Philips CD721 - Page 28

3.4.3. Audio functions
De-emphasis and phase linearity
When de-emphasis is detected in the Q-channel subco-
de, the digital filter automatically includes a de-emphasis
filter section.
Digital oversampling filter
The SAA7345 contains a 2-4 times oversampling IIR
filter. This means that the impulse response of this filter
is infinite.
Concealment
A 1-sample linear interpolator becomes active if a single
sample is flagged as erroneous but cannot be corrected.
The erroneous sample is replaced by a level midway
between the preceding and following.samples. Left and
right channels have independent interpolators.
Hold
J\
f
Interpolation
OK OK Error
Error
Error
When de-emphasis is not required, a phase compensati-
on filter section controls the phase linearity of the digital
oversampling filter to < + 1° within the band 0 to 16 kHz.
The advantage of using an IIR filter is that it is easy and
cheap to implement on a chip. The main disadvantage is
that is inherently not phase linear.
if more than one consecutive uncorrectable sample is
found, the last good sample is held.
A 1-sample linear interpolation is then performed before
the next good sample.
Interpolation
Error OK OK
Conceaiment mechanism
Mute, Attenuation and Fade
A digital level controller is present on the SAA7345 which
performs the functions of soft mute, attenuation and fade.
All these functions are software programmable.
Soft mute is activated by sending the Mute command to
the fade control register, attenuation(-12dB) is activated
by sending the Attenuate command. Attenuation and
mute are cancelled by sending the full scale command.
The audio output is determined by the value of the inter-
nal fade counter.
Level = counter x maximum level
128
The counter is preset to 128 by the Full scale command
if no oversampling is required.
The counter is preset to 120(-0.5dB scaling) by the Full
scale command if either 2 or 4 fs oversampling is ne
grammed in the DAC output register.
The counter is preset to 32 by the Attenuate command.
The counter is preset to 0 by the Mute command.
To control the fade counter in a continupus way, the
step-up and step-down commands are available. They
will increment or decrement the counter by 1 for each
register write operation.
PCS 60 961