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Re: Digital Tweaks and Audiophilia

2) In this day and age, getting the correct data off the disc on time is NO PROBLEM.
True. The main reason is that modern readers separate the tracking and the data collecting functions onto 2 separate laser beams. The tracking beam is servoed to track the ... track, and the data collecting laser only collects data. The servo moves the beam bearing arm. The bandwidth of this servo should be from DC (in order to slowly follow the outside spiraling track) to about 5KHz, in order to deal with vibrations that could access to the reader. By making the 2 fonctions separate, the manufacturer can tweak the servoed mechanical system in order to have a Q of about 0.5 (which means no peak when following some vibration), without any interaction with the read data. Furthermore, the arm being very light, the servomotor task is much easier. On the contrary, ancient CD reader used the same laser mounted on a much heavier arm. So, the servo bandwidth had to be much less than 5kHz, and you got a modulation of the BER (bit error rate) by a peaked version of incoming vibrations (peaked because the high arm mass and low BW forcludes a low Q). In other words, odd sonic behavior. That was the past.

>So the real places for differentiation lie in the clock, filter, and analog circuits of the digital device.
let us talk about the clock first. The data modulation used in CDs lets the raw clock being directly extracted from the read light reflection of pits and holes. You can then servo out mismatch and slow variations of the clock (for example when regularly reading from the inner part of the CD to the outer, the servo needs to slow down the angular speed). However, this raw clock is noisy and jittery, as any little variation in pits and holes width translates immediately in more or less trailing edges in the clock. Which is jitter by definition. Same with dirt, mechanical acceleration from incoming vibration or the motor itself. To deal with it, good quality CDs use a kind of elastic storage in which data is entered at the raw clock rythm, and exited at the rythm of a much more stable clock, externally generated with low jitter design. Technically, it's based onto a FIFO (Fist-In Fist Out memory) as elastic storage, and a PLL (phase locked loop) for the clock generation. A compromise here is that the lower frequency the PLL embedded low-pass filter is, the less jitter the clock exhibits, but also the longer the time to lock onto the raw clock is, and so the larger the FIFO has to be, and eventually the deeper your pocket shall be (a topic to discute). Oh let us give up technics and speak $: raw components prices are around $10 for the FIFO, $3 for the PLL (with an external low-pass), and $5 for miscellaneous components. No need for exotic PCB material here, it's casual speed digital in our era. So a complete "clock enhancer" is HTML tag not allowed

>So the real places for differentiation lie in the clock, filter, and analog circuits of the digital device.
>Lets get a real technical discussion going about digital tweaks.
Yes. Today, I continued your thread, talking about clocks. There are also a big bag of things to tell about clocks... I hope somebody will follow the thread. Don't hesitate to ask any question that should arise about what I wrote, EE jargonning is hard to avoid in these matters, but the topic deserves it.
Another day I'll talk about filter and analog circuits around the DAC (well, EE jargonning again: digital/analog converter)

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