SAA2501 Datasheet, PDF (13/52 Pages) NXP Semiconductors – Digital Audio Broadcast DAB decoder

Philips Semiconductors
Digital Audio Broadcast (DAB) decoder
Preliminary specification
SAA2501
Table 5 APU coefficient index and actual coefficient
APU COEFFICIENT INDEX C
BINARY
00000000 to 00111111
DECIMAL
0 to 63
01000000 to 01111110 64 to 126
01111111
1XXXXXXX
127
128 to 255
APU
COEFFICIENT
2^{–1--C--2--}
–
2
C------–_{-6----3---2--}
0
reserved
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audio
samples
right decoded
audio
samples
LL
LR
RL
RR
left output
audio
samples
right output
audio
samples
MGB493
From Table 5 we learned that up to coefficient index 64 the
step size is approximately −0.5 dB per coefficient
increment, and from coefficient index 64 to index 126 the
step size is approximately −1 dB per increment.
Note that the APU has no built-in overflow protection, so
the application must take care that the output signals of the
APU cannot exceed 0 dB level. For an update of the APU
coefficients, it may be required to increase some of the
coefficients and decrease some others. The APU
coefficients are always written sequentially in the fixed
sequence LL, LR, RL and RR. Therefore, to prevent
internal APU data overflow due to non-simultaneous
coefficient updating, the following steps can be followed:
1. Write LL, LR, RL, RR once, but change only those
coefficients that must decrease; overwrite the
coefficients that must increase with their old value (so
do not change these yet).
2. Write LL, LR, RL, RR again, but now change those
coefficients that must increase, keeping the other
coefficients unchanged.
The consequence of this two-pass coefficient updating is
that the application must keep a shadow of the current
APU coefficients (the L3 APU coefficients data item is
write-only).
Fig.5 Audio Processing Unit (APU).
handbook, full pagewidth
0
0
APU coefficient index
64
126 127
32
(2)
(1)
gain
(dB)
94
(1) Step −0.5 dB per coefficient increment.
(2) Step −1 dB per coefficient increment.
MGB494
Fig.6 Relation between APU coefficient index and gain.
January 1995
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