# [time-nuts] Oh dear

J. Forster jfor at quikus.com
Mon May 7 23:38:20 UTC 2012

```It's kinda a trick question.

The important thing is 'before anyone could detect any differences in the
sound?'

I was involved in making a decision to go with brand A or B speakers in a
roughly 1200 seat auditorium. There was a lot of political pressure to
choose brand B. IMO, brand A sounded better in all cases, except possibly
for some specially chosen 'high pressure' jazz advocated by fans brand B.

So I set up an A/B test. To make the test fair the speakes were colocated
and each set of speakers had their own amps and levels were set to exactly
the same SPL on pink noise.

Nobody but I knew which speakers were on at what time.

The test audience selected brand A as the best sound, but eventually brand
B was installed. Politics won.

Bottom line, when evaluating the claims of audiophools, like you can 'hear
the difference', you have to do truly random, blind, tests A/B tests.

I doubt that you need anything better than a crummy \$2 crystal to clock
audio systems.

YMMV,

-John

================

>
> jfor at quikus.com said:
>> Suppose you have a perfect, ideal clock that puts out 'convert' pulses
>> at an
>> exact rate is used to strobe a high precision A/D.
>
>> Now suppose you add jitter to that perfect clock so that the rate stays
>> the
>> same but time interval between successive pulses varies randomly between
>> P(1-x) and P(1+x).
>
>> How big would x have to be before anyone could detect any difference in
>> the
>> sound?
>
> It's easy enough to work out the right ballpark.
>
> Feed a theoretical sine wave into your A/D.  Set it to the max amplitude
> and
> max frequency that you expect the system to handle.  Look at the zero
> crossing (max slope).  How much time does it take for the signal to
> transition from an output of 0 to an output of 1 (LSB).
>
> If your clock if off by that much in time, the analog voltage that you
> sample
> will be off by 1 bit.
>
>
> It's a big deal at radar frequencies, less so at audio.  You want to make
> sure that you don't use one of the oscillator packages that has a
> programmable PLL.  Their jitter specs are nasty.
>
>
> --
> These are my opinions, not necessarily my employer's.  I hate spam.
>
>
>
>

```