User manual

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selected, then it will take +18 dBu of level to get to a 0 dB reading (full-scale A/D) on the front
panel input meter.
The setting in the Full-Scale Input Sensitivity dialog box will determine how the front panel input
meters display audio levels coming into the CAB 4n. It is very important that these switches be
set correctly. For example, if you know that the nominal levels of your system are going to be
around 0 dBu, and you want to monitor peak levels 12 dB above that, then you will want to use
the +18 dBu setting for the Full-Scale Input Sensitivity. This will allow the front panel meter to
display +18 dBu at the 0 dB LED. Your 12 dB headroom level in this case would show up at
approximately -6 dB on the meter. In order to get the maximum performance from the analog
input modules of the CAB 4n, it is recommended that your Full-Scale Input Sensitivity settings
be properly matched to the audio source and your headroom requirements. What we are looking
at so far is front panel metering. Remember this.....the setting of the Full-Scale Input Sensitivity
switch will always equal the 0 dB LED on the front panel input meter. This is the full-scale
(maximum) level before A/D converter clipping. So, how does this relate to the meters inside of
MediaMatrix? Read on.....
Inside the DPU audio levels are represented by software based meters that can be placed into the
signal flow at any stage. They can also be found within certain devices and can be configured as
input or output meters. These meters include a bar-graph display for quick visual reference.
However, the more accurate way to monitor levels is by using the small text display included
with each meter. This display will give you exact readings with much finer resolution than the
bar-graph display. It is very important to understand that the software meters display actual levels
inside the digital domain (DPU). Even though the front panel meters are in the digital domain,
they can only display the digital signal immediately after the A/D and before the D/A converters.
In the view file, this same signal shows up at the CAB 4n input and/or output blocks/tabs. Once
the signal is processed, split or distributed by other audio devices, the only way to accurately
meter the signal is by placing software meters within the view file signal flow. The relationship of
the hardware and software meters is shown in the following illustration.
The key to understanding how the front panel and software meters relate is in the signal flow. The
levels at the CAB inputs and outputs of the view file are the same levels displayed on the CAB’s
hardware front panel meters, although they are calibrated differently. Once the signal is
processed, digital audio levels inside the DPU should be monitored exclusively within the view
file using software meters. This is because there is an additional 12 dB of headroom, or gain,
available exclusively within the DPU. This additional gain cannot be displayed on the front panel
meters. Before the signal leaves the DPU at the CAB’s output block, it should be attenuated back
to full-scale converter level, or lower, to avoid clipping the D/A converter. Refer to the above
example. The Full-Scale Input Sensitivity switch is set for +18 dBu and a +18 dBu signal is
coming into an input MM modular card in the Cab4n, or full-scale, on the front panel input meter.
The first software meter, placed at the view file input block/Tab, will indicate this level as +23.0
dB peak. The +23.0 dB represents the full-scale peak level, measured inside the DPU. Inside the
DPU, there is an additional 12 dB of headroom available for EQ, summing, etc. This headroom
level is represented by a 12 dB boost at the first Level control and the peak meter connected to it
indicating +35.0 dB (max DPU level). The signal is then attenuated by the second Level control
to avoid clipping the modular card in the CAB 4n and its peak meter again shows +23.0 dB. This
signal level could be output via a modular output card also installed into the CAB4n and the
associated output block/Tab, and returned to the same CAB at full-scale. The front panel output
meter will display 0 dB, regardless of the position of the Full-Scale Output Drive switches.