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In Reply to: Jitter Research, Analysis & Measurement posted by cics on March 24, 2008 at 12:55:55:
The 2 kHz jitter component was too coincidental. It relates to a software function on the cMP Recorder that operates at 2000 transactions per second (i.e. 2kHz). This is Juli@’s Mixer software which seems to induce jitter through digital logic. I decided to reapply all optimizations (esp. given that Cubase was launched using Explorer – the worst way to run Cubase).
Only 2 optimizations could not be done: E1200 processor and Minlogon (Cubase fails to start). A dummy wav file is used for which cMP’s software is set to start Cubase. Cubase allows for both Svchost and Lsass to be suspended. Thread optimizations is best left to Cubase.
With the recorder enjoying these optimizations, there’s marked improvement in measurements. Here’s the output for a 7kHz (filter 2) test tone.
Figure 10. dCS Scarlatti analogue outputs for 7kHz input sine wave. Filter 2. Harmonic distortion and filter artifacts are visible.
Figure 10 reveals no jitter components (including the previous 2kHz component)! Jitter no doubt exists but is buried in the noise floor. Sideband noise increases with increasing input frequencies. A test using 14kHz is more useful at revealing the jitter.
Figure 11. dCS Scarlatti analogue outputs for 14kHz input sine wave. Filters 2 (top) and 3 (bottom).
At 14kHz (figure 11), jitter artifacts become visible. The 2 kHz component is at a much lower level of -135.5db yielding Jpp of just 11ps. Other jitter artifacts at higher levels are visible. Here’s a zoom in picture for the 14kHz output (filter 2).
Figure 12. Zoom in: dCS Scarlatti analogue outputs for 14kHz (filter 2).
Any single jitter component over -130db would yield jitter at 20ps. No such levels exist!
These are the individual jitter components:
- At 2kHz, -135.5db Jpp 11ps
- At 1.605kHz, -132.0db Jpp 16ps
- At 5.904kHz, -134.0db Jpp 13ps
- Jitter like noise close to fundamental (600-800Hz, right channel only), peak at -131.5db Jpp 17ps (actuals: -131.5 17ps, -133.0 14ps, -134.7 12ps, -131.8 16ps, -133.5 14ps x 6)
This yields an overall Jpp of 51ps (RSS).
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Follow Ups
- More accurate cMP Recorder reveals lower Jitter at 51ps - cics 08:10:08 04/07/08 (0)
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