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MDX 5.1
Basic Operation
The Ref Level parameter on the Main page sets the unity
gain point for all channels (unless gain offsets are applied),
see Fig 2. The Thresholds on the DXP pages are relative
to Ref Level, so in this particular drawing, Ref Level is set
at -12dBFS, while most DXP Thresholds are set at -16dB.
If you invoke the Defeat Threshold, gain reverts to unity for
“below radar” input levels. Defeat Threshold is relative to
DXP Threshold. In the drawing, the Defeat Threshold is set
at -20dB
Fig 2. MDX5.1 Level Diagram for different Steer and
Threshold settings.
Defeat Threshold relates to DXP Threshold which relates to
Ref Level. Limit Threshold only relates to Digital Full Scale
output level.
Note, that the lower the DXP Threshold, or the higher a
Steer setting, the more low level boost is applied. The low
level boost can be different in different channels, and even
in different frequency bands.
Also observe that the Limiter threshold setting is not
relative to Ref Level, but always referenced to output full
scale.
Reading the Gain Meters
Gain meters in MDX5.1 indicate absolute gain. The upper
segments of a meter gives an indication of the boost and
frequency response applied to low level signals, while
the lower segments of a meter gives an indication of the
current (dynamic) gain and frequency response, see Fig 3.
In this example, low level signals are subject to a 5dB
boost in the Low and Hi band. The Low frequency band is
currently attenuated by 2dB, while the Mid and Hi bands
are at 0dB gain.
Fig 3. Example of MDX5.1 Gain
Meter.The meter shows max low
level gain and spectral response,
plus current gain and spectral
response.In the example, the Low
band is currently attenuated by 2 dB,
while Mid and Hi bands are at unity
gain (0 dB).
User Tips
At the beginning of a session, it can save time to set an
appropriate difference between the Ref Level parameter
and the Limit Threshold.
Wide dynamic range material for a high resolution delivery
should start with a substantial difference between the two,
for instance 15dB or more.
If the delivery resolution is low, the difference should
also start smaller. For heavily data reduced multichannel
broadcast using Dolby AC3, best results are typically
obtained with a 6-10dB difference between Ref Level and
the Limit Threshold. With less data reduction, such as DTS
codecs, or linear 16 bit formats, the best sound is normally
achieved with a difference of 8-15dB.
When significant data reduction is to be used, also be
careful not to allow peaks going all the way to 0dBFS.
Consider bringing down the Limit Threshold between 1 and
4dB. Judge the quality of loud, spacious material passing
through MDX5.1 plus data reduction plus decoding, while
listening to the output of the data reduction decoder. Pay
special attention to transient distortion, and if the sound
image collapses at high levels.
In general, and especially for music mastering, start by
processing all channels by the same amount. This can
be achieved by assigning all channels to Sidechain 1, or
by using different sidechains with identical settings. Then
conclude if speech in the center channel, ambience in the
surrounds or activity in the LFE channel etc. needs special
attention and processing.
