[time-nuts] from Sputnik to CD

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Oct 8 19:43:44 EDT 2007

Bill Hawkins wrote:
> Bruce,
> You have been more precise than this in other posts. Can you put
> some numbers behind the sigma delta DAC's sensitivity to jitter?
> Twenty years ago, the process control community got all jittery
> about the jitter in the execution period of function blocks. The
> fears have disappeared into the pages of history, because the
> final control element was so much slower than the jitter.
> Any time that someone is uncertain about the effect of some
> measurable quantity (or not, think of N-rays) then they become
> prey to anyone with the voice of power who claims a certain
> effect with great certainty. For example, fundamentalist preachers
> and certain audiophiles. This is human nature, part of what enabled
> a prehistoric leader to gather a group of true believers and lead
> them into battle.
> But I really like the Great Dane's reply.
> Bill Hawkins

It is difficult to be precise, as the jitter sensitivity of a sigma
delta DAC depends on its architecture either continuous time, or
switched capacitor.
It also depends on the gain bandwidth product of the amplifiers used and
a host of other factors as well as the theoretical increase in
sensitivity jitter sensitivity of an equivalent N bit DAC (a 1 bit DAC
is 2^N-1 times as sensitive to clock jitter as an N bit DAC).
There are also fundamental limits to the effect of clock jitter on sigma
delta DAC SNR etc see:
Finding actual plots of the effect of clock jitter on effective DAC SNR
etc is somewhat problematic however the attached plot from
illustrates the clock jitter sensitivity of some sigma delta DAC designs.

The point is really that the sensitivity of the DAC to clock jitter has
to be known when making statements about the amount of clock jitter that
is audibly perceptible.

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