Digital Processing; Dc Removal; Uv22; Emphasis/De-Emphasis - Apogee AD-8000 Operating Manual

24-bit 8-channel digital audio conversion system

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Application notes (2 pages)

Datasheet (1 page)

Application notes (2 pages)

Table Of Contents

Table of Contents

Eight-Channel D/A (Optional)

The optional 8-channel D/A card contains four 24-bit stereo DACs with filtering, digital de-emphasis (includ-

ing de-emphasis override for digital signals with an incorrectly-set emphasis flag), DC removal, automatic soft

mute and gain adjustment circuitry. The same output channels going to the AES/EBU output jacks are also rout-

ed to the 8-channel D/A card.

In the case of both D/A cards, output levels can be switched between +4 dBu and -10 dBV. Emphasis con-

trol is normally automatic, but it can be defeated manually on the card. The ANALOG DEST LED illuminates if

either D/A card is detected.

DC Removal

Although it is much less common today than in the past, you may still occasionally encounter a DC compo-

nent present on incoming signals. If left untreated, this can result in a DC component being overlaid on the out-

put signal. Later, during editing, if a section of digital signal with a DC component is edited on to a portion of

signal without a DC component (or vice-versa), the result will be a loud noise at the edit point.

DC removal is performed by a high-pass filter set at 1 Hz. DC may be removed from both analog and dig-

ital source material. As set at the factory, DC is not removed from either analog or digital sources.

Digital and analog settings may be changed via DIP switches SW6 and SW7 respectively, on the rear panel.

Note: If you wish to remove DC offsets from analog inputs, bear the following in mind. The "over"

threshold setting defaults to four consecutive full-scale samples. When removing DC from analog sig-

nals, we recommend that you set the "over" threshold to one sample, ie set DIP switches 3 and 4 ON

(up). If the threshold is set to a higher value, some overs may go unregistered – the DC removal circuitry inter-

prets any analog clipping longer than a single full-scale sample as a DC component, and consequently removes

it before it can be registered as an over, if the over threshold is set to more than one sample.

The output of the high-pass filter is fed into the metering algorithm. Hence, the audio meters on the front

panel always indicate true audio levels, regardless of any residual DC level. The high-pass filter is set to have

less than 0.1 dB roll-off at 20 Hz.

UV22

UV22 can be applied to channel pairs for digital sources, or individual channels for analog sources, using the

UV22 mode button on the front panel. Depending on the setting on the back panel DIP switch, UV22 encodes

the audio to either 16 or 20 bits (the default is 16 bits). UV22 has two amplitudes, "normal" and "low", set by

the DIP switch SW5 on the back. Set to "low" if program will be reprocessed through UV22 at a later stage; if

not, set to "high". See Appendix II for more information on UV22.

Emphasis/de-emphasis

15/50 µs emphasis is detected and utilized by the AD-8000, but it only directly affects the headphone DAC.

If an expansion stereo or 8-channel DAC is plugged in, then de-emphasis will be detected if present, and

applied to that.

Soft-Muting

Individual channels can be muted. Muting provides an exponential decay to –122 dBFS over 0.25 seconds.

Note: AD-8000 units leave the factory with all DIP switches in the default (down) position. Changes to switch

setting should be made with the power OFF to ensure that changes are registered.

DOWN = OFF (Default)

SW1

"Pro" line level (+4 dBu)

SW2

UV22 to 16 bits

SW3/4

SW5

UV22 "Normal" setting

SW6

Digital Input DC remove DISABLED

SW7

Analog Input DC remove DISABLED

SW8

Polarity Pin 2 hot

SW9

Truncation Logic ENABLED*

AD-8000 Operating Manual