User manual
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Gain Structure
Gain structure is among the most important, often debated and misunderstood topics in the audio
industry. For years, engineers have disagreed with each other about proper levels, gain I/O,
meters and other issues that affect the way we work. When digital audio came into the
mainstream, the topic’s confusion was only compounded, and today, few fully understand the
inner workings of gain structure. This is true within MediaMatrix® as well. Although this manual
does not present an exhaustive discussion on the topic, it is important to understand the basics of
gain structure, and how it relates specifically to the CAB 4n.
In most cases the CAB 4n is shipped from the factory with “unity” gain through the unit. In other
words, if you inject a 1.23VAC 1 kHz sine wave into the input card of a CAB 4n without
adjusting any controls, you will get the same 1.23VAC sine wave on an output card when the
signal paths are directly connected and without audio processing. It is important to note, however,
that pending the modular card installed unity gain settings could differ and should be checked
before testing the unit. Because the CAB 4n has such flexible capability it is primarily intended
for larger MediaMatrix systems where the gain structure may be required to be set up differently.
For more information on gain structure with the Cab 4n and MediaMatrix systems, please refer to
the relevant User’s Manuals. Adjusting the gain structure for the CAB 4n is possible both at the
analog and digital gain stages. All controls for adjusting the input and output gain stages are
found in the respective Inputs and Outputs NWare Block Tabs for CAB devices.
Setting Full Scale Input and Output Sensitivity
To fully understand how MediaMatrix® gain structure works, you should have a clear, high level
view of how the Full-Scale Input Sensitivity and Full-Scale Output Drive settings relate to the
front panel and software-based audio meters. First and foremost, it is important to understand
what the levels mean. The Full-Scale Input Sensitivity and Full-Scale Output Drive settings refer
to separate analog circuits within the MM Modular Cards loaded into the CAB 4n. These circuits
are in addition to the fully adjustable analog input and output gain stages. Both the Full-Scale
Input Sensitivity and Full-Scale Output feature four user-definable settings. These settings
represent MAXIMUM (Full-Scale) levels, NOT nominal settings. Keep this in mind as we
progress. In a digital system, 0 dB indicates the “top” of the useable dynamic range, just below
full-scale. For the inputs, full-scale indicates the maximum level BEFORE converter clipping. On
the front panel input meter, 0 dB has been calibrated to be <1 dB below full-scale. Anything over
this level is clipping, or in digital parlance, an “OVER”. Remember that the front panel input
meter is in the digital domain, just after the analog to digital (A/D) converter. The position of the
Full-Scale Input Sensitivity switch determines how much analog input signal is required to get the
front panel input meter to read 0 dB. Or, in other words, to get the A/D converter to full-scale
maximum input level.
The CAB 4n typically provides greater than 105 dB of dynamic range. This is the range of 100%,
clean audio above the published noise floor, and below the threshold of converter clipping (Full-
Scale). The adjustable sensitivity switches simply “shift” this dynamic range upwards or
downwards before the signal gets into the MediaMatrix DPU. This is done primarily to
accommodate the connection of third party components and to match source output levels. The
setting of the Full-Scale Input Sensitivity will determine what level is required to drive the A/D
converter to full-scale and illuminate the 0 dB front panel input meter. For example, if +18 dBu is