13
Using the 20/20 AD
20/20 AD User Guide
Adjust the front panel Gain knobs until the peak input levels are balanced and
come within a few dB of clipping. For now, make sure that the DISPLAY switch
in the DIGITAL COMP/LIMITER section is set to OFF. In normal operation, each
level meter will indicate the level at the output of the converter in dB referenced
to digital full scale (dBfs). A lighted green or yellow LED indicates that the signal
has exceeded the level printed next to it. The red 0dB LED lights when the signal
exceeds -0.5dBfs. The red OVL LED indicates audible clipping, (clipping which
lasts for several samples).
I dB SCALE
Turning on the 1dB SCALE switch increases the resolution of the top end of the
scale shown on the level meters. In this mode, each LED corresponds to 1dB.
Meter Ballistics
Each meter consists of a peak meter (indicated by an isolated LED) and an
average meter (indicated by a growing and shrinking bar). The peak meter's
attack time constant is instantaneous, and its decay time constant is roughly 1
second. The average meter's time constants approximate those of a VU meter,
with attack and delay time constants of 10 mS.
PK HOLD
When you press PK HOLD, the meters will indicate the highest level received
after PK HOLD was pressed. To reset the peak, turn PK HOLD off and then on
again. Remember that the level meters will display either the normal scale or the
1dB scale, dependent on the setting of the1 dB SCALE switch.
ATTEN
The ATTEN button should only be used on balanced input signals.
If a Gain knob is turned completely counter clockwise and the level meter still
indicates an overload, you must either press the ATTEN button (which reduces
the input level by 6dB), or reduce the signal level at its source. Note that ATTEN
should not be used unnecessarily as it causes a slight reduction in the Common
Mode Rejection Ratio (CMRR) and thereby increases the potential for hum.
4-CHAN
When the 4 CHAN switch is on, the 20/20 AD converts four channels from analog
to digital, with a dynamic range of better than 16 bits. When the 4 CHAN switch
is off, the 20/20 AD only converts CH1 and CH2, buts adds 12dB (2 bits) to the
dynamic range. Your selection of mode depends primarily on how many
channels you need to digitize, and on whether or not you need the extra dynamic
range. If you plan to record to 16-bit media without using the on-board digital
compressor, 2-channel mode will give you duplicate digital outputs.
EMPH
The EMPH switch adds CD-standard 15/50
µ
sec pre-emphasis to each input
signal before digitization.
Normal Level Meter settings
(1 dB SCALE and DISPLAY set to OFF)
OVL
0dB
-3dB
-6dB
-12dB
-18dB
-24dB
-30dB
-36dB
-42dB
-60dB
- red -
- red -
- yellow -
- yellow -
- green -
- green -
- green -
- green -
- green -
- green -
- green -
OVL
0
3
6
12
18
24
30
36
42
60
1 dB Level Meter settings
(1 dB SCALE set to ON,
DISPLAY set to OFF)
- red -
- red -
- yellow -
- yellow -
- green -
- green -
- green -
- green -
- green -
- green -
- green -
OVL
0
3
6
12
18
24
30
36
42
60
OVL
0dB
-1dB
-2dB
-3dB
-4dB
-5dB
-6dB
-7dB
-8dB
-9dB
Front panel
Adjustments
