TABLE OF CONTENTS As mentioned, SDO and SDI carry both commands and audio data. The commands and their associated responses are low-bandwidth; the audio data is high-bandwidth. The two forms of data are interleaved, but the outbound protocol is slightly different than the inbound protocol.
To understand how these forms are multiplexed together, it is important to first understand the concept of a stream. A stream is a connection between a DMA buffer in the controller and a codec. A stream contains one or more audio channels from the same audio program. For instance, a stereo program would contain two channels.
Streams can either be output streams conveyed over SDO or input streams conveyed over SDI. Given the multipoint topology of SDO, it is not surprising to learn that output streams can be broadcast to multiple codecs. For instance, the same audio stream can be rendered concurrently on both headphones and speakers.
These streams transport audio samples in both directions across the link. Although the audio samples take up the majority of the link bandwidth, they are useless without ancillary control data that is used to modify and direct the flow of the audio samples. The command information originates from the controller, which issues 32-bit commands known as verbs. Figure 4.13 shows the structure of verbs.
The verb contains a 4-bit codec address that indicates the codec to which the command is targeted. A value of all ones in this field ( Fh
