4 oversampling modes, 5 interpolation filter, 6 de-emphasis – Cirrus Logic CS4384 User Manual
Page 25: Figure 20. de-emphasis curve, Figure 20, Cs4384
DS620F1
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CS4384
4.4
Oversampling Modes
The CS4384 operates in one of three oversampling modes based on the input sample rate. Mode selection
is determined by the M4, M3 and M2 pins in Hardware Mode or the FM bits in Software Mode. Single-Speed
Mode supports input sample rates up to 50 kHz and uses a 128x oversampling ratio. Double-Speed Mode
supports input sample rates up to 100 kHz and uses an oversampling ratio of 64x. Quad-Speed Mode sup-
ports input sample rates up to 200 kHz and uses an oversampling ratio of 32x.
The auto speed-mode detect feature allows for the automatic selection of speed mode based off of the in-
coming sample rate. This allows the CS4384 to accept a wide range of sample rates with no external inter-
vention necessary. The auto speed-mode detect feature is available in both Hardware and Software Mode.
4.5
Interpolation Filter
To accommodate the increasingly complex requirements of digital audio systems, the CS4384 incorporates
selectable interpolation filters for each mode of operation. A “fast” and a “slow” roll-off filter is available in
each of Single-, Double-, or Quad-Speed Modes. These filters have been designed to accommodate a va-
riety of musical tastes and styles. The FILT_SEL bit is used to select which filter is used (see the
for more details).
When in Hardware Mode, only the “fast” roll-off filter is available.
Filter specifications can be found in
, and filter response plots can be found in
to
.
4.6
De-Emphasis
The CS4384 includes on-chip digital de-emphasis filters. The de-emphasis feature is included to accommo-
date older audio recordings that utilize pre-emphasis equalization as a means of noise reduction.
shows the de-emphasis curve. The frequency response of the de-emphasis curve will scale proportionally
with changes in sample rate, Fs if the input sample rate does not match the coefficient which has been se-
lected.
In Software Mode the required de-emphasis filter coefficients for 32 kHz, 44.1 kHz, or 48 kHz are selected
via the de-emphasis control bits.
In Hardware Mode only the 44.1 kHz coefficient is available (enabled through the M2 pin). If the input sam-
ple rate is not 44.1 kHz and de-emphasis has been selected then the corner frequencies of the de-emphasis
filter will be scaled by a factor of the actual Fs over 44,100.
Gain
dB
-10dB
0dB
Frequency
T2 = 15 µs
T1=50 µs
F1
F2
3.183 kHz
10.61 kHz
Figure 20. De-Emphasis Curve