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Old 17th July 2004, 08:10 PM   #11
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Default Parallel dacs.

Have a look at Accuphase www.accuphase.com They seem to think it is worth the effort.
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Old 17th July 2004, 09:13 PM   #12
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I suppose its worth testing because if it does bring about a benafit then its a fairly simple thing to do.

I think there would come a time when you would create more problems then you solve when you keep increasing the number of chips. But it is something worth considering.
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Old 17th July 2004, 09:29 PM   #13
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I just looked at the accuphase web site - in one of their CDplayers (didnt look at more pdf files are rather large and even on DSL i cant be bothered :P) they use 12 DAC chips , six per channel, obviously they use a balanced output but six per channel seems a bit OTT. I would have thought 4 per channel would be a sensible place to stop.
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Old 17th July 2004, 09:59 PM   #14
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They used to have one model with 16 dacs per channel.
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Old 18th July 2004, 11:39 AM   #15
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Konnichiwa,

Quote:
Originally posted by pinkmouse

Jim, Thorsten, knock off the personal stuff now please.
I am not sure what you are refering to. When Mr. Wilson used the exact same phrase applied to me in another thread you did not feel any need to moderate. Surely what is allowed one way cannot be wrong the other?

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Old 18th July 2004, 12:03 PM   #16
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Quote:
Originally posted by Kuei Yang Wang
When Mr. Wilson used the exact same phrase applied to me in another thread you did not feel any need to moderate.
I didn't see that thread, but I did see this one.
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Old 18th July 2004, 12:07 PM   #17
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Konnichiwa,

Quote:
Originally posted by 5th element
I would have thought 4 per channel would be a sensible place to stop.
I have thought about this too. What you get is an improvement in dynamic range of 3db per doubeling the number of DAC chip's in parallel. BTW, Voltage output DAC's can also operate parallel but they cannot be "stacked" the way it is possible with current output DAC's.

So, if lets start with a single PCM 1704 which is a notional 24 Bit DAC but has only -110db Noisefloor yet all the neccesary parts to have the LSB at (theoretically) -144db. So the DAC's possible performance at low level drowns in noise. Using 4 DAC's per channel will give us -116db Noisefloor, still 4db worse than 20 Bit equivalent performance. So, if we use 8 DAC's per channel we now have -119db for the noisefloor, nearly 20 Bit equivalent performance.

If we do further to 16 or even 32 DAC's per channel we could extend possible performance past -120db THD & N and thus would reap the full dynamic range from high resolution Digital Recordings. The fact that such recordings are few and far inbetween makes this of course largely academic.

And of course in the context of CD Standard audio with a limit of -93db on the recording (due to the 1/2 lsb uncertainty) all this is past academic. Hence one needs to understand what one wants to achieve.

A reason why one may parallel multibit converters for CD Audio is the use oif inexpensive DAC's with poor low level linearity, such as TDA1543. If you use one you have according to the datasheet -83db THD & N (@ -60dbfs) guaranteed and -90db as "typhical" performance. If we take -83db then using 8 DAC's in parallel will get us to -92db THD & N inherently and -99db for the "typhical" case, which would suggest that the recording becomes the limiting factor.

One might argue that using the TDA1543 in a DAC is retrograde and daft, however, if correctly applied the TDA1543 can produce a full scale output similar to any other CD player with only a Resistor as I/V Converter and at reasonable linearity. So there is good sense for using this obsolete DAC Chip with multiples in parallel.

Another reason to use multiple DAC's with CD is that many older DAC's as well as "economy" type DAC's tend to have a large variation of the exact (absolute) level produced at the lower and upper Bit's. This adds ceratain added non-linearity. As making DAC Chip's is a process that is goverend by the classic gaussian "bell" curve using one DAC chip will result in a performance that is all over the place, using multiple chip's in parallel will in effect place the result pretty reliably in the vicinity of the "peak" of that bell curve, making performance more consistent at very low and very high levels.

Now how any of the above sounds is a debatable issue. Whenever DAC Chip's are paralleled (and no matter how this is done either) I percieve some losses, especially in the are Id calle "immedicay". This is the key quality displayed by SE (and good PP) Valve Amplifiers and full range driver speaker display a great ofr and which is invariably lacking in systems using Multiway Speakers.

On the plus side, using multiple DAC's can sound smoother, more relaxed and refined. I found parallels here to using multiple other devices (Valves, Transistors, Op-Amp's) in parallel, in subjective terms you gain in some areas and loose in others.

Personally I'd say that the use of paralleled DAC's or not is governed by a number of parameters and depends entierly on the goals desired. To simply dismiss the use of multiple DAC's in parallel as stupid illustrates a deplorble lack of understanding of rather basic principle in Digital Audio, equally the blanket recommendation of paralleling up DAC's as cure for all digital evils shows a lack of understanding.

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Old 18th July 2004, 11:19 PM   #18
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Hello to all:

Is some ways, paralleling DAC chips could be compared to
the technique of "stacking" digital camera images to produce
a better image.

If you were to take 8 consecuitve frames of a still life image
with, say, a 2 mega pixal camera on a tripod, the 8 frames
could be aligned, summed and averaged using a suitable
program like "Registax" (free on the web). I have tried this
and note the following improvements:

1. Less noise in picture
2. Better dynamic range (light to dark)
3. Better details in dark or shadow areas
4. Better clarity
5. Improved color accuracy

Its like trading up to a digital camera with an improved
sensor chip. The improvement is not linear, so going with
an extremely large number of images (in most cases) does
not greatly improve on the technique.

Aside from the improvements of paralleling DAC chips (and
note that some of the Burr Brown chips already have 2
paralleled internal DAC circuits!), it is interesting to note
that at least Burr Brown grades some of their DAC chips.
The highest grade chips have the best S/N, lowest distortion,
and better linearity. And the sound that the highest grade
chips produce can be better than the average versions. I
once talked to an engineer at Wada, and he said that they
were using 4 BB1704 DAC chips in parallel (select grade)
at that time. And he said that it was well worth seeking
out the highest grade chips.

Fastcat
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Old 19th July 2004, 03:02 PM   #19
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fastcat95

Increasing the dynamic of multiple stacked images for audio or video signals.

You can add multiple converted images together, the process is called stacking. Stacking can improve the Dynamic range and S/N of a signal however it is different than paralleling Dacís on top of each other.

The process of stacking requires a filter algorithm that can process arrays of data. These arrays of data images or then stacked one on top of another what happen then is an a processor subtracts the noise that is random in the signal. The signal should repeat but the noise can jump around
so the algorithm remove the noise that does not totalize to the highest value.

Example: using an 8 bit word to simulate the signal you can see how this process works.

10101001
10101101
10101011
10101000
------------
10101001 signal

One of the issue with paralleling DACís is the decision points where the lower order bits turns is not always the same. When this happens, and it will in lower cost DACís it is possible for several types of errors to happen, depending on the DAC type being used. We can have offset bit error, a gain error, a differential nonlinearity error, and possibly create a non-monotonicity error in the bit we are converting.

Depending again on the DAC topology that being used it is possible to increase the current noise in the output. If we take two transistors and parallel them we get an increase in both current and current noise. If the noise is filtered out by the aliasing filter and there are no bit linearity issues it is possible to improve performance. However, I donít believe you can keep getting a 3dB improvement while you adding noise currents in the output.

Thorsen, I looked over the BB EVM-1702 and the measurements described donít match the methods used in the data sheets for the PCM 1702. In fact
the specifications for the data sheet or better in many cases. Maybe you know is the EIAJ filter the same as the A weighted filter?

fastcat95,
You mentioned, Wadia used the best grade parts for this application. However, to assure your getting the best performance from the DAC they should be graded for bit linearity to realize a performance improvement.

Another improvement is possible if we have missing codes in one dac and none in the other. We will find that most of the converters never meet their maximum linearity specís, it difficult to get true 18-20bit but after that all betís or off. What to look for in a dac is guarantee bit performance. I just have not seen consumer grade parts guarantee bit performance. Of course we still have not mentioned the noise and performance needed in a power to get to this level as well as the grounding. I Do like the ISO isolators used in the BB EVM-1702 that should help reduce the digital noise.

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Old 19th July 2004, 09:05 PM   #20
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Konnichiwa,

Quote:
Originally posted by jewilson
One of the issue with paralleling DACís is the decision points where the lower order bits turns is not always the same.
I know few DAC's where this could be expected. All multibit DAC's known to me are SUPPOSED to update the current output on the change of WS/LE. Now if they do not settle their current very quickly they would not be able to provide any kind of performance. I will agree that there may a minimal difference in the slope time between chip's, but with the same FAB and methodes this again should be minimal and small enough to avoid any notable glitches.


Quote:
Originally posted by jewilson
When this happens, and it will in lower cost DACís
I would severely question that. 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....

Quote:
Originally posted by jewilson
it is possible for several types of errors to happen
Virtually anything is possible in this universe, but most things are HIGHLY unlikely.

Quote:
Originally posted by jewilson
If we take two transistors and parallel them we get an increase in both current and current noise.
Yes, the absolute current and the absolute noise current both rise. BUT the relative noisecurrent (compared to tho the operating current) goes DOWN.

Quote:
Originally posted by jewilson
However, I donít believe you can keep getting a 3dB improvement while you adding noise currents in the output.
Based on a variety of published measurements - you can.

Quote:
Originally posted by jewilson
Thorsen, I looked over the BB EVM-1702 and the measurements described donít match the methods used in the data sheets for the PCM 1702. In fact the specifications for the
data sheet or better in many cases. Maybe you know is the EIAJ filter the same as the A weighted filter?
The EIAJ specification of dynamic range etc. is more stringent (IIRC) but also remember that the EVM is charaterised as complete system, the 1702 per datasheet as "chip only".

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