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Author Topic: DAC guts stuff, etc.  (Read 11920 times)

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Marvey

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Re: DAC guts stuff, etc.
« Reply #50 on: October 02, 2014, 07:41:50 PM »
Quote from: firev1 on October 02, 2014, 06:06:12 PM
I believe JA did it but slightly differently in Fig 6. http://www.stereophile.com/content/quality-lies-details-page-5

He claims that it would be excellent but not necessarily a make or break measurement for a DAC though. Getting to parts and their influence on sound quality, I wonder how would the proposed linearity measurements changed with say different PSUs or parts?

Yes, JA measured linearity with 1kHz sine. Different from bit / word code vs. output linearity. Basically steady DC output instead of Sine @ certain dBFS.
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Azteca X

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Re: DAC guts stuff, etc.
« Reply #51 on: October 02, 2014, 08:03:50 PM »
Given Bruno Putzeys approach to DACs I'd love to hear him weigh in on this thread.
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ultrabike

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Re: DAC guts stuff, etc.
« Reply #52 on: October 02, 2014, 08:12:37 PM »
Quote from: purr1n on October 02, 2014, 07:41:50 PM
Yes, JA measured linearity with 1kHz sine. Different from bit / word code vs. output linearity. Basically steady DC output instead of Sine @ certain dBFS.

Makes sense, for the same reason he is using dBFS on the x-axis instead of DAC code, and dBr instead of LSBs.

Interesting that JA mentions the practical lower limit for linearity is about 18 bits.

I do wonder how a ladder DAC will perform using this different approach to linearity measurements. It's not classical for ladder DACs based on what I recently read. I expect not too different though and might be more practical for delta sigma DAC measurements. I mean, linearity might be frequency dependent (hopefully slightly), but music is not just 1kHz tones or DC static signal. I do agree is probably not as "clean" or "elegant" or "valid" an approach as the DC one, but it's proly a bit more practical for some architectures...

Measured differences (between approaches even) might indeed correlate to perceived performance differences.
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Clemmaster

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Re: DAC guts stuff, etc.
« Reply #53 on: October 11, 2014, 01:06:35 AM »
LE and THD look actually better with software trimming (DAC8581):



It is not impossible software trimming is used in the well known NOS DAC (in the "glue logic" that formats S/PDIF inputs to the DAC's word input).

cf p.11 IMPROVING DAC8581 LINEARITY USING EXTERNAL CALIBRATION
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Hands

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Re: DAC guts stuff, etc.
« Reply #54 on: October 11, 2014, 01:18:24 AM »
This is interesting. Said DACs have similar looking THD measurements. 2nd order right in between -70-80dB, 3rd a bit higher, 4th drops a bit, 5th is up a bit from 4th, 6th drops from 5th.

Not an exact match by any means, but the pattern is similar.
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Clemmaster

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Re: DAC guts stuff, etc.
« Reply #55 on: October 11, 2014, 02:46:57 AM »
Meaning no trimming is done? Doh...
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Hands

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Re: DAC guts stuff, etc.
« Reply #56 on: October 11, 2014, 03:01:28 AM »
No idea. Mind offering a brief explanation or link on what software trimming is exactly?
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Marvey

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Re: DAC guts stuff, etc.
« Reply #57 on: October 11, 2014, 03:07:14 AM »
Measuring the linearity of the errors of the DAC per bit-code and then programmatically adjusting it so the error is less. It's not 100% perfect as linearity will fluctuate with temperature and other factors. Obviously it seems it can only be done with R2R DACs.
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Hands

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Re: DAC guts stuff, etc.
« Reply #58 on: October 11, 2014, 03:21:11 AM »
Ah, OK, thanks for the explanation.
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Solderdude

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Re: DAC guts stuff, etc.
« Reply #59 on: October 11, 2014, 08:11:49 AM »
Here is the lengthy explanation:

http://liu.diva-portal.org/smash/get/diva2:559515/FULLTEXT01.pdf

page 47 and up .. put your math cap on !

For Delta Sigma a correction is not needed for linearity as it is enclosed in the algorithm.



picture is from this article:  http://archive.siliconchip.com.au/cms/A_112766/article.html

The Delta Sigma CS4398 chip for instance has great linearity (as shown above).
When considering 16 bit accuracy it isn't even 0.1 LSB 'off' at -100dB
When considering a 24 bit accuracy it is about 2 LSB 'off' at -100dB (it will get worse below -100dB of course)

Even though the Delta Sigma is just an 'approximation' the 'approximation' is more accurate than most R2R DACs
An R2R chip output has 'steps' which has to be 'smoothed' if you want to get rid of HF garbage.
Most NOS DACs that can handle make no effort and expect the speaker or headphone to do the smoothing for you, otherwise your ears will.

Smoothing can be done by either up-sampling (inventing = approximation of in-between values) or by a rather steep analog filter which also basically 'connects' between the output time steps more smoothly (and thus 'approximating' in-between values) by an RC-time constant.

In the end it seems to come down to differences in the 'in-between' values and how they are generated and linearity problems in R2R DACs because of tolerances of the MSB and the bigger bits below the MSB compared to the LSB.
No idea how this would translate to audible effects and can't wait till someone has a clear answer.

« Last Edit: October 11, 2014, 11:12:53 AM by Solderdude »
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