SAA7712H Datasheet, PDF (11/44 Pages) NXP Semiconductors – Sound effects DSP

Philips Semiconductors
Sound effects DSP
Preliminary specification
SAA7712H
8 FUNCTIONAL DESCRIPTION
8.1 Analog outputs
8.1.1 ANALOG OUTPUT CIRCUIT
Depending on the configuration of the equalizer sections,
the SAA7712H has 2 or 4 analog outputs which are
supplied by the same power supply. Each of these outputs
has a voltage and a current pin (see Fig.3). The signals are
available on 2 outputs (OUT0 and OUT1), or 4 outputs
(OUT0, OUT1, OUT2 and OUT3).
handbook, halfpage
OUT0_I
(OUT1_I)
BIT 0 to 13
MSB
Vref
DAC
MGS208
OUT0_V
(OUT1_V)
Fig.3 Analog output circuit.
8.1.3 DACS
Each of the four low noise high dynamic range DACs
consists of a signed-magnitude DAC with current output,
followed by a buffer operational amplifier.
8.1.4 UPSAMPLE FILTER
To reduce spectral components above the audio band, a
fixed 4 times oversampling and interpolating digital filter is
used. The filters give an out-of-audio-band attenuation of
at least 29 dB. The filter is followed by a first-order noise
shaper to expand the dynamic range to more than 105 dB.
The band around multiples of the sample frequency of the
DAC (4f_s) is not affected by the digital filter. A capacitor
must be added in parallel with the DAC output amplifier to
attenuate this out-of-band noise further to an acceptable
level.
In Fig.4 the overall frequency spectrum at the DAC audio
output without external capacitor or low-pass filter for the
audio sampling frequencies of 38 kHz is shown. In Fig.5
the detailed spectrum around f_s is shown for an f_s of
38, 44.1 and 48 kHz. The pass band bandwidth (−3 dB) is
1_⁄2fs.
8.1.2 DAC FREQUENCY
The sample rate (f_s) of the selected source is the frame
rate of the DSP. The word clock for the upsample filter and
the clock for the DACs, at 4f_s, are derived internally from
the word select of the selected audio source.
1999 Aug 05
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