WM8756

Advance Information

AI Rev 1.3 October 2001

13

SYSTEM CLOCK FREQUENCY (MHZ)

SAMPLING

RATE

(LRCIN)

128fs

192fs

256fs

384fs

512fs

768fs

32kHz

4.096

6.144

8.192

12.288

16.384

24.576

44.1kHz

5.6448

8.467

11.2896

16.9340

22.5792

33.8688

48kHz

6.114

9.216

12.288

18.432

24.576

36.864

96kHz

12.288

18.432

24.576

36.864

Unavailable

Unavailable

192kHz

24.576

36.864

Unavailable

Unavailable

Unavailable

Unavailable

Table 11 System Clock Frequencies Versus Sampling Rate

DSD MODE

When pin 14, DSDB pin is held low, the device is reconfigured to operate as a DSD or ‘bitsteam’

compatible DAC. That is, the input audio data is in a sigma delta modulated form, or pulse density

modulated. In this case the only signals required are the bitstream for each channel supported, and the

oversampling clock.

WM8756 supports this mode when run at a 64x oversample rate. That is, the bitstream data is supplied at

a rate of 64 bits per normal word clock. Of course no word clock is provided, and the actual spectral

content of the data is determined by the noise shaping that was used to create the bitstream. WM8756

can support six channels of bitstream or DSD audio. Data BITSTREAMS and the 64fs clock are applied

to pins 3-9 and 10, if the DSDCLK128 pin is used. Signals applied to the PCM input pins 1,2, 45-48 are

ignored. The DSDB signal controls an internal multiplexor which switches the signals on the DSD input

pins into the DAC rather than the PCM signals.

In DSD mode operation, the entire digital filter on WM8756 is disabled, and the bitstream data is applied

directly to the multi-bit switched capacitor DAC’s in the analogue part of the device. There, rather than

operate as oversampled multi bit DACs, the DAC inputs are reconfigured to act as analogue FIR filters, so

providing both D to A conversion of the bitstream data, and analogue smoothing of the sampled waveform

with no phase distortion. Filter responses of the analogue filter that results are shown in Figure 26 - Figure

29. Note in DSD mode software controlled functions such as digital volume control and phase reversal,

are not available. The FIR filter response is designed such that by adding only a 3

post DAC filter, which may be implemented with a single op-amp, the Scarlet book specified filter

requirements may be met, saving cost over the 5

It is normally desirable to use an external analogue post-DAC filter, particularly in the case of DSD

operation due to the presence of high frequency energy as a result of the aggressive high order noise

shaping used in the creation of the modulated DSD datastream. The analogue FIR filter used in WM8756

provides useful filtering of this noise, but it may be desirable to add further post filtering using active RC

filters. Figure 26 - Figure 29 show the overall filter response of the combined DAC filter operating in DSD

mode with an external 3

PCM DIGITAL AUDIO INTERFACE

PCM audio data is applied to the internal DAC filters via the PCM Digital Audio Interface. 5 popular

interface formats are supported:

•

Left Justified mode

•

Right Justified mode

•

I

•

DSP Early mode

•

DSP Late mode

All 5 formats send the MSB first and support word lengths of 16, 20, 24 and 32 bits except that 32 bit data

is not supported in right justified mode. DIN0/1/2 and LRCIN are sampled on the rising, or falling edge of

BCKIN.

In left justified, right justified and I

inputs. Audio Data for each stereo channel is time multiplexed with LRCIN indicating whether the left or

right channel is present. LRCIN is also used as a timing reference to indicate the beginning or end of the

data words.

In left justified, right justified and I

the selected word length. LRCIN must be high for a minimum of word length BCKINs and low for a