Front panel Adjustments, PK Hold, Atten, Chan

20/20 AD User Guide

Using the 20/20 AD

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.

Front panel

Adjustments

Normal Level Meter settings

(1 dB SCALE and DISPLAY set to OFF)

- red -	OVL	OVL
- red -	0	0dB

- yellow -	3	-3dB

- yellow -	6	-6dB

- green -	12	-12dB

- green -	18	-18dB

- green -	24	-24dB

- green -	30	-30dB

- green -	36	-36dB

- green -	42	-42dB

- green -	60	-60dB

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∝ sec pre-emphasis to each input signal before digitization.

1 dB Level Meter settings (1 dB SCALE set to ON, DISPLAY set to OFF)

- red -	OVL	OVL
- red -	0	0dB

- yellow -	3	-1dB

- yellow -	6	-2dB

- green -	12	-3dB

- green -	18	-4dB

- green -	24	-5dB

- green -	30	-6dB

- green -	36	-7dB

- green -	42	-8dB

- green -	60	-9dB