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Old 21st October 2006, 06:10 PM   #1
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Default PCM 1794 Architecture/Concept??

It may well be on that awful TI website, but it didn't jump out at me.
It may have been menitoned here before, but it escaped my attempt at searching here.

Anyhow, what is the principle behind the 1794's architecture?

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Old 22nd October 2006, 07:08 AM   #2
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Default Re: PCM 1794 Architecture/Concept??

Quote:
Originally posted by bear
It may well be on that awful TI website, but it didn't jump out at me.
It may have been menitoned here before, but it escaped my attempt at searching here.

Anyhow, what is the principle behind the 1794's architecture?

_-_-bear

There was an AES paper about Advanced Segment DAC architecture
when the PCM1738 came out. I'll try and dig it up.

Meanwhile there is some info here about the architecture and
in general a really interesting paper about DACs.

http://www.iet.ntnu.no/~ivarlo/files...t_audiodac.pdf

cheers

Terry
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Old 22nd October 2006, 07:52 AM   #3
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Read the PCM1794 datasheet. Namely, the "Theory of Operation" section. Basically, the upper 6 bits are passed through, while the lower 18 bits are dithered by third-order delta-sigma modulator to the level of a single upper bit. The result is fed to current-output DAC.
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Old 22nd October 2006, 05:44 PM   #4
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Terry,

thanks for the link!
... am downloading it now... dial-up is slooooooowwwww...

Andrej,

I thought I downloaded the datasheet and didn't see much about the "theory of operation" on the one I got - they just said something about their "bla bla bla Advanced Segment...". I figured there must be another paper somewhere on their site that explained it.

The delta-sigma thing gives me pause.
Dunno if there is any reason for it, but all the delta sigma based DACs that I have heard have "something funny" about the sound when A/B'd against the "ladder dacs"... I dunno. Maybe by going back to a current output the issue is avoided - assuming the internal opamps in most of the delta-sigma dacs is the source of the problem I've noticed??

Ah well, guess I have to read more to try to understand it...

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Old 23rd October 2006, 12:54 AM   #5
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Two basic technical problems with delta-sigma modulators are noise and limited resolution. They work esentially on the same principle as pulse-width modulators: intermediate levels are achieved by operating the switch at higher frequency (64x for PCM1794), and keeping it open n cycles out of 64 to give n/64 output level when integrated. Since it switches the full reference level all the time, there is a significant amount of noise in the output. That is shifted away out of the audio band by the real delta-sigma algorithm (which is a bit trickier - it uses a digital feedback loop to shape modulation noise). If you look at the spectral noise density plots above 20kHz, you'll see the noise floor raising - by as much as 30dB for PCM1794. Also, to achieve high resolution delta-sigma modulators rely on long integration. If you are integrating output over just 1 sample - you have resolution of 1/64 - roughly 6 bits worth.
As a side effect of switching strategy, they are also more susceptible to jitter and transient distortions in I/V analog stage. They are generally only usefull for low frequency applications. In RF and telecomm, ladder DACs are the norm.

On the positive side, delta-sigma modulators offer better linearity and are less expensive to make. The later does not apply to pricing of PCM1794, somehow.
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Old 23rd October 2006, 02:07 PM   #6
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Hmmm... that's what I was wondering about...

So, that leads me to the question, what does the 1794 do better than a 1704?? Or, why would anyone want a 1794?

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Old 23rd October 2006, 02:40 PM   #7
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Quote:
Originally posted by bear
Hmmm... that's what I was wondering about...

So, that leads me to the question, what does the 1794 do better than a 1704?? Or, why would anyone want a 1794?

_-_-bear

It measures a lot better. 1/2 distortion at 0dBFS and 3 x dynamic range.

It appears 1704 is the limit of R2R linearity in a chip and BB must
have felt they needed to come up with an answer to competitors
dacs such as AD1853 etc.

My gut feeling is the 1704 is easier to get VG results with, lower
OP current, very low OOB RF noise into I-V, less susceptibility to
jitter.

Maybe the 1794 is a jewel waiting to be uncovered but only with
very careful implementation???

Whilst you are in dac research mode it's worth a look at the
Lavry DA 924, 24 bit / 96k fully discrete ladder dac.
The manual is a very interesting read.

http://www.lavryengineering.com/white_papers/DA924m.pdf

Dan's a very smart guy.

cheers

Terry
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Old 23rd October 2006, 05:20 PM   #8
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There are many reasons to use PCM1794. First, it can be mono or dual channel, whereas 1704 is only mono. From a practicality standpoint, not everyone can afford to have dual mono operation. Second, PCM1704 is obsolete, while PCM1794 is currently in production. Third, PCM1794 accepts I2S and is very easy to connect to modern receivers and ASRC's, whereas PCM1704 needs some external logic. Obviously, these are all technical issues, that may or may not affect sound, but these are important to take into consideration, especially if you are DIYing, and FIYing (F=Funding). Also, if cost is a consideration, you may have to compromise on your implementation of 1704, whereas with 1794 you may be able to have a better design at the same price point.
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Old 23rd October 2006, 07:43 PM   #9
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Quote:
Originally posted by ezkcdude
PCM1704 is obsolete, while PCM1794 is currently in production.
What's your source for your claim the PCM1704 is obsolete? ‘Not recommended for new designs’ is not the same as obsolete. In fact, TI is in the process of converting the PCM1704 process to lead-free and new parts are scheduled for production next month. If the part was truly obsolete, why would TI bother making it lead-free?

Since when are DIYers concerned about avoiding ‘obsolete’ parts? The TDA154x DACs have been obsolete for years but they still enjoy cult status amongst the DIYers here.
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Old 23rd October 2006, 10:04 PM   #10
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Quote:
Originally posted by jbokelman


What's your source for your claim the PCM1704 is obsolete? ‘Not recommended for new designs’ is not the same as obsolete. In fact, TI is in the process of converting the PCM1704 process to lead-free and new parts are scheduled for production next month. If the part was truly obsolete, why would TI bother making it lead-free?
My bad . I didn't know an ROHS version was coming out.

The other points still apply, and as for using obsolete components, it's just not my personal preference. I'm not a fan of cults or status.
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