By Various
I currently have an Emu 1820M audio
interface and a Line 6 Pod XT Pro modelling guitar preamp, and am about to
get a PreSonus Digimax FS eight‑channel mic preamp. The thing is,
I haven't got a clue as to how you would connect all these devices
together, or even if it is possible.
Via SOS web site
SOS contributor Martin Walker
replies: It's often possible to connect gear in several ways, but there will
nearly always be a 'best' way that should ensure the highest audio
quality. First of all, avoid unnecessary A‑D and D‑A conversions.
With your gear, for instance, if you
simply connected your Pod analogue out to one of your 1820M analogue inputs,
this would pass the signal through the D‑A converter in the Pod and then an A‑D
converter in the Emu, which would compromise the signal slightly. It's far
better to connect the S/PDIF digital output of your Pod to an S/PDIF input on
the Emu, bypassing these two conversion stages. The PreSonus Digimax FS also
provides separate analogue outputs for each of its eight mic preamps, but once
again you're better off connecting the PreSonus ADAT output to the Emu ADAT
input, rather than tying up every Emu analogue input!
Next, with any combination of
digitally connected devices you have to decide which should be set to its
'internal' clock setting, and thus provide the master clock signal, via its
S/PDIF, ADAT or AES/EBU outputs, to the other devices. These, in turn, should
all be set to 'external' clock and become 'slaves', locking everything together
in perfect digital sync.
Choosing the device to provide the
master clock is the key to achieving the best audio quality. Theory states that
you should always choose as master the device whose clock offers the lowest
jitter levels, which in this case would be the PreSonus, with its JetPLL jitter‑reduction
technology. However, in practice this choice is often more complicated.
The two most critical points, as far
as digital clocking are concerned, are when analogue signals are converted to
digital by the A‑D conversion process during recordings, where any digital 'shaking'
(jitter) will result in a permanently 'blurred' recording that can't be
corrected or improved later on. When digital audio is converted back to
analogue, so we can hear it through loudspeakers or headphones, any further
digital shakiness will blur existing recordings.
Like many budget interfaces, Emu's
1820M works rather well on its Internal clock, but its jitter levels increase
if you switch to external clock, however good that external clock is. So for
best results when using a budget interface, you should generally allow it to be
the master device during playback and when recording through its analogue
inputs.
Connecting
the elements of a digital audio system together is not always
a straightforward process, with clocking and jitter just two of the issues
that should be taken into account. Some devices, such as this PreSonus Digimax
FS preamp and A‑D converter, even have jitter‑reduction technology that can
improve the jitter performance of other devices in the system.
The beauty of the
jitter‑reduction technology featured in the PreSonus Digimax FS (and various
other devices) is that even when slaved to the Emu's more jittery clock it will
nevertheless significantly reduce its jitter levels on the way in, so your mic
recordings will still sound pristine. To do this, just connect a cable between
the Emu S/PDIF or ADAT output and the corresponding digital input on the
Presonus (it doesn't matter which, since only the embedded clock signal is
being utilised). The Pod XT manual doesn't mention jitter reduction, so when
recording guitar you should probably switch that to be the master device and
the Emu to slave.
Whenever you're faced with several
clocking choices, try each one in turn and listen carefully. The one offering
lowest jitter should provide a stereo image that's both wider and deeper;
you should notice more 'air' and space in recordings, and you should also be
able to hear further into the mix, with distant sounds revealed better.
Published August 2009
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