Thursday, 11 February 2016

Clock 的迷思

Upgrading a DAC clock is a fool’s errand

You can buy and install a so-called femto-clock but the clock signal that reaches the DAC chips is more likely a pico- or a nano-clock. The reason is the intervening logic gates used to select and divide the oscillator’s output to provide different bit- and word-clock rates as required. 
意思大概是說, 當你換了一個 femto鐘之後, 可能到了DAC chip 內它只是一個 pico 或 nano 鐘, 原因係中間的一些 logic gate 已經把 clock 的訊號劣化了.

femto clock = 千萬億分之一秒 = 0.000 000 000 000 001 秒
pico clock = 兆分之一秒 = 0.000 000 000 001 秒
nano clock = 十億分之一秒 = 0.000 000 001 秒
Most USB DACs have two oscillators, one for sample rates that are multiples of 44.1K and the other for multiples of 48K. In most cases the oscillators are always running, each taking gulps of current from a common power supply and leaving behind a residue of their respective clock rates.
大多數的 USB 解碼是有兩個鐘, 一個負責44.1KHz 和它的倍數, 另一個負責48KHz 和它的倍數

To select which oscillator is used to clock the DAC chips the circuit uses a multiplexor (mux).

In its simplest form a mux has three inputs, one selector, two data, and one output, data. The selector signal chooses which input data is passed to the output.
 最簡單的數據多工器有3個輸入,一個選擇輸入, 兩個數據輸入(44.1k和48k)

Now, the two pristine OCXO ‘femto’ clocks are dumped onto the same silicon substrate and pass, side-by-side, through a common logic gate, the mux. 
那兩個clock都輸入到mux中再經過如galvanic isolator 才去到DAC chip. 所以解釋到何以換了這些鐘得出來的效果也不是很顯著.

換鐘的確是會有改善的, 想想便明白, 之前的鐘可能本身只是 nano 的級數, 過了 mux 也是會劣化了, 不一定是到了 milli (0.001), 到了 micro (0.000 001) 也比 pico 鐘劣化後的 nano 差千倍了.
另外, 上文講出了兩點值得留意:
1. 用了 mux 便劣化了鐘的表現, 那麼可否不用 mux?
2. cpu, usb bus 等的其他訊號對鐘也有會劣化的影響

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