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. The selector signal chooses which input data is passed to the output. In its simplest form a mux has three inputs, one selector, two data, and one output, data. To select which oscillator is used to clock the DAC chips the circuit uses a multiplexor (mux).
However, OCXOs are always on and their output cannot be turned off. The crème de la crème DACs use oven controlled oscillators (OCXO) because they are the most stable. 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. Most USB DACs have two oscillators, one for sample rates that are multiples of 44.1K and the other for multiples of 48K. The output of a latch is data not a clock. It is used to capture the state of the input data at the instant the clock changes state, usually the rising edge. In its simplest form a latch is logic gate with two inputs, clock and data, and one output, data. At the heart of any reclocking scheme is a latch. I remember one that asks, "When is a clock not a clock? … When it passes through combinatorial logic."īlizzard proposes reclocking as a cure for all that ails USB audio. Clocks were treated with great respect because the slightest perturbation would jeopardize the entire system. The most often heard topics of conversation in the hallways and conference rooms pertained to clock distribution and signal integrity. I was one of only a few programmers there and I was surrounded by dozens of the best digital hardware engineers in the business. Thirty years ago, I worked for a startup in the mainframe business. 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.
DIY WORD CLOCK GENERATOR INSTALL
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.
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