The Mysteries of Metering: Navigating the Path from Analog to Digital
The focus of this "Support Report" is metering. As a manufacturer of both analog and digital products, UA Customer Support receives many calls and e-mails concerning the union of these two distinctly different worlds. One of the most common points of confusion revolves around the difference in calibration between analog and digital metering.
For the purpose of this demonstration I will be monitoring a signal in three different stages of the audio chain: (1) the preamp, (2) the converter, and (3) the DAW. On the front end of the chain I have chosen the LA-610 MkII Channel Strip.
When setting up the preamp on the LA-610 MkII, the starting point is to set the MODE switch to BYP (Bypass) and the METER switch to PREAMP. The output of the preamp is now being monitored with no effect from the compressor. The stepped GAIN switch in the upper left hand corner of the faceplate adjusts the gain of the input stage in stepped 5dB increments. Turning the GAIN switch clockwise raises the gain. Because this also has the effect of reducing negative feedback, the GAIN switch also alters the amount of the input tube’s harmonic distortion—a major contribution to the "warm" sound characteristic of tube equipment. "0" is a good starting point when setting up the preamp.
The -15dB pad has no effect in Hi-Z mode, so I will leave it in the down position. The Polarity switch will almost always be set to IN Ø. Polarity reversal may be useful in cases where more than one microphone is used in recording a source. The +48V switch has no effect in HI-Z mode so it should be left in the OFF position. Next in line is the LEVEL pot, which determines the amount of signal sent to the final preamp output stage. Once signal is being sent to the unit, turning this knob clockwise will start to register a signal on the VU meter. Turn the LEVEL knob clockwise until the meter is peaking at or just above "0." This indicates a strong signal outputting from the preamp side of the unit.
Now that the preamp is set, it’s time to move on to the compressor. Start by switching the MODE select to COMP and the METER select to GR (Gain Reduction). The needle on the VU meter will rise to the zero point. I start with the PEAK REDUCTION knob at "0" and slowly rotate it clockwise while monitoring the VU meter. The needle will begin to bounce to the left, indicating the amount of compression being applied. Between -3 dB and -5 dB is the nominal range of compression for this situation; once that is reflected on the meter it’s time to move to the output stage.
Once the preamp and compressor are set up, I can start to play my bass line and begin monitoring it on the VU meter of the LA-610 MkII. I have the meter set to OUTPUT so I can view the final output stage of the unit. When OUTPUT is selected, a meter reading of "0" corresponds to a level of +4 dBu at the rear panel line output jack. This is a standard line level signal and it is what we are after.
Note: A complete definition of "line level signal," as well as step-by-step instructions for the setup of the LA-610 MkII Channel Strip can be found in the April, 2009, "Support Report."
A strong signal that peaks at or slightly above "0" on the VU (Example 1) is ideal. Now that I have a strong signal outputting from the LA-610 MkII it’s time to look at the next step in the chain, the Universal Audio 2192 Master Digital Audio Interface. As stated earlier, I have a +4dBu line level signal outputting from the channel strip. When I run that into the analog input of the 2192 and check the analog input meter I am seeing a level of -18 dB (Example 2)! This is where many of the questions arise.
Digital metering is registered in dB "full scale"—written as dBFS. A value of 0 dB on the 2192 analog input meter is equal to digital "full scale" code (0 dBFS), which reflects an analog signal level of +22 dB. This is why the +4dBu signal registers at -18 dBFS. The extra headroom is provided on the digital side to avoid digital clipping, which creates undesirable effects in the sonic quality of a track. This is especially important in the case of fast-moving transients, which can elude your compressor and push your signal out of manageable range. UA calibrates the 2192 to provide 18 dB of headroom in the transition region from nominal to clipping.
The tricky part of understanding the relationship between analog and digital metering is that not all digital meters are calibrated to the same value. These meters exhibit a range from product to product of 0 VU @ +4 dBu = -14, -16, -18, -20 dBFS. Due to this variance, there may be discrepancies between the digital meters on the converter, the interface, and in the DAW, even though the signal remains at the same level throughout. For example, when running S/PDIF out of my 2192 into my FireWire interface and then on to my laptop, I see a level of -16 dBFS (Example 3). It appears that I am experiencing a mysterious 2dB increase in gain, when it’s actually attributed to the difference in calibration.
That wraps up this "Support Report." Turns out that the path from analog to digital isn’t so mysterious after all. Once familiar with the calibration specs of the digital unit—generally listed in its owner’s manual—it’s smooth sailing.