SAA7724H Datasheet, PDF (25/84 Pages) NXP Semiconductors – Car radio digital signal processor

Philips Semiconductors
Car radio digital signal processor
Preliminary specification
SAA7724H
6.3 AD decimation paths (DAD)
The AD decimation paths for both the level and audio are
achieved in the DAD block; (see Fig.12). There are two
DAD blocks implemented for the SAA7724H.
The DAD block consists of a Level Decimation Filter (LDF)
which handles the AUX decimation and an Audio
Decimation Filter (ADF) which handles the AUDIO
decimation.
handbook, full pagewidth
LDF
1-BIT CODE
FILTER
aux ( n ) _sel_lev_voice
1-BIT CODE
FILTER
ADF
CEAD
INTERFACE
CEAD
BLOCK
CONTROLLER
MGW202
ch ( n ) _dc_offset
(n) is 1 or 2.
Fig.12 DAD block diagram.
6.3.1 LDF AND AUX DECIMATION PATH
The input signal has a sample frequency of 128 × f_s and
comes from a 1st-order ADC. The first part of the
decimation is done using a CIC filter. For the AUX
decimating filter a 2nd-order CIC filter is implemented.
A branch is also available from this filter for a signal having
a sample frequency of 8 × f_s. This signal also passes a
built-in high-pass filter section to make it adequate for level
IAC detection purposes. With a sampling frequency of
8 × 42.1875 kHz the −3 dB point of this filter is at
approximately 60 kHz.
The CIC filter decimates the sample frequency by 64. The
new output sample rate is 2 × f_s. The -s---i-xn----x-- roll-off of the
CIC filter needs to be compensated for, therefore, a roll-off
compensation filter is utilized.
The last stage of the AUX decimation filter is the realization
of the appropriate bandwidth characteristic. The bits
aux1_sel_lev_voice and aux2_sel_lev_voice selects
between the level characteristic and the audio
characteristic for voice input.
The transfer characteristics, level and audio, of the AUX
decimation filter are illustrated in Fig.13. It should be noted
that the figure corresponds with a 38 kHz sample rate. For
the SAA7724H a 42.1875 kHz sample rate is used, the
horizontal values need to be scaled with a factor of
4----2---.-3-1--8-8----7---5--
Remark: The absolute gain or attenuation of the graphs in
Fig.13 has no meaning. The relative levels however have.
When bit aux1_sel_lev_voice or aux2_sel_lev_voice is
logic 1, the coefficient for audio processing is active.
When bit aux1_sel_lev_voice or aux2_sel_lev_voice is
logic 0, the coefficient for level processing is selected.
2003 Nov 18
25