and therefore determining it's own word clock. Curently very few DACs on the market are capable of properly implimenting the asychronous mode because of the coplexity of the computer code necessary to make it work properly. Gordon Raskin, whose first career was in the computer world is one of the few people on the planet with a good understanding of bother sides of the problem. Currently his custom code is available in products from his company, Wavelength Audio and the upcoming QB-9 from Ayre.
  2. Re-sampling: CD's are at a sampling rate of 44.1kHz which means that every second is broken up into 44,100 slices with each slice being described by a series of 16 "1" and "0." Microsoft in their infinite wisdom decided that most of their products translate everything to 48kHz or 48,000 by adding essentially garbage in a part of the program called the "K Mixer." According to the Wavelength web page the "K" mixer can be by-passed or mapped through as it's called (proceed at your own risk) by the following steps. "Go Control Panel => Sounds and Audio Device => Select Hardware Tab => Select USB Audio Device.

    Click Properties Button => Select Properties Tab => Click the “+” to the left of [+] Audio Devices that will pop down USB Audio Devices.
    Select USB Audio Devices and click Properties.

    Check both Use audio features of this device and Do not map through this device.
    ==> [ Apply ] <=="

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