Metric Halo Mobile I/O - Manual (Page 103)

ULN-8 Users Guide
103
1. Analog Audio
2. AES Digital Audio
3. Clock Sync
4. FireWire
5. Power
6. MIDI
7. SMPTE
Analog Audio Connections
The analog I/O connections on the ULN-8 have been engineered for maximum flexibility in that they support
both balanced and unbalanced connections with a wide range of input and output levels and a wide range
of matching impedances. The microphone inputs have an impedance of 3.3k Ω, and the line inputs are 10k
Ω. With that in mind, there are a number of aspects of the design that you should take into account when
interfacing with the ULN-8. All multichannel audio connections are made using industry standard DB25 con-
nectors, which are pin- compatible with cables using the Tascam/Digidesign standard.
Wiring details for this connector can be found in the DB25 Pinouts appendix.
Whenever possible, use balanced connections with the ULN-8. The performance of balanced interconnects
is much higher and much more resistant to noise interference and electrical (power) wiring problems. The
expense of balanced interconnects is not substantially higher than unbalanced connections, so if the gear that
you are interfacing with supports balanced connection — use it. If you cannot utilize balanced interconnects,
there are connection schemes that you can use that will maximize performance.
On input, at line level, it is sufficient to simply use standard unbalanced (TS) connections. If you are interfacing
with the Mobile I/O XLR inputs, you will need to ensure that pin 3 is grounded in the unbalanced adapter
cable. More information about adjusting the input level can be found in the MIO Console software chapter.
ULN-8 DB25 cables should be wired pin 2 or Tip hot. The front and rear 1/4” connectors are wired Tip hot.
The headphone connector is wired Tip/Left, Ring/Right, Sleeve/Ground.
Connecting to the DIs
The DI inputs provide an excellent high impedance input for directly connecting instruments to the ULN-8.
These inputs are unity-gain summed with the corresponding line input from the back panel. This means that if
you have signal connected to both the DI and line input for a given channel the ADC will see a mix of those
two inputs. If you do not wish to have signal from one of the connectors you need to physically disconnect
that input or otherwise ensure that no signal is present at the unused input.
You may notice that the MIO Console has an input selection for “Inst”. This can be used as a visual reminder
that you are plugged into the DI, but has no operational effect on the input. Selecting “Inst” on any channel of
the ULN-8 sets the input to Line +4; the only DI input is available from the front panel jacks for inputs 1 and 2.
Each DI input supports balanced connections via TRS cable. You can connect a source to the DI via a balanced
interconnect (if the source is actually a balanced output), an unbalanced interconnect (if the source is unbal-
anced like most instruments), or a telescoping shielded cable (which is an alternate unbalanced connection
that provides additional shielding).
Since the DI inputs provide an ultra-high input impedance, if you connect an unbalanced source to the DI via
a balanced cable you will float the negative input and will effectively inject a large DC offset into the output of
the DI which will cause the associated ADC to mute. As a result you must use either a TS cable or a telescoping
shield cable with unbalanced sources.
When you remove the connector from the DI jack, the DI connects its inputs to ground internally to remove any
possible residual DC offset or pickup noise. If you disconnect the cable from the source but leave it connected
to the DI input, the cable will function as an antenna and will inject noise into your inputs and may cause