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Old 4th November 2011, 12:04 PM   #17541
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Quote:
Originally Posted by rsdio View Post
Any time the bit depth is reduced, there should be dither. Going from analog to digital is just a unique case of reducing from infinite bit depth to finite bit depth. Thus, the reason for dithering in from of an ADC is exactly the same as dithering between a 24-bit master and a 16-bit CD. Finally, if a DAC employs digital processing, then it's probably handling the internal calculations at 48-bit or even 80-bit, and thus it needs dither.

There's a lot to be said for avoiding dither in your DAC, but the only way to do that properly is to first do away with the digital processing in the DAC so that dither is no longer necessary. As Thorsten points out, it's increasingly difficult to find a DAC without digital processing and dither. This is why you'll find people making 16-bit DAC products to this day (I only wish they wouldn't expect me to buy a nonlinear tube output stage along with the 16-bit DAC, since I'm no fan of the particular distortions introduced by tubes).
I totally agree that dither in front of an ADC is useful and also when reducing the bit resolution.

My question is: is it useful to put dither in the output of a DAC ?
In more details: suppose you have an ideal non-oversampling DAC with an ENOB of 20 bits. Suppose that the DAC receives exactly 20 bits as digital input and outputs 2^20 levels. The error of the output signal is smaller than 1 LSB.
In this case you can either:
-let the DAC output the analog "equivalent" of the original digital signal with no processing (no dither)
or
-add dither to the digital input signal.

Is it useful to add dither to the digital input signal ? I would think it isn't, but I am not sure.
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Old 4th November 2011, 12:10 PM   #17542
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Quote:
Originally Posted by rsdio View Post
Sorry to sidetrack, but is anyone aware of a 24-bit DAC with balanced outputs (current or voltage) and 192 kHz or greater sampling rate capabilities?
If memory serves the PCM1792 but should check the datasheet.


chrissugar
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Old 4th November 2011, 12:11 PM   #17543
Jakob2 is offline Jakob2  Germany
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Quote:
Originally Posted by fireworks View Post
Nice example !
But it seems to me that you're demonstrating the usefulness of dither IN FRONT of an ADC, not inside a DAC ? Unless there is something I'm missing ?
Yes, up to a certain point a lot of the discussion seems to be a misunderstanding, as ThorstenL and abraxalito have some concerns regarding wrt to the changeover from multibit DACs to Delta-Sigma-DACs combined with heavy dithering and noise shaping.

And because their concerns are perception based it should not be easily dismissed.

Quote:
By the way, what is a good book that presents the modern ADC and DAC theory and practice including dithering, filtering, etc. ? Preferably one at undergraduate or low graduate level, but a more advanced level would also work.
A good introduction overall is still:
John Watkinson, The Art of Digital Audio



@ rsdio,

Widrow and Kollar have written a comprehensive book:

Quantization Noise - A book on quantization
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Old 4th November 2011, 12:31 PM   #17544
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Quote:
Originally Posted by Jakob2 View Post

Widrow and Kollar have written a comprehensive book:

Quantization Noise - A book on quantization
Very nice link: the chapter on dither is freely available on the website !
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Old 4th November 2011, 12:36 PM   #17545
rsdio is offline rsdio  United States
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Quote:
Originally Posted by fireworks View Post
My question is: is it useful to put dither in the output of a DAC ?
In more details: suppose you have an ideal non-oversampling DAC with an ENOB of 20 bits. Suppose that the DAC receives exactly 20 bits as digital input and outputs 2^20 levels. The error of the output signal is smaller than 1 LSB.
In this case you can either:
-let the DAC output the analog "equivalent" of the original digital signal with no processing (no dither)
or
-add dither to the digital input signal.

Is it useful to add dither to the digital input signal ? I would think it isn't, but I am not sure.
Excellent question. For the exact situation that you describe, dither would be completely unnecessary. It would not add anything but pointless noise. This assumes that your data sampling rate is the same as your DAC sampling rate.

However, there is one technical flaw in your description. "No processing" is not quite correct, because you need an anti-alias a.k.a. reconstruction filter at the output. That's arguably analog processing, but it is processing nonetheless. You cannot (correctly) just use the DAC output without further processing. Also, depending upon whether the DAC has some sort of sample-and-hold to isolate the output from the intermediate changes between sample codes, you might need to add a S/H before the low-pass filter. With a parallel multibit DAC, changing the 20-bit code can result in some bits affecting the output before or after the other bits, such that there is a settling time before the output is accurate. Sample and hold is generally used to prevent this noise from spoiling the output. For example, if the code were to change from 0x400000 to 0x3FFFFF, it might temporarily output a voltage for 0x000000 or 0x7FFFFF, which would represent an absolutely horrible spike in the waveform that is unrelated to the input.
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Old 4th November 2011, 12:51 PM   #17546
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Quote:
Originally Posted by ThorstenL View Post
Incorrect. RTFDS.

It is specified for 8 * OS at 96Khz so it's maximum input to the Chip is I2S
Incorrect. RTFDS.
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Old 4th November 2011, 12:58 PM   #17547
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Quote:
Originally Posted by rsdio View Post
Excellent question. For the exact situation that you describe, dither would be completely unnecessary. It would not add anything but pointless noise. This assumes that your data sampling rate is the same as your DAC sampling rate.
Thanks for confirming my guess.

Quote:
Originally Posted by rsdio View Post
However, there is one technical flaw in your description. "No processing" is not quite correct, because you need an anti-alias a.k.a. reconstruction filter at the output. That's arguably analog processing, but it is processing nonetheless. You cannot (correctly) just use the DAC output without further processing. Also, depending upon whether the DAC has some sort of sample-and-hold to isolate the output from the intermediate changes between sample codes, you might need to add a S/H before the low-pass filter. With a parallel multibit DAC, changing the 20-bit code can result in some bits affecting the output before or after the other bits, such that there is a settling time before the output is accurate. Sample and hold is generally used to prevent this noise from spoiling the output. For example, if the code were to change from 0x400000 to 0x3FFFFF, it might temporarily output a voltage for 0x000000 or 0x7FFFFF, which would represent an absolutely horrible spike in the waveform that is unrelated to the input.
By "no processing", I meant no processing of the digital signal which is fed to the input of the DAC, I didn't mean what happens at the DAC output.
I also totally agree that the output filter is an integral part of a DAC, so any discussion about the output signal of a DAC should include the filter.
This also holds for any sample and hold circuit at the output of a DAC (and before the output filter).

Anyway, thanks for your polite and reasoned answer.

Last edited by fireworks; 4th November 2011 at 01:01 PM.
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Old 4th November 2011, 02:00 PM   #17548
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Hi,

Quote:
Originally Posted by rsdio View Post
Thanks for the correction. My thought processes were in ADC mode, not DAC mode, and in an oversampling A/D you do not have access to the intermediate data. You've been trying to convince us that everything is sigma-delta these days, and I didn't expect you to point to something like the PCM1704. Sorry I didn't spend more time on the data sheet.
First I always pointed out that ALMOST everyone does only DS these days, I have repeatedly mentions the PCM1704 as one of the very few exception. In fact, I pretty much always qualify things, as I am quite critical of those who do not. So I am rather surprised...

Quote:
Originally Posted by rsdio View Post
Unfortunately, DAC is much easier to design this way than ADC. If know of an A/D chip that employs the equivalent quality 24-bit conversion, then please share!
Sadly, I do not.

I have been thinking of bodging several S&H circuits and several PCM1704 into something that is fast enough, but it would be a mess, I'd probably be better off with much faster industrial 16 Bit DAC's in a different architecture...

Quote:
Originally Posted by rsdio View Post
Frankly, you usually come across as someone who doesn't understand what he's talking about, at least when it comes to anything digital audio.
Is that I because I refuse to swallow the Bull (and I mean the whole bull) that is used to sell DS systems?

The point is that I DO understand and hence naturally object.

Quote:
Originally Posted by rsdio View Post
Most recently, you've claimed that nobody makes chips that are R2R any more, and you even went so far as to say most all converters today are sigma-delta internally.
I claimed that ALMOST nobody makes R2R for Audio today and I listed the exceptions I know. If you read more into my writing than what I write that is regrettable, but not my fault.

Quote:
Originally Posted by rsdio View Post
You totally caught me off guard by suggesting an actual chip that fits your own requirements. I wish you hadn't waited until I put my foot in my mouth to say something useful.
Because everytime I do for example suggest that Multibit DAC Chip X by Maker Y is a good part and recommended I get ten people jumping on my comment and pointing that Chip A by Maker O is so much better and the oldfashioned multibit is no good.

Then I point out that the modern miracle chips are not really 32 Bit, but just 12 Bit and the rest is just "fuzzy distortion" (which it is) and then more people come into the argument on the side of orthodox faith and tell me I know nothing about digital audio.

It gets boring after a few times, so I skip part of it nowadays.

Quote:
Originally Posted by rsdio View Post
If you're only going to use your knowledge for evil, then how do you sleep at night?
I do like to use my knowledge to help people, but too many people do not want to be helped and others seems to feel the need to protect poor idiots that might follow unorthodox suggestions from such a horrific faith (like having actually a system who's sound satisfies them, even if in an orthodox sense it does not measure too well), so I get to do much less of that than I like.

But even so, I sleep very well, thank you.

Quem di diligunt, adolescens moritur, vivere pergam.

Ciao T
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Old 4th November 2011, 02:06 PM   #17549
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Hi,

Quote:
Originally Posted by abraxalito View Post
Incorrect. RTFDS.
I did read the fantastic datasheet.

Page seven, paragraph 3.

Ciao T
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Old 4th November 2011, 02:32 PM   #17550
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Quote:
Originally Posted by ThorstenL View Post
Page seven, paragraph 3.
Look closer - where does it mention I2S?
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