[time-nuts] Oh dear
attila at kinali.ch
Mon May 7 15:59:04 UTC 2012
On Mon, 07 May 2012 08:20:55 -0700
Dan Rae <danrae at verizon.net> wrote:
> I see nothing odd about wanting to get the best possible source for the
> Master Clock for your master recordings.
> My son does run a small studio and for him I was able to make a version
> of that unit, for a lot less money of course. If he says it improves
> the sound of the recordings, and his customers agree, I am inclined to
> believe him.
The thing is, that an Rb is good for one thing: Have a long term
stable and accurate frequency source that is better than 1 to some
billions for measurement or other stuff that take more than a few
hours or have to be repeated exactly in a couple of weeks.
For audio, you need a frequency source that is stable over a couple
of hours (probably a working day) and shows "low" jitter. Where as low
jitter is quite high in time-nuts terms and stable not stable at all.
A cycle-to-cylcle jitter of a couple of ns is not audioable at all,
but any Rb will have a much lower jitter. Or to have a different look at it,
you want to have very low phase noise, as this phase noise is mixed in
over the ADCs into your signal. But as we know, the phase noise of
an Rb is not defined by the Rb physics package, but by the OCXO they use.
(yes i know that the close in phase noise is defined by the reference
and not by the OCXO, but the "base level" is the OCXO, not the reference)
As for stability. You want the instruments to sound the same over an
recording. Ie the human ear has to preceive the recorded sound as the
same. The frequency resolution of the human ear is somewhere around 3Hz.
This makes for 150ppm (at 20kHz). Even a 32kHz tuning fork crystal
achieves an absolute accuracy that is better than this. Its stability is much
better than this....
Of course, you want to have enought headroom for other non ideal components.
So, lets say, go for a factor of 10, then we are at 15ppm. For absolute
accuracy, that's already a good XO. For stability, still most XO should
Or to say it differently: Using some good OCXO with low or very low
phase noise would be more than enough for even the most high end
audio equipment. You don't even have to discipline it, as a even
quite bad OCXO has variations much lower than 1ppm, which is definitly
not something anyone can hear.
IMHO getting a 20-50USD OCXO from ebay, some good, low noise power supply
(audio power supplies with low noise in the <40kHz region), some distribution
amplifier with low noise figure and you are set. All in all probably at
a cost of 200-300USD including rack mount. If you want to have "high fidelity"
you can use an GPSDO to get your OCXO within a couple mHz.
To summarize: Nobody here does want to insult anyone who does professional
audio recordings. But having the knowledge of what the stability and
accuracy numbers for an ordinary Rb mean, and being able to put that into
perspective with the not so good capabilties of the human sensory systems,
one wonders why people spend an awfull lot of money for something that has
no audiable effect over something a lot cheaper. Not to mention that other
things have a much higher impact on audio quality than the reference
oscillator: Like temperature and humidity during recording (do you control
them as well to the ppm level?), or the tuning of the instruments which
wanders quite a bit during use.
Why does it take years to find the answers to
the questions one should have asked long ago?
More information about the time-nuts