Grimm Audio CC1, Manual

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to lock and has a 0.1Hz bandwidth. Typical AES/EBU receiver chips lock within a few
samples and have a bandwidth of around 10kHz.
If the local clock is very clean, a narrowband PLL is the best choice because all but
the lowest-frequency jitter in the external sync is rejected. A converter designed
along those lines will sound stellar under all conditions. If the external sync is very
clean, a wideband PLL is the best choice because the local oscillator’s own errors
will be corrected. This is the case where a good external sync like the CC1 improves
a budget converter, or even a pricey one, beyond expectations.
If the designer guesses wrong however, a too- fast PLL might end up forcing an
otherwise fine local oscillator to reproduce faithfully every bump and hiccup in the
external sync signal. Equipment constructed along these lines sound good in mas-
ter mode but will only improve in slave mode if the external sync is stabler than the
internal oscillator. An unstable external sync actually makes it sound worse.
Alternatively a too-slow PLL might not correct a local oscillator of suboptimal qual-
ity. In that case, jitter performance is bad regardless of the quality of the external
clock. And here lies the rub: a slow PLL will always make a converter sound the
same, but not necessarily good. If a converter is insensitive to external jitter, that
alone is no indication that its internal jitter is low. A slow PLL shuts the door to
external jitter, but also to any improvement to be had from external clocking with a
very stable source.
In short, one cannot expect an external clock to work miracles everytime. If the PLL
of the receiving device is slow, the sound quality will be independent of the quality
of the external clock, for better or for worse. If the PLL is fast, real improvements
can be had.
By example, the graph below shows the result of measurements on a well known
DAW converter. The jitter performance, measured at the converter chips’ clock pin,
improves substantially at jitter frequencies below 200 Hz when slaved to a CC1.
4f
1f
100p
30p
10f
10f
100f
100f
1p
1p
10p
10p
Jitte
r
(s
)
Jitter
density
(s/sqrtHz
)
20 20k 50 100 200 500 1k 2k 5k 10k
Hz
DAW slaved to CC1
DAW (master)
CC1
Jitter performance of a well known DAW converter