SAA2500H 查看數據表（PDF） - Philips Electronics

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SAA2500H

MPEG Audio Source Decoder

Philips Electronics 

Philips Semiconductors

MPEG Audio Source Decoder

Preliminary specification

SAA2500

When using the master input with FSCLKM = 0, the

SAA2500 selects the active sample rate autonomously,

and generates the signals SCK and WS from its crystal

clocks. After a device reset, the SAA2500 selects a sample

rate of 44.1 kHz by default.

SCK and WS may, and will only, show phase or frequency

changes in any of the following 3 situations:

1. When the SAA2500 establishes synchronization with

the coded data input bitstream.

2. When the active input interface is changed from the

master input with FSCLKM = 0 to the slave input (i.e.

the timing source for the generation of SCK and WS is

switched from the crystal clocks to FSCLKIN).

3. When the active input interface is changed from the

slave input to the master input with FSCLKM = 0 (i.e.

the timing source for the generation of SCK and WS is

switched from FSCLKIN to the crystal clocks); the

sample rate is set to the last selected sample rate that

was used with the master input (the last selected

sample rate is memorised while using the slave input).

In all other cases, SCK and WS keep on running without

phase or frequency changes, and the sample rate

selection remains unchanged.

Handling of errors in the coded input data

The SAA2500 can handle errors in the input data. Errors

are assumed to be present in 3 cases:

1. If errors are indicated with the coded input data error

flag CDSEF and/or CDMEF.

2. On CRC failure if ISO/MPEG error protection is active.

3. If input bitstream syntax errors are detected.

Errors in the input data have an effect on the decoding

process if the corrupted data is inside the header, bit

allocation or scale factor select information field in a frame

(then the SAA2500 will mute) or inside the scale factor field

(then the previous scale factor will be copied). Errors in

other data fields are not handled explicitly. If the

ISO/MPEG CRC is active, only the CRC result is

interpreted: CDSEF/CDMEF un-reliability indications for

bit allocation and scale factor select information are

neglected.

In applications where the ISO/MPEG CRC is always

present, the protection bit (which itself is not protected) in

the ISO/MPEG header may be overruled by making L3

settings flag CRCACT HIGH. In this manner, the SAA2500

is made robust for data errors on the protection bit.

Subband filter signals

The decoded subband signals are output, so that they can

be processed. The optionally processed subband signals

are put back into the SAA2500 for synthesis filtering.

Baseband audio processing

The baseband audio de-emphasis as indicated in the

ISO/MPEG input data is performed digitally inside the

SAA2500. The incorporated 'Audio Processing Unit'

(see Fig.5) can be used to apply inter-channel crosstalk or

independent volume control per channel. The APU

attenuation coefficients LL, LR, RL and RR may be

changed dynamically by the host microcontroller, writing

their 8 bit indices to the SAA2500 over the L3 control bus.

The coefficient changes become effective within one

sample period after the coefficient index writing.

To avoid clicks at coefficient changes, the transition from

the current attenuation to the next is smoothed. The

relation between the APU coefficient index and the actual

coefficient (i.e. the gain) is given in Table 5.

September 1994

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