Femto clocks ,every unit should have one

Don't believe the hype. In the real world ALL so-called Femto clocks are just marketing BS when applied to audio.

This specifically applies to clocks intended for SONET applications (Read internet backbones), which need to have very low phase noise (jitter) above 12KHz, but care zero about phase noise below 12KHz. Which is why clock jitter for these clocks is specified usually as RMS 12KHz - 1MHz (or higher) and a very low level of phase noise / jitter at these frequencies is essential. This is why this specification recently became important and specified (many vendors simply adjusted their datasheets for what is the part that previously was a "picoclock")

Now here is the killer. it is practically impossible to make a crystal oscillator that is fixed frequency and does not measure at the max. a few 100 Femtoseconds RMS phase noise/jitter under the tests conditions. The cheapest clock you find in China will do that.

For Audio use what matters most is audioband (20Hz-20KHz) jitter. In this case a really basic "Tent-clock" rated at 3pS P-P Jitter RMS IIRC from a commonly available source with a cheapo 3-Pin chip regulator will beat the pants of ANY Femtoclock most designers are aware of.

None of them provide even 50pS peak-Peak jitter 10Hz-20KHz.

The GMT System, which uses a programmable clock that was not optimised for SONET, but for an application that needs low audio band phase noise levels., when measured it "Femto Clock" style, shows around 200 Femto Seconds RMS phase noise.

Most any clock in audio (1 rung above lo-fi) use if measured "Femto Clock Style" is a Femto Clock .

Finally, jitter is NOT the end-all be-all of good sound in digital.

Audioengr, Implementing a Femto clock will have little effect on your designs in my opinion. Kind of like a cop out :-) I have researched your designs as best I could and they are brilliant. One of the best there is and I don't know you from Adam just your work. Obviously Silver is a better conductor than Copper but seriously no way you or anyone will hear the difference in your well designed products. But if you want to hear a difference you will. Stick to what you do don't pander.

I agree with most of what Wisnon just said. I wouldn't claim that every chinese CPU clock can beat any femtosecond clock, but maybe some of them.
These canned clock's are usually made with a logic gate as the active element, in a Pierce coupling. Such as the 74HCU04 The equivalent input noise is relatively high. I don't think anyone would use 74HCU04 as a RIAA amplifier?
Noise on the input of the active gate, transfers directly to jitter in the clock signal.
Also the input level is half that of the Vsupply. So half of the noise on the supply rail is transferred to the input. That's why the regulator is of great importance.
Much better is the Colpitts oscillator, built with discrete transistors. Here you can get somewhat better noise (jitter) performance.
I agree with Wisnon also that jitter is not necessarily a make or break factor, the problem is when the music signal is transferred to the clock, over the 5V supply lines. That is going to kill depth perspective, and sound stage. And strangely if you have a jitter instrument, you will find this in many CD players or DAC's. Random jitter is far less intrusive, and like low order distortion in tube amplifiers, it can mask other distortion / jitter.

Wisnon, Drusstheaxe, and Larsclausen, nice of you guys to chime in. :-)
Perspectives other than conventional and unconventional wisdom are most welcome here. It makes my head hurt, trying to get it around all of this but it's most appreciated.

All the best,
Nonoise

ITs very interesting to read these comments on what matters with digital reproduction, but its also very hard for most anyone I would think to get their arms around what works and does not work better in the digital domain. Makes getting ones arms around traditional audio issues like turntable setup and impedance matching seem like childsplay.

Keep it coming though. More information is always better than less.