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发表于 2016-9-27 00:07:21
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本帖最后由 Astralark 于 2016-9-27 00:15 编辑
虽然外接钟有问题，本来就是给 Pro Audio 远距离传输统一用的
Why external clocks are junk for home audio applications
Hi guys,
It's extremely annoying reading about these 10Mhz external clocks with extremely low phase noise specs. The phase noise of the OCXO's in these clocks are absolutely irrelevant when it comes to the jitter performance of what it's clocking. Since we want educated clients and forum members, here's why these external clocks are total junk when it comes to home audio applications, no matter how low the phase noise is on the internal clock:
1: Most are all based on a 10Mhz OCXO. It's not hard to get extremely low phase noise at 10Mhz. But 10Mhz is useless for audio. You need multiples of 22/24Mhz to be useful for audio. So they must use a PLL generator to generate these frequencies, using the 10Mhz clock as a reference. However the phase noise/jitter of the generated frequency is far higher than the reference clock. So the only figure that is relevant is the phase noise of the frequency generated taken from the external output. But nobody ever publishes that figure!!
2: Even if you still have decent phase noise performance from the PLL generated clock output, by th e time the clock goes through the 2 connections, cable, and all of the connections in the DAC, so much jitter is added, that locating a $20 Crystek an inch away from the DAC chip will result in far lower jitter/phase noise at the only place it actually matters. Even if you are using a $20000 external clock.
The reason these external clocks were made from the beginning was to clock multiple devices in a studio environment. It was the only way to sync up the clocks between all of the ADC's, DAC, mixing boards etc, in a studio before AES67 came along. So it was out of necessity, not for performance reasons.
So why are these clocks sold to the audiophile market? To suck your money out of your wallet that's why. If it sounds better to you, there's only 2 reasons why:
1: The clock it's replacing in your DAC is total junk, so using the external actually does make a difference for the better.
2: Expectation bias based on price and beliefs.
The only good solution is putting that ultra low jitter clock into the DAC very close to what it's clocking. And not a 10Mhz clock either, it must be a multiple of 22/24 Mhz if you don't want to use a jitter adding PLL generator. |
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发表于 2016-9-26 23:51:48
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