Jerry Avins <jya@ieee.org> wrote in message news:<40246155$0$20044$61fed72c@news.rcn.com>...
> robert bristow-johnson wrote:
>
> > In article fMTUb.19737$w_5.11049@newssvr24.news.prodigy.com, Larry McFarren
> > at invalid@address.com wrote on 02/06/2004 16:35:
> >
> >
> >>I came across a link today to an article about the merits of a non-OS DAC over
> >>an OS DAC. Of course, I'm referring to the filter part of the DAC. Anyway,
> >>this article doesn't make much sense to me:
> >>
> >>http://www.sakurasystems.com/articles/Kusunoki.html
> >>
> >>
> >>What is your take on this? Is the author correct when he says that 16-bit
> >>accuracy is not possible if you oversample? I don't believe that this article
> >>was very well written. It didn't convince me that non-OS is the way to go. Let
> >>me know what you guys think.
> >>
> >
> >
> > i'm going over this, but my first take is that the above statement is
> > incorrect in the most meaningful sense. the only reason i might consider
> > conventional DACs is so you can time-multiplex them between channels.
> >
> > r b-j
>
> "The only reason for audio, I imagine. For servos, other criteria rule.
>
> Jerry
A brief read indicates numerous dubious claims.
1) The argument that oversampled DACs have greater sensitivity to
jitter is completely wrong. He is ignoring the fact that jitter
sensitivity is proportional to the average step-size from one sample
to the next (for example, if there is no change at all in the DAC
output, then obviously it doesn't matter when this happens). The
analysis goes like this;
a) assume random jitter. The instantaneous error at each sample point
is proportional to the CHANGE in output level times the instantaneous
time error. These errors must be summed in an RMS fashion to get the
total error magnitude.
b) The CHANGE in output level falls in direct proportion to the
oversampling ratio (assuming your worst case signal is always 20KHz
full-scale).
c) The instantaneous jitter error events occurs more often for higher
oversampling rates, but ...
d) The jitter spectrum is spread over a larger bandwidth.
Point "b" gives audio-band noise FALLING LINEARLY with oversampling
rate..
Point "c" gives a full-spectrum error magnitude that rises as the
SQUARE ROOT of oversampling ratio...
Point "d" gives an in-band noise component that FALLS by the SQUARE
ROOT of oversmapling ratio.
So "c" and "d" cancel each other out, and the result is that jitter
errors FALL LINEARLY WITH OVERAMPLING RATIO.
Now if you want to be pessimistic, you could assume that the jitter
errors were not random, but had long-term correlations. This
potentially eliminates the oversampling reduction factor ("d"),
resulting in in-band jitter errors that fall only by the square-root
of the oversampling ratio.
The author of the article in question seems to assume that every DAC
output event starts from 0, goes to the full value, and returns to
zero again. If this were the case, then point "b" would be eliminated.
But DACs are not designed this way (I have some credentials in this
area). Most DACs have switched-capacitor output filters to remove
sigma-delta noise, and the step-to-step change is mostly dominated by
the interpolated signal itself. Some higher-end DACs use multi-bit
noise-shapers that output directly, with no filtering. Such DACs have
a bit higher step-to-step change statistics for small signals, but for
large high-freqiuency signals (always the worst-case for jitter) the
step size is still dominated by the interpolated signal itself.
By the reasoning presented in the paper, there would be no
oversampling DACs on the market with more than 16-bit performance for
"common" amounts of jitter. The fact is that you can buy such
converters from numerous manufacturers at ridiculously low prices, and
measure their performance with clock jitters as high as 500 ps, and
still easily exceed 16-bit performance.
Bob Adams
Reply by Jerry Avins●February 6, 20042004-02-06
robert bristow-johnson wrote:
> In article fMTUb.19737$w_5.11049@newssvr24.news.prodigy.com, Larry McFarren
> at invalid@address.com wrote on 02/06/2004 16:35:
>
>
>>I came across a link today to an article about the merits of a non-OS DAC over
>>an OS DAC. Of course, I'm referring to the filter part of the DAC. Anyway,
>>this article doesn't make much sense to me:
>>
>>http://www.sakurasystems.com/articles/Kusunoki.html
>>
>>
>>What is your take on this? Is the author correct when he says that 16-bit
>>accuracy is not possible if you oversample? I don't believe that this article
>>was very well written. It didn't convince me that non-OS is the way to go. Let
>>me know what you guys think.
>>
>
>
> i'm going over this, but my first take is that the above statement is
> incorrect in the most meaningful sense. the only reason i might consider
> conventional DACs is so you can time-multiplex them between channels.
>
> r b-j
"The only reason for audio, I imagine. For servos, other criteria rule.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by robert bristow-johnson●February 6, 20042004-02-06
In article fMTUb.19737$w_5.11049@newssvr24.news.prodigy.com, Larry McFarren
at invalid@address.com wrote on 02/06/2004 16:35:
> I came across a link today to an article about the merits of a non-OS DAC over
> an OS DAC. Of course, I'm referring to the filter part of the DAC. Anyway,
> this article doesn't make much sense to me:
>
> http://www.sakurasystems.com/articles/Kusunoki.html
>
>
> What is your take on this? Is the author correct when he says that 16-bit
> accuracy is not possible if you oversample? I don't believe that this article
> was very well written. It didn't convince me that non-OS is the way to go. Let
> me know what you guys think.
>
i'm going over this, but my first take is that the above statement is
incorrect in the most meaningful sense. the only reason i might consider
conventional DACs is so you can time-multiplex them between channels.
r b-j
Reply by Larry McFarren●February 6, 20042004-02-06
I came across a link today to an article about the merits of a non-OS DAC over
an OS DAC. Of course, I'm referring to the filter part of the DAC. Anyway,
this article doesn't make much sense to me:
http://www.sakurasystems.com/articles/Kusunoki.html
What is your take on this? Is the author correct when he says that 16-bit
accuracy is not possible if you oversample? I don't believe that this article
was very well written. It didn't convince me that non-OS is the way to go. Let
me know what you guys think.
-Larry