Cirrus Logic AN282 User Manual
AN282
The 2-Channel Serial Audio Interface:
A Tutorial
INTRODUCTION
It may come as a surprise to those trying to make their initial investigation into audio systems design that there is a
de-facto standard for transferring audio data within a system. Despite the differing naming conventions used within
the industry, these apparently different interfaces are essentially identical. For the sake of simplicity, we will use the
term Serial Audio Interface (SAI) in this discussion. The Serial Audio Interface is by far the most common mechanism used to transfer two channels of audio data between devices within a system; for instance, from the analogto-digital converter to the Digital Signal Proc essor (DSP) and then the digital-to-analog converter, as shown in
Figure 1.
Left / Right Clock
Analog-to-Digital Converter Digital-to-Analog Converter
DSP
Figure 1 System Block Diagram
Serial Clock
Serial DataSerial Data
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1. THE SERIAL AUDIO INTERFACE
The Serial Audio Interface is comprised of two control clocks, the Left / Right and Serial Clocks, and the Serial audio
data line. Despite the many different names used for the various clocks, their uses and requirements are nearly identical. A generalized Serial Audio Interface is shown in Figure 2. Some form of this diagram can probably be found
in every audio converter and DSP data sheet that has ever been published.
Left/Right Clock
Serial Clock
1.1 Left/Right Clock
The Left/Right (LRCK) Clock is known by several names, including Word Clock, Frame Clock, Frame Sync,
and probably several others. Despite the differ ent name s, the use and require ments for this clocking signal
are identical. In all applications, the function of this clock signal to id entify the audio system sample rate and
frame the two channels of audio data that exist on the single audio data line. As a result of the first mentioned function, the required frequency of the Left/Right Clock signal is always at the system audio sample
rate, such as 44.1 kHz, 48 kHz, etc. The high and low times of this clock are used to separate or delineate
the Left and Right channel data.
1.2 Serial Audio Data
The industry standard for representing Pulse-Coded-Modulation (PCM) audio data is a word comprised of
16 to 32 bits (16- and 24-bit data are the most common) coded in a two’s-complement format. The audio
data word is always transmitted with the Most Significant Bit (MSB) first. The only common exception to the
two’s-complement format is when the audio data is represented in one of the many compressed audio formats.
1.3 Serial Clock
Audio Data
Figure 2 Generic Serial Audio Interface Diagram
Audio Data
The Serial Clock (SCLK) is also often referred to as the Bit Clock. As with the Left/Right Clock, the function
is the same regardless of the name. The sole purpose of the Serial Clock is to shift the audio data into or
out of the serial audio port. The m inimu m req uire d freq uency f or t he Seri al Cl ock is direc tly pro port ional to
the system audio sample rate and the audio word length. Recall that there are two channels of audio data
presented in each period of the Left/Rig ht Clock, a nd the frequency of the Left/Right Clock must be at the
audio sample rate. Therefore, the minimum required Serial Clock frequency is twice the audio sample rate
times the number of bits in each audio word.
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