dB levels and ADC level calibration?

[A Fak]

Please help me understand.

So I reset my AD16x so it goes back to factory specs’s +4dBu, and have my DAC1 set to Calibrated mode which is +4dbu. I plugged the right output of my DAC1 into input 1 of my AD16x. I opened up logic and inserted a test oscillator set to 1 kHz sine wave. I also inserted the Sonalksis FreeG (a meter and fader plugin). When the oscillator was on and the signal was at 0db the ad16x clipped (red light came on). I had to pull down the fader to -4.04db so it wouldn’t clip??

Maybe I just don't understand, but shouldn’t I be able to output +4db before my converters clip?

Apogee AD16x manuel:

RESET
To reset the AD-16X, power up the unit while pressing the DOWN button. This provides a quick method to return to factory default settings when exploring the unit’s functionality and capabilities. When reset, the conversion level is set so a +4 dBu analog input results in a –16 dBFs digital output.


Benchmark DAC1 manuel:

OUTPUT LEVEL SWITCH – Three Position Toggle:

Calibrated (UP) – Analog output levels are controlled by 10-turn rear-panel trim controls
Off (CENTER) – Analog outputs are muted, headphone outputs remain active
Variable (DOWN) – Analog output levels are controlled by front-panel gain control

The Output Level switch does not affect the operation of the headphone jacks (the headphone outputs are never disabled and the headphone level is always controlled from the front panel).

CALIBRATION POTENTIOMETERS:
The calibration potentiometers are 10-turn trimmers and are accessible through the rear panel using a small screwdriver. These trimmers provide a 2 dB per rotation adjustment with a total control range of + 9 to +29 dBu at 0 dBFS (full-scale digital input). Factory calibration has been set so that the output level at the balanced XLR connectors is +4 dBu @ -0 dBFS (+4 dBu when the digital input signal is at -0 dBFS, and -16 dBu at -20 dBFS). This is lower than the most common reference level for studio applications, but is appropriate for most powered monitors. The factory-preset levels may be increased by 5 dB or decreased by 15 dB in order to conform to other studio reference levels. This range of levels is also well suited for direct connection to the balanced linelevel inputs on most power amplifiers. Most professional equipment will work well at these levels.

Note: The Calibration Potentiometers have no effect on the output levels when this switch is set to Variable

[steve_flower]

Seems like the DAC1 has been calibrated to give +24dBu @ 0dBFS. Check the internal attenuation jumpers, it sounds like they are set to 0dB, rather than the factory default of -20dB.

That said +24dBu = 0dBFS is better than the default +4dBu unless you're connecting directly to speakers...

[chris319]

Seems like the DAC1 has been calibrated to give +24dBu @ 0dBFS.

That would mean +4 dBu = -20 dBFS -- in other words, you have 20 dB of headroom before clipping.

[steve_flower]

Originally Posted by chris319

That would mean +4 dBu = -20 dBFS -- in other words, you have 20 dB of headroom before clipping.

Yep, and if outputting 0dBFS from the oscillator it would clip by 4dB:
0dBFS out of the DAC = +24dBu

0dBFS on ADC = +20dBu.

So 0dBFS from the DAC is equal to +4dBFS on the ADC.