Built In Level Calibration Using One Led; Digital Headroom Explained - Apogee AD-1000 Operating Manual

AD-1000 Operating Manual
Precise Calibration using One LED
When you need to make a mix from your analog mixing console, it is necessary to calibrate your AD-1000 to
the output of the mixing console. The AD-1000 incorporates a unique feature to enable you to simply match
the digital output to within a hair width of your console's meters. The –12 LED is much more flexible than the
labeling would make it appear. When you input an analog signal into the AD-1000, the –12 LED's threshold
becomes variable and coincides with the headroom setting of the digital oscillator. This permits precise match-
ing of an analog input to the internal digital oscillator.
If the internal digital oscillator is set to the default –15dBfs position (all three switches in default off) the –12
LED will remain off until the input level reaches –15dB (Peak) below full scale digital output. Passing the thresh-
old turns the LED On. In addition, the LED will blink rapidly to tell you when the signal is within the 0.05dB of
the –15dB threshold.
If you are inputing a calibrated analog sine wave oscillator tone from your console (typically +4dBu for an ana-
log meter zero) you can now adjust the selected gain pots on the AD1000 to make the "–12" LED blink. You
have now calibrated the AD-1000 to within ±0.05dB of the "perfect" digital oscillator. Switching between the
analog input (such as +4) and the digital oscillator will show perfect calibration.
The calibrated headroom can be varied to best suit your application by changing the digital oscillator head-
room. The blinking point for the "–12" LED moves right along with it. The mastering world usually chooses –12
or –14dB, tracking tends to be done with –15 to –8 dB and the film world tends to play it safe with up to 20dB
of headroom.
Headroom
(See diagrams overleaf.) A hotter-sounding compact disc can be the difference between having a hit or being
forgotten. A hotter-sounding CD means not wasting headroom. In analog recording we define a nominal oper-
ating level and allow enough headroom above to avoid clipping the analog circuitry. This nominal level is usu-
ally referred to as 'zero' for the 0dB calibration on analog meters. The analog zero usually represents a nominal
+4 dBu output level, i.e. when the meter indicates 0 it is really putting out a level of +4 dBu.
With digital audio, the precise distortion or clipping point is known. This is the point where we run out of num-
bers to represent the analog input. This maximum positive (or negative) level is often called an 'over' due to the
popular labeling of digital meters. The 'over' indicators illuminate when a digital maximum is reached, usually
for a total of more than 3 samples in a row.
NOTE: Some DAT recorders such as the Panasonic 3700 and 3900, indicate 'overs' with analog inputs, but do
not indicate them with digital inputs such as from the AD-1000.
In digital audio we must decide on how much headroom we want above our nominal level (the zero from ana-
log world) before we hit an 'over' or digital clip. The aim is to use as much of the dynamic range as possible.
Any wasted headroom means we are closer to the noise floor than necessary.
We require more or less headroom, depending on the material being recorded. Mastering engineers typically
choose 12 or 14 dB headroom over their nominal input level because they usually have their dynamics tightly
controlled. In tracking situations, 15 and 16 dB are the most popular, with some users going as high as 18 and
20 dB. The headroom of analog inputs or some DAT recorders are often fixed, such as at 18 dB for the Panasonic
3700 and 3900. When using the AD-1000, the headroom is easily adjusted with the front panel controls (purple
knobs – essential for high-quality digital audio) or front panel multi–turn pots, and the built in oscillator for gen-
erating the a digital test tone at a known digital level and using the precise threshold to input tones and adjust
the gain pot or CAL pot. .
Digital audio levels are often referred to the maximum level (or full scale, f.s.). "Zero dB full scale" is a maximum
level when referring to digital levels. With 16dB headroom, the nominal level would be then sitting down at
–16dB referenced to full scale (f.s.)
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