[time-nuts] Re: Clock specs for audio (was: High precision OCXO supplier for end costomers)

[Bernd Neubig]

We are receiving such inquiries from "Audio nuts" rather frequently, but
also from professional high-end audios-studio equipment makers. There
argument is often, that the spatial transparency of the sound, i.e. how
exactly you can locate the sound source (instrument in an orchestra) would
be noticeably improved by such ultra-low noise OCXO sources. So it should be
more about time or phase (jitter?) than about frequency....

As the customer and his belief is "king" at AXTAL - as long as doable and
payable - we have developed our AXIOM45ULN series, where the best phase
noise option guarantees a PN level of -115 dBc/Hz @ 1 Hz. But this kind of
performance can only be achieved by a crystal selection with rather low
yield. Therefore, as a manufacturer you need enough customers who accept
that "less is sufficient" and will buy the OCXO made from the other
crystals. We also are getting a few parts with -120 dBc/Hz @ 1 Hz out of a
larger lot, but we rather keep them than selling them to everybody.

Best regards
Bernd


-----Ursprüngliche Nachricht-----
Von: Attila Kinali [mailto:attila at kinali.ch] 
Gesendet: Montag, 10. Januar 2022 15:42
An: Discussion of precise time and frequency measurement
<time-nuts at lists.febo.com>
Betreff: [time-nuts] Clock specs for audio (was: High precision OCXO
supplier for end costomers)

On Mon, 10 Jan 2022 12:35:17 +0100
Attila Kinali <attila at kinali.ch> wrote:

> That said, are yo sure you need such stringend phase noise requirements?
> It's audio. Nobody is going to hear whether the noise is -60dBc or 
> -80dBc @ 1Hz, much less -120dBc.

To give here a bit more background: psychoacoustic masking, which is the
relevant metric here, mans that we cannot discern sounds that are close to
eachother with one of them being louder than the other. Depending on who you
listen to, it's usually a sound masking another sound at a distance of 100Hz
up to 20dB to 40dB lower. Even if we account for someone with golden ears
and use 60dB, that would translate to a noise spec of -60dBc @ 100Hz offset.
That's a spec that almost all XO do fulfill. A good VCXO (40-100MHz) is
somewhere around 90-100dBc @ 100Hz.
Any OCXO will fulfill that spec too, even the tiny DIL-14 ones (most are at
-110-140dBc @100Hz @10MHz).

And this doesn't take into account that we are arguing about audio frequency
specs at HF frequencies. I.e. if we use the 10MHz clock and use it to derive
a sampling clock for an ADC to sample a 20kHz signal, the noise performance
improves by another ~25dB... at least (if the design is done right, it can
be up to 50dB)

What is more important than close in noise, though, is broadband noise
performance and spurs. For someone with good ears, it's not unheard of to be
able to discern far away noise and spurs down a -100dB-120dB. Especially the
spurs can be quite hard to control, depending on what clock synthesis system
is used.

Another important spec, especially for recording, is accuracy of frequency.
An offset of just 1ppm becomes 3.6ms if you record for an hour. That's
something most people can hear already. But whether this actually elevant or
not depends on how the recording is done. The usual way is to have a central
master clock that feeds all clocked devices, such that all of them have the
same notion of time/frequency. In that case, quite high frequency deviations
can be tolerated, way beyond what a simple XO would deliver.

				Attila Kinali
--
In science if you know what you are doing you should not be doing it.
In engineering if you do not know what you are doing you should not be doing
it.
        -- Richard W. Hamming, The Art of Doing Science and Engineering
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