Philips CD721 - Page 32

5.4. Functional description
General
The TDA1549 CMOS D/A bitstream continuous calibrati-
on converter incorporates internal digital filtering which
increases the oversampling rate of 4fs input data to 96fs,
and removes the spectral data components around 4fs,
8fs and 12fs.
Input
The circuit accepts four times oversampled data in 18-bit
2’s complement standard Japanese format with MSB
first. Left and right data-channel words are time multi-
plexed.
A second-order noise shaper operating at 96fs outputs a
five-bit data bitstream to the D/A converters. The filtering
required for waveform smoothing and out-of-band noise
reduction is achieved by simple first-order analog post-
filtering. ,
»
The bit clock(BCK) operates at 192fs i.e. 48 times the
word select(WS) frequency of 4fs.
BCK JUL LL LLL
DATA (tT
LEFT CHANNEL DATA
ws
fig 5.1 : Input format
Oversampling filter
The oversampling filter consists of:
RIGHT CHANNEL DATA
eC ee
HAS1159
- A seventh order halfband low pass FIR filter which increases the oversampling rate from 4 times to 8 times. This
removes the spectral components around 4fs and 12fs.
-
A
linear
interpolation
section
which
increases
the
oversampling
rate
to
16
times.
This
removes
the
spectral
components around 8fs.
- A sample and hold section which provides another 6 times oversampling to 96 times. The zero-order hold
characteristic
of
this
sample
and
hold
section
plus
the
first
order
analog
filtering
remove
the
spectral
components
around 16fs.
The passband ripple is within 0.1dB. The stopband attenuation is >50dB around multiples of the sampling frequency.
ee
ees
0
ee
ie
eee
ene
ae
mn
0 4ts
Input spectrum(4fs)
Sts
12fs 16fs
faeces:
cco
Be
eas
See
oy
0 4ts
4fs to 8fs oversampling filter characteristic
12fs 16fs
jena oem ae es
0 4fs
8fs to 16fs oversampling filter characteristic(interpolation filter)
8fs
12fs 16fs
|
aa
eer
ee
0 4ts 8ts
16fs to 96fs sample-and-hold plus first-order postfiltering characteristic
Fig 5.2 : Filter and noise shaper characteristics
12fs 16fs
HAS1160
PCS 60 967