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Old 28-09-2014, 12:26 PM   #27
dqwong
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This is interesting: it takes 24hours for a DAC to reach it's stability(jitter wise).

Cold — At startup, we see at 1Hz, a dBc (decibels relative to the carrier signal) of -73. However, notice the spike from 6Hz to 11Hz. There is a significant rise in phase noise within this range. Also notice how ragged the response is from 1Hz to 100Hz. This volatility will have effects on the DAC chip’s ability to recreate an accurate waveform. Jitter is measured at 3.7pSeconds.
15 Minutes — Here we begin to see a slight drop in noise at 1Hz (-.8dBc), as well as a reduction of the cold-related spike (6Hz to 11Hz). Things appear as though they’re about to begin tracking nicely. Jitter is measured at 1.4pSeconds.

1 Hour — We begin to see the settling process occur. The measured performance takes a significant turn for the worse. At 1Hz, phase noise has risen by 11dBc. However, the good news is that the initial spike from 6 to 11Hz has not returned. But overall, we are seeing a much noisier signal. Additionally, notice that the response is more ragged than it was when tested in the 15-minute range. This will have a very negative impact on the DAC’s ability to output an accurate (and pleasant) musical signal. Jitter has risen significantly to 5.7pSeconds.

24 Hours — At this point and beyond, the DAC appears to have reached a stasis. Notice that the response is now very stable, that there is a smooth line from 1Hz all the way out to 1kHz. At its greatest point, the delta in phase noise has improved by 17.2dBc over the initial cold test. We can now begin to critically listen to this DAC, confident that our findings will be accurate and illuminating. Jitter is dropped and settled at an astonishingly low level of 0.85pSeconds.

Audio-based jitter is most accurately measured from 1Hz to 100Hz range. This is where averaging cannot correct or hide errors as easily as it does at say 1KHz. Ironically, most published jitter measurements are given at this frequency range.

How does a high level of phase noise affect our music? While it’s hard to draw a simple conclusion that will apply to all DACs, it’s absolutely clear that higher levels of phase noise create higher levels of jitter. And higher levels of jitter correlate to poor sound; the more jitter, the worse the sound.

If you’re auditioning a new DAC – whether for personal purchase, store merchandise, or critical review – be patient and give that contestant a day to warm up, so that it can truly strut its stuff.
AudioQuest White Paper: Evaluation of Digital Devices and Proper Warm-Up for Ideal Listening and Measurements | AudioStream
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