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Old 29th April 2006, 04:44 PM   #1
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Default DAC Settling Time

Aside from undermining clockmonger business, why is the subject of settling-time ignored here? The diyAudio experts dismiss it as inconsequential yet they are obsessed with jitter.

Quote:
Originally posted by Kuei Yang Wang
Most DAC's settle their output current extremely quickly, the amount of time spend setteling compared to the amount of time at "steady state" MUST be MINIMAL and resonably CONSISTENT sample to sampel for the darn thing to work at all....
Here’s a collage of traces looking at the DAC-AH’s left channel reproducing a 10KHz, dithered sine wave. The ‘scope is triggered on the falling edge of FSYNC. The right channel is mute. There is a 220ns delay between the trigger point and when the data starts to change. Of course, there is some jitter in the timing of that change time. Every clocked circuit has jitter. Jocko and the clockmongers are convinced they can hear that jitter, which in this picture amounts to less than the width of the orange cursor line. They say those tiny timing errors result in audible amplitude errors. What Jocko and the clockmongers don’t seem to hear is the very long period of time the DAC’s outputs are at the wrong level. In the case of the DAC-AH, it can take over 5us to settle a relatively small step. That’s one-quarter of the total sample period! If I was concerned about amplitude errors in the DAC output I would work on reducing DAC settling time, measured in microseconds, long before I bothered with clock jitter, measured in picosecond. It’s a tough problem to solve but that’s what makes it interesting. There are at least three approaches that are within the reach of serious DIY.

Here’s another view with 10022.727273Hz. The signal frequency beating with the sample rate makes the stationary pattern shown.
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Old 29th April 2006, 06:35 PM   #2
phn is offline phn  Sweden
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Nice post. Very enlightening.

But isn't that the history of high-price audio? Somebody picks something everybody can understand, like solid-core wire, and runs with it. People spend big money on the Philips CD-Pro2, which just HAS to be better, while Meridian, perhaps the world's premier CD player manufacturer, uses €20 CD ROM drives!

This jitter thing is beyond my competence so I have nothing to say in that regard. But do I believe in it? Hell, no. How can I believe anything in an industry where 90 pct is lies and superstitious belief? (My lack of competence means I can't discard it either.) I apply Occam's razor here, and, in particular, that early audiophile, Buddha: "Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense."
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Old 30th April 2006, 12:53 AM   #3
awpagan is offline awpagan  Australia
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interesting!

with my limited knowledge of dac's i have a question on settling time.

does the dac resync during data transfer?
if so how often and why?

I thought settling time was on startup.
Being once to dac, decocder are sync'd they should stay that way unless something interrupts the signal?

allan

ps it's quite audible on some interruptions, eg weak laser.
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Old 30th April 2006, 10:09 PM   #4
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Quote:
why is the subject of settling-time ignored here?
I have suggested many times use of very low input bias current / fast settling OP's (8066) for I/V.

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Old 30th April 2006, 10:31 PM   #5
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Quote:
Originally posted by awpagan
with my limited knowledge of dac's i have a question on settling time.
The digital samples come off the CD at the rate of 44,100 per second. Each sample is a number that represents the signal level for each sample period. It is the DAC's job to convert of those numbers into an electic current that is proportional to each value. Ideally, the current should change instantaneously at precisely the right moment. The moment is defined by one of the DAC clocks. If there is jitter in the clock, the current will change a little earlier or a little later than is should. This timing error, usually measured in picoseconds or 0.000000000001 seconds, is what Jocko and the clockmongers worry about.

Regardless of any error in the timing of the clock, DACs do not change the output current instantaneously: Far from it. The time it takes the DAC to change the output, from where it is to where it should be, is called settling time. It is usually measured in micro- or nanoseconds. The larger the difference in value between one sample and the next, the longer the settling time. Also, the closer the output gets to its desired value, the slower it changes. In other words, the output changes very quickly, at first, but then slows down and may even wander around before slowly creeping up on the correct destination. (You can see this behavior in the photos.) For that reason, settling time is usually specified for a given range, often one-half of full scale, and a given “nearness” to the correct final value, often ½ LSB. For example: the settling time for the TDA1541 is typically 1us to reach +/- 1 LSB.

In the case of the DAC-AH, it can take over ¼ of the sample interval for the DAC to reach the correct output value. During that time, the output is the wrong value. If the DAC has the wrong value for 5us, does it really matter if it has the wrong value for 0.000005000001 or 0.000004999999 seconds? Wouldn’t it be better to fix the big error, settling time, before trying to fix the tiny error, jitter, which is a million times smaller?
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Old 30th April 2006, 10:38 PM   #6
awpagan is offline awpagan  Australia
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thx Uli

so we are looking at the speed of the dac or the responce time?

allan
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Old 30th April 2006, 10:48 PM   #7
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What about S/H ?
Used to eliminate spikes, it should also correct for settling time errors.

The settling time issue and jitter depends on the oversampling rate.

I have measured a few and some rather old CD players that use BB DACs with 1 kHz 0 dB signal.
All have clean spectrum without any harmonics down to below -93 dB.

Oh jitter & settling time where are you ?

Big differences are in low level linearity...
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Old 30th April 2006, 11:03 PM   #8
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Very fast DAC outputs result in HF noise. All those step changes and all that ringing is just added noise.
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Old 30th April 2006, 11:38 PM   #9
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Default Re: DAC Settling Time

Quote:
Originally posted by Ulas
Aside from undermining clockmonger business, why is the subject of settling-time ignored here? The diyAudio experts dismiss it as inconsequential yet they are obsessed with jitter.
Far from being an expert I just want to add that only these 2 first sentences are enough to disattract from any further reading of the thread.

Why do some people have to provoke to proove some point ?
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Old 30th April 2006, 11:52 PM   #10
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I have played a lot with 24/96 dual differential stereo (4 X 1704) DAC configuration.

I think jitter / clock accuracy should be considered separately from DAC’s settling time / resolution. There’s no point in repeating all over again about the jitter and its influence to sampling / reconstruction of the original data (time domain).

If we consider 24bit word length and 96kHz or higher sampling rates, then the settling time and I/V conversion are very important to accurately follow all those fast step-changes coming out of DAC’s. I have tried many OP configurations and types. It appears that the best results were obtained with extremely low input bias current / fast settling time OP’s. The fastest dual OP I used was 8066 which sounded MUCH better as an I/V conversion OP than 627 or 825.

However, even highly modified DAC / IV / analog filtering and buffering stage sounded very artificial compared to NOS DAC. In my opinion, the simplicity of NOS DAC is to blame for such a nice sound. It makes you wonder why oversample / increase word length…. when all the details are there even with 16bit DAC’s. It’s a different story with high resolution sources like DVD-A and SACD, but we are talking here about trusted 16bit audio CD.

I will have a nice NOS DAC design of my own (it has proven to be extremely popular and good sounding configuration and many have shown a huge interest), which details I am keeping for myself at the moment – my suggestions on forums have been copied already and copyrighted by others – so no more comments about this one…

Extreme_Boky
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