Cirrus Logic CS4244 - Manual (Page 39)

DS900F1 39
CS4244
4.6.5 Volume Control
The CS4244 includes a volume control for the DAC1-4 signal path. The implementation details for the vol-
ume control and other associated peripheries for DAC1-4 is shown in
Figure 29 below. Digital volume
steps, adjustable noise gating, muting, and soft ramping are provided on each DAC channel.
4.6.5.1 Mute Behavior
Each DAC channel volume is controlled by the sum (in dB) of the individual channel volume and the mas-
ter volume registers. The channel and master volume control registers have a range of +6 dB to -90 dB
with a nominal resolution of
6.02
/
16
dB per each bit, which is approximately 0.4 dB. The sum of the two
volume settings is limited to a range of +6 dB to -90 dB. Any volume setting below this range will result in
infinite attenuation thus muting the channel.
A DAC channel may alternatively be muted by using the mute register bits, the power down bits, or the
Noise Gate feature. For any case when the mute engages (volume is less than -90 dB, power down bit is
set, mute bit is set, or Noise Gate is engaged), the CS4244 will mute the channel immediately or soft-ramp
the volume down at a rate specified by the
MUTE DELAY[1:0] bits depending on the settings of the DAC1-
4 ATT. bit in the "DAC Control 3" register . This behavior also applies when unmuting a channel.
4.6.5.2 Soft Ramp
The CS4244 soft ramp feature (enabled using the DAC1-4 ATT. bit) is activated on mute and unmute tran-
sitions as well as any normal volume register changes. To avoid any potential audible artifacts due to the
soft ramping, the volume control algorithm implements the ramping function differently based upon how
the user attempts to control the volume.
If the user changes the volume in distant discrete steps such as what would happen if a button were
pressed on a user interface to temporarily add attenuation to or mute a channel, then the volume is
ramped from the current setting to the new setting at a constant rate set by the
MUTE DELAY[1:0] bits.
Alternatively, if the user controls the volume through a knob or slider interface, a volume envelope is sam-
pled at a slow, not-necessarily uniform rate (typically 1-20 Hz) and sent to the CS4244. In this case the
ramping algorithm detects a short succession of volume changes attempting to track the volume envelope
and dynamically adjusts the soft-ramp rate.
If the CS4244 were to use a constant ramp rate between the volume changes it receives, its output volume
envelope may either lag behind the user-generated envelope if the ramp rate is set too low (possibly not
reaching the peaks and dulling the envelope) or the output volume envelope may cause a stair-case effect
resulting in audible zipper noise if the ramp rate is set too high. By instead adapting the soft-ramp rate to
fit the envelope given by the incoming volume samples, the envelope lag time is limited and the zipper
1
0
Noise Gate
Soft Ramp 0
1
x
DACx Data
+
DACx Volume
Register Setting
Master Volume
Register Setting
Interpolation
Filter
Modulator DAC
Limiter
(+6 to -90 dB)
INV DACx
DAC1-4 Noise
Gate Threshold
MUTE DACx
DAC1-4 ATT
AOUTx
Figure 29. Volume Implementation for the DAC1-4 Path