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PDF SAA2500H Data sheet ( Hoja de datos )
| Número de pieza | SAA2500H | |
| Descripción | MPEG Audio Source Decoder | |
| Fabricantes | Philips | |
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INTEGRATED CIRCUITS
DATA SHEET
SAA2500
MPEG Audio Source Decoder
Preliminary specification
File under Integrated Circuits, IC01
September 1994
Philips Semiconductors
1 page  
Philips Semiconductors
MPEG Audio Source Decoder
Preliminary specification
SAA2500
SYMBOL
TMS
TDI
FSCLK384
FSCLKM
MCLK24
PIN DESCRIPTION
40 boundary scan test mode select input
41 boundary scan test data input
42 sample rate clock frequency indication input
43 sample rate clock source selection for the master input
44 master clock frequency indication
TYPE
I
I
I
I
I
handbook, full pagewidth
handbook, full pagewidth
RESET 1
FSCLK 2
FSCLKIN 3
MCLK
VDD
GND
4
5
6
MCLKOUT 7
MCLKIN 8
X22OUT 9
X22IN 10
STOP 11
SAA2500
(QFP44)
33 TB
32 TI
31 TO
30 WS
29 SCK
28 GND
27 TA
26 SD
25 L3MODE
24 L3DATA
23 L3CLK
MGB490
Fig.2 Pin configuration.
FUNCTIONAL DESCRIPTION
Coding system
The perceptual audio encoding/decoding scheme defined
within the “ISO/IEC 11172-3 MPEG Standard” allows for a
high reduction in the amount of data needed for digital
audio whilst maintaining a high perceived sound quality.
The coding is based upon a psycho-acoustic model of the
human auditory system. The coding scheme exploits the
fact that the human ear does not perceive weak spectral
components that are in the proximity (both in time and
frequency) of loud components. This phenomenon is
called masking.
For layers I and II of ISO/MPEG the broadband audio
signal spectrum is split into 32 sub-bands of equal
bandwidth. For each sub-band signal a masking threshold
is calculated. The sub-band samples are then
re-quantized to such an accuracy that the spectral
distribution of the re-quantization noise does not exceed
the masking threshold. It is this reduction of representation
accuracy which yields the data reduction. The
re-quantized sub-band signals are multiplexed, together
with ancillary information regarding the actual
re-quantization, into a MPEG audio bitstream.
September 1994
5
5 Page 
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
11
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet SAA2500H.PDF ] | |
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