May 1994

17

Philips Semiconductors

Preliminary specification

Stereo filter and codec

SAA2003

Audio fade processor

The fade processor is controlled by the system microcontroller. It achieves level control, or fading, by multiplying the

audio samples with a 17 bit accuracy fade coefficient, which is selected by an 8-bit fade counter. The fade coefficients

⁄

where: 0

≤ FC ≤ 255.

In encode mode, audio samples are taken from input SD1 and scaled before sub-band filter processing, and sent to

output SD2.

In decode mode, audio samples are scaled following reconstruction by the sub-band filter, and sent to outputs SD1

and SD2.

Table 10 Fade processor operating modes.

The operating mode of the fade processor is controlled by two extended registers

Table 11 Fade processor mode control.

The fade rate can be set to either fast or slow modes. In fast mode the attenuation changes rate at one step per audio

sample. In slow mode the rate of change of level is controlled by the fade rate bits P3 to P0. In slow mode, the fade

counter is stepped up or down according to a clock derived from the WS pin.

MODE

FUNCTION

Fade rate

controls rate of automatic increments and decrements

Step down

increases attenuation by one increment

Step up

reduces attenuation by one increment

Full scale

sets gain to unity, incrementing from current level automatically

Mute

sets gain to zero, decrementing from current level automatically

−12 dB

sets gain to

−12 dB, decrementing or incrementing from current level automatically

A3

A2

A1

A0

D3

D2

D1

D0

MODE

0011

P3

P2

P1

P0

set fade rate

01000001

step down

01000010

step up

010001

X

0

full scale slow

010001

X

1

full scale fast

010010

X

0

mute slow

010010

X

1

mute fast

010011

X

0

−12 dB slow

010011

X

1

−12 dB fast

01000000

no action

Attenuation (dB)

20 log cos

–

π FC

×

510

------------------





dB

()

=