[time-nuts] femtosecond jitter anyone?
Chris Mack / N1SKY
sometimesyoufeellikeanut at twentylogten.com
Thu Apr 9 13:59:59 UTC 2009
On Apr 8, 2009, at 11:59 PM, Tom Van Baak wrote:
>> The incoming clock source (master house clock) to this box / design
>> of interest is in another rack mount box external to this design on
>> the other side of the room and is anywhere from 44.1kHz up to a 10MHz
>> Rubidium (see also http://www.antelopeaudio.com). This clock source
>> on the other side of the room also drives other equipment to be in
>> synch for any framing on AES/EBU digital.
> Can you comment more on the antelopeaudio box? I admit I know
> little about high-end audio, but it seems clear to me that the several
> companies that over the years have incorporated "atomic clocks"
> into their digital audio work might be misguided; confusing what is
> long-term accuracy with short-term stability.
> Sure, the word "atomic clock" sounds really cool. But high-quality
> OCXO typically have much better jitter and short-term stability than
> the telecom-style Rb oscillators that are used by audio companies.
> Or do I misunderstand?
> It would seem to me that very low jitter (or phase noise out to, say,
> 100 kHz, or ADEV from, say 10 us) is much more important for
> audio work than specifications about absolute frequency accuracy
> or long-term drift (such as what telecom Rb oscillators offer).
> Now if an audio company used surplus Sulzer, or FTS 1200, or
> Oscilloquartz BVA oscillators in their design, or even H-masers,
> well, that would make sense. But Rb? Something doesn't feel right
> about this.
For audiophile or consumer in the home, a Rb clock may be overkill
especially if the price tag is huge... But the market will bear what
the market will bear and there is already a user base. To note, the
jitter does not matter as much in the transport chain up until the
converter. Once at the converter jitter is paramount, but the jitter
in the thermally hot chip will probably trump everything else. This
is especially true for 16-bit 44.1kHz at the consumer level.
However, this is for mastering, not audiophile or consumer
applications. Mastering is the final creative and scientific step
before an album is manufactured, after the recording studio phase is
complete; every commercially released album is mastered (mastering
does not use that huge mixing desk / console like you see on TV; the
less equipment in front of the mastering engineer, the better,
because of acoustic comb filtering bouncing off the gear).
Mastering also comprises the possibility of archival.
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.
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....
The Antelope Rb unit is $6k which is a little overboard in price
considering one can get a Rb on ebay in the $200 range (age of the Rb
unknown perhaps). However, Antelope also has OCXOs that discipline
to the Rb for the long term (although they fail to disclose the
useful life of a Rb clock and have brain-dead specs for phase noise;
only one data point at 10kHz -140dBc)... caveat emptor in 20 years
(hmmm let's see, $6k amortized over 20 years = $300/year)? The
Antelope Rb and OCXO are all 75 ohm BNC.
The normal mode of operation in a recording studio or mastering suite
is to synchronize gear together with one master house "word" clock
usually 44.1kHz, 48kHz, 88.2kHz or 96kHz (I usually use 88.2kHz;
twice the CD sampling rate, then Sample Rate Convert (-140dB to
-170dB dynamic range) before PQ coding a red book standard audio CD
at a dithered 16-bit, 44.1kHz). This is all on 75 ohm BNC usually or
AES11 "digital black" on higher frequencies characteristic of AES
framing on 110 ohm differential. These house "word" clocks may have
jitter in the nanosecond range to maybe 10ps or so...
The math for full 24-bit accuracy for 88.2kHz sampling rate (44.1kHz
BW) shows 0.43 ps of timing uncertainty. Even if it were 20 bits of
accuracy, 6ps would be the limit... The converters I am using are
124 dB and 132 dB of dynamic range for ADC and DAC respectively.
There could be some dynamic range gains with the proposed
modifications to the standard application circuit for these
converters, thusly a better clock desired or local clock cleaning of
the house word clock wether it be word clock from crystals or Rb.
I believe Mr. Lavry for his 127dB ADC uses an OCXO which helps
achieve the specs. http://www.lavryengineering.com/
Hope this paints the landscape in the studio world (different from
the consumer / audiophile at home).. We use more bits and higher
sampling rates in the studio to give better results in aliasing
filters during processing and the like. then SRC and dither to the
final lower resolution product, keeping an eye for the higher
resolution formats of the future.
More information about the time-nuts