Word Clock In And Out; Digital Output Front Panel Controls; Applications; Mic Preamp Input Impedance - Focusrite ISA428 MkII Handbuch

Word clock in and out

The optional ADC can be synchronised to an external word clock. By pressing the front panel Ext switch, the synchronisation mode
can be switched between standard external word clock and 256X external word clock. Both types of external word clock should be
connected to the ISA428 MkII ADC card at the Word Clock In BNC connector.
The Word Clock Out BNC connector either regenerates the external word clock connected at the Word Clock In BNC connector, or
transmits the internal sample frequency of the ADC card. Where the ISA428 MkII is being used as a slave device within a larger
digital system, the Word Clock Out BNC connector can be used to pass on the external word clock signal to the next device. When the
unit is not slaved to another device and is in internal clock mode, the Word Clock Out BNC connector outputs the sample frequency
selected on the ISA428 MkII front panel.

Digital output front panel controls

Clock select
Pressing this switch allows the user to select between sample frequencies of 44.1kHz, 48kHz, 88.2kHz, 96kHz,
176.4kHz, and 192kHz.
Ext
Pressing this switch allows the ISA428 MkII to be slaved to an external word clock source. Selecting 256X allows
the ISA428 MkII to be slaved to an external clock running at 256 times faster than the sample frequency and enables
connection to systems such as the Digidesign 'Superclock' or other 256X slave clock devices.
Lock LED
When lit, LOCK indicates that the unit is successfully synchronised to an external clock. Note: When using 256X
external clock, no lock indication is given, if audio can be heard in this mode then 256X clock is locked.

Applications

Mic preamp input impedance

A major element of the sound of a mic pre is related to the interaction between the specific microphone being used and the type of
mic pre-amp interface technology it is connected to. The main area in which this interaction has an effect is the level and frequency
response of the microphone, as follows:
Level
Professional microphones tend to have low output impedances and so more level can be achieved by selecting the higher impedance
positions of the ISA428 MkII mic pre-amp.
Frequency response
Microphones with defined presence peaks and tailored frequency responses can be further enhanced by choosing lower impedance
settings. Choosing higher input impedance values will tend to emphasise the high frequency response of the microphone connected,
allowing you to get improved ambient information and high end clarity, even from average-performance microphones.
Various microphone/ISA428 MkII pre-amp impedance combinations can be tried to achieve the desired amount of colouration for the
instrument or voice being recorded.
To understand how to use the impedance selection creatively it may be useful to read the following section on how the microphone
output impedance and the mic pre¬amp input impedance interact.

Switchable impedance: In depth explanation

Dynamic moving coil and condenser microphones
Almost all professional dynamic and condenser microphones are designed to have a relatively low nominal output impedance
of between 150Ω and 300Ω when measured at 1kHz. Microphones are designed to have such low output impedance because the
following advantages result:
• They are less susceptible to noise pickup.
• They can drive long cables without high frequency roll-off due to cable capacitance.
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