WM8770
Production Data
AUDIO DATA SAMPLING RATES
In a typical digital audio system there is only one central clock source producing a reference clock to
which all audio data processing is synchronised. This clock is often referred to as the audio system’s
Master Clock. The external master system clock can be applied directly through the MCLK input pin
with no software configuration necessary. In a system where there are a number of possible sources
for the reference clock it is recommended that the clock source with the lowest jitter be used to
optimise the performance of the ADC and DAC.
The master clock for WM8770 supports DAC and ADC audio sampling rates from 256fs to 768fs,
where fs is the audio sampling frequency (DACLRC or ADCLRC) typically 32kHz, 44.1kHz, 48kHz or
96kHz (the DAC also supports operation at 128fs and 192fs and 192kHz sample rate). The master
clock is used to operate the digital filters and the noise shaping circuits.
In Slave mode the WM8770 has a master detection circuit that automatically determines the
relationship between the master clock frequency and the sampling rate (to within +/- 32 system
clocks). If there is a greater than 32 clocks error the interface is disabled and maintains the output
level at the last sample. The master clock must be synchronised with ADCLRC/DACLRC, although
the WM8770 is tolerant of phase variations or jitter on this clock. Table 6 shows the typical master
clock frequency inputs for the WM8770.
The signal processing for the WM8770 typically operates at an oversampling rate of 128fs for both
ADC and DAC. The exception to this for the DAC is for operation with a 128/192fs system clock, e.g.
for 192KHz operation where the oversampling rate is 64fs. For ADC operation at 96kHz it is
recommended that the user set the ADCOSR bit. This changes the ADC signal processing
oversample rate to 64fs.
SAMPLING
RATE
System Clock Frequency (MHz)
128fs
192fs
256fs
384fs
512fs
768fs
(DACLRC/
ADCLRC)
DAC ONLY
32kHz
44.1kHz
48kHz
4.096
5.6448
6.144
6.144
8.467
8.192
11.2896
12.288
24.576
12.288
16.9340
18.432
36.864
16.384
22.5792
24.576
24.576
33.8688
36.864
9.216
96kHz
12.288
24.576
18.432
36.864
Unavailable Unavailable
192kHz
Unavailable Unavailable Unavailable Unavailable
Table 6 System Clock Frequencies Versus Sampling Rate
In Master mode BCLK, DACLRC and ADCLRC are generated by the WM8770. The frequencies of
ADCLRC and DACLRC are set by setting the required ratio of MCLK to DACLRC and ADCLRC using
the DACRATE and ADCRATE control bits (Table 7).
ADCRATE[2:0]/
DACRATE[2:0]
MCLK:ADCLRC/DACLRC
RATIO
000
001
010
011
100
101
128fs (DAC Only)
192fs (DAC Only)
256fs
384fs
512fs
768fs
Table 7 Master Mode MCLK: ADCLRC/DACLRC Ratio Select
PD Rev 4.1 June 2005
18
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
