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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