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

7.4.3 SAMPLE CLOCK GENERATION

When the ‘external sample clock’ mode of the clock

generator is used, the application must know the sample

rate. FSCLKIN has to be applied, with a frequency which

is a multiple of the sample rate. The (sample rate

dependent) output interface timing signals will be

generated from FSCLKIN. This mode will normally be

used in applications with a fixed sample rate. Should the

sample rate change, then a soft reset is strongly advised.

When one of the remaining clock generator modes is used,

the SAA2502 selects the active sample rate automatically,

and generates the required sample rate related timing

signals from its MCLKIN and X22IN clock inputs. Soft

resets at sample rate changes are generated

automatically. After a hard reset, a sample rate of 44.1 kHz

by default is selected. Such default setting may be

overruled using the control interface.

SCK, WS and SPDIF will show frequency changes in any

of the following 3 situations:

• When the SAA2502 establishes synchronization to the

coded data input bit stream at a sample rate different

from the one previously selected

• When the current (default) sample rate is overruled by

the control interface

• When the clock generator mode is changed, resulting in

a switch from or to the ‘external sample clock mode.

In all those situation the phase of WS and the data content

of SPDIF will be continuous.

In all other events SCK, WS and SPDIF remain operating

without phase or frequency changes and the sample rate

selection remains unchanged.

7.4.4 DECODER PRECISION

During decoding several multiply operations are carried

out on coded samples. The results of these operations

have to be rounded in order to keep the word length

required for internal number representation within

reasonable limits. Accumulation of these rounding errors is

kept at a very low level in order to assure precise audio

output samples. SAA2502 precision is specified using the

output of the MPEG reference decoder based on double

precision floating point calculations as a reference.

Differences between that reference decoder and SAA2502

output manifest themselves as white noise.

Two contributions to this noise may be identified:

• Noise resulting from internal rounding on intermediate

results

• Noise resulting from rounding of final output samples

to 16, 18, 20 or 22 bits (depending on selected output

accuracy).

Table 8 shows the effective noise level figures. (unit is

1 LSB of 22-bit accuracy output). Except for 22-bit

accuracy, output rounding is by far the dominant effect.

Consequently the SAA2502 may be considered a

professional level high precision decoder.

Table 8 Effective noise level figures

OUTPUT ACCURACY

(BITS)

22

20

18

16

INTERMEDIATE

ROUNDING

0.6

0.6

0.6

0.6

OUTPUT ROUNDING(1)

0.3

1.2

4.6

18.5

Note

1. The output rounding part of this precision is valid only for I2S and SPDIF outputs.

TOTAL NOISE LEVEL

0.7

1.3

4.7

18.5

1997 Nov 17

18

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