Insert In
Activating the INSERT switch (illuminated when active) breaks the signal path of the channel, so that the signal
connected to the ADC Input 25-pin connector on the rear panel routes to the digital output card (if connected) rather
than the direct mic, line or instrument signal. Note that the mic, line or instrument signal will still route to the analogue
output on the rear panel with this switch active. This switch is designed to allow the input signal to be routed to other
hardware for processing (out of the analogue output) and then back into the 828 (using the ADC input) for digital
conversion. The
Meters Pre ADC switch on the front panel allows the ‘return’ signal level to be viewed on the LED
strip meters before conversion (see the Metering section below for details).
HPF
Pressing the HPF switch makes the High Pass Filter for that channel active in the audio path. This is useful for
removing any unwanted bass caused by proximity effect or rumble. The filter provides a 75Hz knee frequency with
18dB/octave roll-off.
Input Impedance
Pressing the Z In switch steps through each of the four transformer pre-amp input impedance values, as indicated
by the corresponding LEDs. By selecting different values for the impedance of the ISA 828 transformer input, the
performance of both the ISA 828 pre-amp and the microphone connected can be tailored to set the desired level and
frequency response. The impedance values are as follows:
Low – 600
Ω
ISA 110 – 1k4 Ω
Med – 2k4 Ω
High – 6k8 Ω
A guide to setting input impedance is available in the Applications section.
The impedance switch is also active on the instrument input. In this case, pressing the Z In switch toggles between
High and Low impedance settings. The impedance values are as follows:
Low – 470 k
Ω
High – 2.4 MΩ
Metering
The vertical columns of LEDs indicate the peak signal levels of channels 1-8 in one of two modes, defined by the state
of the ‘Meters Pre ADC’ switch on the front panel:
Mode 1. Meters Pre ADC switch disengaged
This is the default state and the mode to use when no digital output card is installed. Meters 1 to 8 indicate the analogue
level directly after the Gain stages set on the front panel. 0dBFS (reached when the red LED is lit) indicates that a signal
level of +22dBu is present at the analogue output. -18dBFS (unless recalibrated on the rear panel, see description on
page 8) therefore indicates that there is a signal level of +4dBu at the analogue output.
7
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