[time-nuts] femtosecond jitter anyone?

J.D. Bakker jdb at lartmaker.nl
Fri Apr 24 12:08:34 UTC 2009

[Another late reply -- Easter and taxes got in the way]

At 09:59 -0400 09-04-2009, Chris Mack / N1SKY wrote:
>Imagine this: if you were the mastering engineer for Elvis 50 years 
>ago, and if today's digital technology was available back then, would 
>you want to archive the King's record inside Iron Mountain on MP3?  
>Not really, you would want the most resolution possible for the re-
>release onto 32-bit 384kHz whiz-bang chip media in the future.  And 
>cumulative error for successive re-releases over the decade for 
>frequency drift and re-archival is maybe another concern.

What cumulative errors are going to impact digital->digital transfers??

>For example, I am currently using a Rosendahl Nanoclocks (http://
>www.rosendahl-studiotechnik.com/nanoclocks.html) for my house master 
>word clock and a $6k Eventide Orville for processing of audio.  The 
>Eventide used to measure the Nanoclocks at 88200 Hz... now it 
>measures 88201 Hz 4 years later... something drifted....  The 
>Rosendahl uses normal ambient crystals (although they have trimmer 
>caps) and use 74ACT00 logic to drive the distribution outputs.  The 
>Eventide, I assume uses normal ambient crystals too for the myriad of 
>DSPs in it....

(What do you mean by 'ambient crystal'? AT cut with minimal df/dT at 
room temperature?)

So your Nanoclock drifted relative to your Eventide. Even assuming 
that the absolute frequency shift of your converter clock was this 
much, that translates to a pitch shift of 0.02 cent. Find me *anyone* 
who can hear that in an ABX test. For that matter, try finding a 
string instrument that stays within 0.02 cent of its tuning after a 
minute or two of playing.


>I believe Mr. Lavry for his 127dB ADC uses an OCXO which helps 
>achieve the specs.  http://www.lavryengineering.com/

The fact that one good converter uses an OCXO doesn't mean that you 
have to use an OCXO to build a good converter. The Grimm one 
mentioned by Chris C gets by just fine without one, for example.

[plus, all things being equal an ovenized oscillator has higher 
Johnson noise than one running at room temperature]
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