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Data Sheet

ADAU1761

Rev. D | Page 45 of 93

DSP CORE

SIGNAL PROCESSING

The ADAU1761 is designed to provide all audio signal processing

functions commonly used in stereo or mono low power record

and playback systems. The signal processing flow is designed

using the SigmaStudio software, which allows graphical entry

and real-time control of all signal processing functions.

Many of the signal processing functions are coded using full,

56-bit, double-precision arithmetic data. The input and output

word lengths of the DSP core are 24 bits. Four extra headroom

bits are used in the processor to allow internal gains of up to

24 dB without clipping. Additional gains can be achieved by

initially scaling down the input signal in the DSP signal flow.

ARCHITECTURE

The DSP core consists of a simple 28-/56-bit multiply-accumulate

(MAC) unit with two sources: a data source and a coefficient

source. The data source can come from the data RAM, a ROM

table of commonly used constant values, or the audio inputs to

the core. The coefficient source can come from the parameter

RAM or from a ROM table of commonly used constant values.

The two sources are multiplied in a 28-bit fixed-point multiplier

and then the signal is input to the 56-bit adder; the result is usually

stored in one of three 56-bit accumulator registers. The accumu-

lators can be output from the core (in 28-bit format) or can

optionally be written back into the data or parameter RAMs.

COEFFICIENT SOURCE

(PARAMETER RAM,

ROM CONSTANTS)

DATA OPERATIONS

(ACCUMULATORS (3), dB CONVERSION,

BIT OPERATORS, BIT SHIFTER, ...)

DATA SOURCE

(DATA RAM,

ROM CONSTANTS,

AUDIO INPUTS)

OUTPUTS

TRUNCATOR

Figure 67. Simplified DSP Core Architecture

PROGRAM COUNTER

The execution of instructions in the core is governed by a program

counter, which sequentially steps through the addresses of the

program RAM. The program counter starts every time that a

new audio frame is clocked into the core. SigmaStudio inserts a

jump-to-start command at the end of every program. The

program counter increments sequentially until it reaches this

command and then jumps to the program start address and

waits for the next audio frame to clock into the core.

FEATURES

The SigmaDSP core was designed specifically for audio processing

and therefore includes several features intended for maximizing

efficiency. These include hardware decibel conversion and audio-

specific ROM constants.

STARTUP

Before the DSPRUN bit is set or any settings are written to the

parameter RAM, the DSP core must be enabled by setting the

DSPEN bit in Register R61 (Address 0x40F5).

The following steps should be performed every time that a new

program is loaded to the SigmaDSP core, or any time that the

DSPRUN bit is disabled and reenabled.

1. Set the DSPSR[3:0] bits in Register R57 (Address 0x40EB)

to 1111 (none).

2. Set the DSPRUN bit in Register R62 (Address 0x40F6) to 0.

3. Download the rest of the registers, the program RAM, and

the parameter RAM.

4. Set the DSPRUN bit in Register R62 to 1.

5. Set the DSPSR[3:0] bits in Register R57 to the operational

setting (default value is 0001).

Changing any register setting or RAM can cause pops and

clicks on the analog outputs. To avoid these pops and clicks,

mute the appropriate outputs using Register R29 to Register R32

(Address 0x4023 to Address 0x4026). Unmute the analog out-

puts after the startup procedure is completed.

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