SAA2502 データシートの表示（PDF） - Philips Electronics

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SAA2502

ISO/MPEG Audio Source Decoder

Philips Electronics 

Philips Semiconductors

ISO/MPEG Audio Source Decoder

Preliminary specification

SAA2502

7.4.8 BASEBAND AUDIO PROCESSING

Baseband audio de-emphasis as indicated in the MPEG

input data stream is performed digitally inside the

SAA2502. The included ‘Audio Processing Unit’ (APU)

see Fig.19, may be used to apply programmable

inter-channel crosstalk or independent channel volume

control.

The APU attenuation coefficients LL, LR, RL and RR may

be changed dynamically by the microcontroller, writing

their 8-bit indices to the SAA2502 through its control

interface. The coefficient changes become effective within

one sample period after writing. To avoid audible clicks at

coefficient changes, the transition from the current

attenuation to the next is smoothed. The relationship

between the APU coefficient index and the actual

coefficient (i.e. the gain) is shown in Fig.14 and in Table 9

For coefficient index 0 to 64 the step size is −3⁄16 dB and

for coefficient index 64 to 255 the step size is −3⁄8 dB.

The APU has no built-in overflow protection, so the

application must assure that the output signals of the APU

cannot exceed the 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 a fixed

sequence LL, LR, RL and RR. Therefore, to prevent

(temporary) internal APU data overflow, the following

sequence of steps may be necessary:

1. Write LL, LR, RL and RR, but change only decreasing

coefficients. Overwrite increasing coefficients with

their old value (therefore do not change these yet).

2. Write LL, LR, RL and RR again, but now change

increasing coefficients, keeping the other ones

unchanged.

Table 9 APU coefficient index and actual coefficient.

APU COEFFICIENT INDEX C

BINARY

00000000 to 00111111

DECIMAL

0 to 63

01000000 to 11111110 64 to 254

11111111

255

APU

COEFFICIENT

2–3--C--2--

2–--(--C-----1-–--6--3----2---)--

0

handboolke,fht adlfepcaogeded

audio

samples

right decoded

audio

samples

LL

LR

RL

RR

left output

audio

samples

right output

audio

samples

MGB493

Fig.13 Audio Processing Unit (APU).

handbook, h0a0lfpage

64

APU coefficient index

254 255

−12

(1)

(2)

gain

(dB)

−83.25

(1) Step −3⁄16 dB per coefficient increment.

(2) Step −3⁄8 dB per coefficient increment.

MGE480

Fig.14 Relation between APU coefficient index and

gain.

1997 Nov 17

21

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