3 powered-down mixer inputs, 4 avoiding mixer clipping, Cs42l73 – Cirrus Logic CS42L73 User Manual
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DS882F1
CS42L73
4.11.3 Powered-Down Mixer Inputs
If an input to the digital mixer is powered down (refer to register controls
“Power Control 1 (Address 06h)”
and
“Power Control 2 (Address 07h)” on page 83
), that input must be muted. The CS42L73
does not automatically mute mixer inputs that are powered down. If a mixer input is not to be used and is
not muted upstream, set the input’s attenuation to mute.
To minimize audio disturbances, it is recommended that the mute on the mixer input (that is to be powered
down) be applied (at the mixer or upstream) using a soft ramp and that the power-down only occur after
the attenuation has ramped fully to mute.
4.11.4 Avoiding Mixer Clipping
Digital mixers are essentially adders. As such, when more than one input is fed into a mixer the potential
for overflow exists, depending on the bit word length of the inputs and the mixer and depending on the
input value range. For example, if two full-range, signed 4-bit channels were mixed to a signed 4-bit result,
whenever the sum of the two inputs falls outside the -8 to +7 range, the hypothetical mixer would overflow
causing undesired output signal distortion (wrapping).
No mixers within CS42L73’s digital mixer are susceptible to overflow because they all have a sufficient
number of accumulator bits. If any mixer’s result exceeds the bit width of the signal data path, the result
is forced either to the full-scale maximum or the minimum value, which ensures the signal is clipped vs.
being distorted (by the wrapping effect of truncating the accumulator result to fit into the data path width).
Attention is required to ensure clipping does not occur within the digital mixer. Of course, if the digital mix-
er is fed a signal that was clipped elsewhere, its output reflects that external clipping.
The three mixers in
that provide mono versions of input stereo channels (the Input Path, XSP,
and ASP inputs) are impervious to clipping by design. They have -6 dB of attenuation applied to their in-
puts (see
“Mixer Attenuation Values” on page 65
). Mathematically this amounts to InputA/2 + InputB/2,
which illustrates that, given the input and mixer output bit widths are the same, the result can never clip.
Refer to the mixers on the lower-right side of
that are used to provide mono versions of XSP
and VSP output channels. They rely on the input attenuation settings of the stereo mixers that feed them
to avoid clipping. If the XSP or VSP output is configured as mono, the user must program the sourcing
stereo mixer’s attenuators to provide mixer outputs that are at least 6 dB down from full scale. This will
prevent mixer-caused clipping of the signals that are sent to the XSP and VSP.
All the other mixers are susceptible to clipping. For these mixers, the recommended minimum premixer
attenuation level settings (refer to
“Mixer Attenuation Values” on page 65
) to avoid mixer clipping are pro-
vided in
For this table, full-scale inputs are assumed (no preattenuation) and that there is no relative volume adjust-
ment between inputs. If any inputs are at less than full scale, less attenuation can be set while still avoiding
mixer clipping. If there is to be a relative volume adjustment between the inputs, less attenuation can be set
for one or more inputs so long as the other input(s) are attenuated sufficiently to avoid clipping (e.g., with
three full-scale inputs, one input could be attenuated by 6 dB, if the other two are attenuated by 12 dB).
Table 12. Digital Mixer Nonclipping Attenuation Settings
Number of Active
Channels into Mixer
Max Signal Strength
Allowed per Input
Minimum Attenuation (dB)
Setting Allowed per Input
1
1
0
2
1/2
6
3
1/3
10
4
1/4
12