Datasheet

ADAU1761
Rev. C | Page 46 of 92
NUMERIC FORMATS
DSP systems commonly use a standard numeric format.
Fractional numeric systems are specified by an A.B format,
where A is the number of bits to the left of the decimal point
and B is the number of bits to the right of the decimal point.
The ADAU1761 uses numeric format 5.23 for both the
parameter and data values.
Numeric Format 5.23
Linear range: −16.0 to (+16.0 − 1 LSB)
Examples:
1000 0000 0000 0000 0000 0000 0000 = −16.0
1110 0000 0000 0000 0000 0000 0000 = −4.0
1111 1000 0000 0000 0000 0000 0000 = −1.0
1111 1110 0000 0000 0000 0000 0000 = −0.25
1111 1111 0011 0011 0011 0011 0011 = −0.1
1111 1111 1111 1111 1111 1111 1111 = (1 LSB below 0)
0000 0000 0000 0000 0000 0000 0000 = 0
0000 0000 1100 1100 1100 1100 1101 = 0.1
0000 0010 0000 0000 0000 0000 0000 = 0.25
0000 1000 0000 0000 0000 0000 0000 = 1.0
0010 0000 0000 0000 0000 0000 0000 = 4.0
0111 1111 1111 1111 1111 1111 1111 = (16.0 − 1 LSB)
The serial port accepts up to 24 bits on the input and is sign-
extended to the full 28 bits of the DSP core. This allows internal
gains of up to 24 dB without internal clipping.
A digital clipper circuit is used between the output of the DSP
core and the DACs or serial port outputs (see Figure 68). This
circuit clips the top four bits of the signal to produce a 24-bit
output with a range of 1.0 (minus 1 LSB) to −1.0. Figure 68
shows the maximum signal levels at each point in the data flow
in both binary and decibel levels.
4-BIT SIGN EXTENSION
DATA IN
1.23
(0dB)
1.23
(0dB)
1.23
(0dB)
5.23
(24dB)
5.23
(24dB)
SERIAL
PORT
SIGNAL
PROCESSING
(5.23 FORMAT)
DIGITAL
CLIPPER
0
7680-068
Figure 68. Numeric Precision and Clipping Structure
PROGRAMMING
On power-up, the ADAU1761 must be configured with a clock-
ing scheme and then loaded with register settings. After the codec
signal path is set up, the DSP core can be programmed. There
are 1024 instruction cycles per audio sample, resulting in an
internal clock rate of 49.152 MHz when f
S
= 48 kHz.
The part can be programmed easily using SigmaStudio, a graphical
tool provided by Analog Devices (see Figure 69). No knowledge
of writing line-level DSP code is required. More information
about SigmaStudio can be found at www.analog.com.
07680-069
Figure 69. SigmaStudio Screen Shot