it seems digital design is harder than analog design.
good Q&A Charles Hansen, Gordon Rankin:
http://www.audiostream.com/content/draft#Jwd0w77ZLMYORVJa.97
http://www.audiostream.com/content/theres-no-such-thing-digital-conversation-charles-hansen-gordon-rankin-and-steve-silberman-p#HKAQvHqIkeqmVDhM.97
Charlie Hansen: "All of this can be boiled down to a simple phrase. “All of the problems with digital are analog problems.” This is the primary reason that digital audio has taken so many decades to come close to the sound of analog. The thing to remember is that digital systems are not immune to degradation due to noise. They tend to be much more highly resistant to noise than analog systems, but noise in any system will cause performance degradations."
Gordon Rankin: "But since digital audio is a streaming system, the timing
of the bits is critical. If the bit changes to the correct state but at
the wrong time, this is equivalent to changing to the wrong level at the
correct time. Another area to tackle is what is referred to as signal
integrity. The signal leaves a transistor or IC chip and it has to make
its way across the PC board, component-by-component so that the signal
is degraded as little as possible. When you are talking about what makes
one transport a “good sounding” one, again we are talking about
treating so-called “digital” products as very high speed analog
circuits. The clock frequencies in these units are typically between 10
and 100 MHz. When considering a square wave, a convenient rule of thumb
is that the bandwidth must extend in both direction (higher and lower
frequencies) by a factor of at least 10x to preserve the waveform
fidelity.
So designing a high performance digital circuit means that you are
essentially designing high performance analog circuits that have a
bandwidth extending up to at least 100 MHz, and in some cases all the
way to 1 GHz. The traditional rules of PCB layout connectors, signal
routing, ground planes, solder joints, PCB materials, and even PCB
coatings break down at these high frequencies."
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