2.0 Functional Description
Fusion 878A
2.15 Audio A/D
PCI Video Decoder
2-50
Conexant
100600B
2.15 Audio A/D
2.15.1 Muxing and Anti-aliasing Filtering
Before entering the audio A/D, the TV, FM, and microphone/line audio inputs are
selected by A_SEL and multiplexed. The MUX selects are break-before-make. If
A_SEL is set to 3, no mux is enabled. Thus the SMXC pin can be used as a direct
connect to the pre-amp (bypass MUX) if only one analog input is required. Refer
to
0x10C—Audio Control Register (GPIO_DMA_CTL)
in
for register
information.
The SMXC pad leads directly to the single-ended differential converter. The
resistive load seen by the audio inputs is approximately 20 k
Ω.
2.15.2 Input Gain Control
The audio frequency (AF) output level from the TV tuners ranges from 250
mV
RMS
to 750 mV
RMS
, typically riding on a 2 VDC offset. If the A/D nominal
operating point is 0.5 V
RMS
(1.414 V
p-p
), then the input gain needs to vary from
–3.5 dB to +6.0 dB.
The input signal is gained in discrete linear steps via A_GAIN[3:0].
shows the calculated gain values. The A_GAIN value is set in
0x10C—Audio Control Register (GPIO_DMA_CTL)
Table 2-12. Gain Control
(1 of 2)
A_GAIN
Input
GAIN
dB
Nominal
Input V
rms
V
p-p
0
0.500
–6.02
1.000
2.828
1
0.667
–3.52
0.750
2.121
2
0.833
–1.58
0.600
1.697
3
1.000
0.00
0.500
1.414
4
1.167
1.34
0.429
1.212
5
1.333
2.50
0.375
1.061
6
1.500
3.52
0.333
0.943
7
1.667
4.44
0.300
0.849
8
1.833
5.26
0.273
0.771
9
2.000
6.02
0.250
0.707
10
2.167
6.72
0.231
0.653
11
2.333
7.36
0.214
0.606
12
2.500
7.96
0.200
0.566
