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CobraNet

Routing Primer

Steve Gray, Cirrus Logic, Inc.

Configuring CobraNet

systems to properly route audio is fairly straightforward once the underlying

concepts are understood. This document explains the concepts behind end to end routing and how to

configure CobraNet

interfaces to route audio channels. Use this document as a companion to the available

CobraNet

device Data Sheets and Reference Manuals.

The variables referred to in this document are described in the CobraNet

Programmer’s Reference. They

are known as Management Interface (MI) variables and can be read and written via the hardware Host

Management Interface (HMI) or via SNMP over the network.

The signals referred to are described in the Datasheet or Hardware Reference Manual applicable to the

particular CobraNet

interface in use.

Referring to Figure 1 below, let’s consider the components of CobraNet

audio routing.

Fig. 1 Simplified Routing Schematic

CobraNet

Page 1 of 9

Summary of content (9 pages)

PAGE 1
CobraNetTM Routing Primer Steve Gray, Cirrus Logic, Inc. Configuring CobraNetTM systems to properly route audio is fairly straightforward once the underlying concepts are understood. This document explains the concepts behind end to end routing and how to configure CobraNetTM interfaces to route audio channels. Use this document as a companion to the available CobraNetTM device Data Sheets and Reference Manuals.
PAGE 2
Components of Routing Bundles A Bundle is the basic unit of data for transporting audio over a CobraNetTM network and can contain from zero to eight audio sub-channels. Sample size, sample rate and transmission latency all have an impact on processor bandwidth and packet sizes required to process and transmit the audio data. These factors can limit the number of audio channels that can be processed by a CobraNetTM interface.
PAGE 3
Bundle to Audio Routing Channel Mapping – txSubMap and rxSubMap The RxSubMap and TxSubMap variable arrays are used to map Audio Routing Channels to and from Bundle sub-channels. As described above, an Audio Routing Channel can be thought of as assigned to a specific time slice within a Synchronous Serial Interface. Therefore, assigning an Audio Routing Channel to a bundle subchannel is equivalent to assigning an SSI time slice to a bundle sub-channel.
PAGE 4
Example: We have two standard CM-2 modules to be configured as a transmitter and a receiver. We want to send the audio data with standard latency, a 48-kHz sample rate and 20 bit sample size. We would like to send two audio channels from the transmitter to the receiver in one unicast bundle. Bundle number 273 is a valid unicast bundle number suitable for use. SSI interface 1 is connected to a 2 channel A to D on the transmitter and SSI interface 1 is connected to a 2 channel D to A on the receiver.
PAGE 5
Appendix A - MI Variables used in routing Refer to the CobraNetTM Programmer’s Reference for more detailed information on these variables. txBundle rxBundle: Array variables which set the bundle number that a transmitter or receiver will use. There is one instance of these variables for each Receiver or Transmitter. Defaults: All variables = 0 for standard CobraNetTM interfaces. May be initialized to other values depending on product.
PAGE 6
modeRateControl: Must be set the same on both the sending and receiving interfaces. There is one modeRateControl variable per CobraNetTM interface and its value affects all audio channels. All channels on a particular CobraNetTM node must be set to the same latency (1 1/3, 2/23 or 5 1/3) and sample rate (48-kHz or 96-kHz). Sample sizes (16, 20 or 24 bits), as specified by txSubFormat, can vary by channel, and are not affected by modeRateControl.
PAGE 7
Appendix B – SSI Interface Theory There are typically four Synchronous Serial Interfaces in a CobraNetTM interface. This number can differ depending on product and/or configuration. SSI interfaces are configured at the factory to carry two to eight audio channels each, in each direction. The number of audio channels multiplexed on an SSI will be reflected in the rate of the audio bit clock (SCLK). The word clock (FS1) will run at the configured sample rate of 48kHz or 96-kHz.
PAGE 8
Appendix C – Default SSI to Audio Routing Channel Mapping 2 x 2 Interface SSI Interface SSI Timeslice 1 1 2 Audio Routing Channel (Receive) 33 34 Audio Routing Channel (Transmit) 1 2 8 x 8 Interface SSI Interface 1 2 3 4 SSI Timeslice 1 2 1 2 1 2 1 2 Audio Routing Channel (Receive) 33 34 35 36 37 38 39 40 Audio Routing Channel (Transmit) 1 2 3 4 5 6 7 8 Audio Routing Channel (Receive) 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 Audio Routing Channel (Transmit) 1 2 3 4 5 6 7 8 9 10 11 12 13 1
PAGE 9
Appendix B – continued 32 x 32 Interface SSI Interface 1 2 3 4 SSI Timeslice 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 CobraNetTM Audio Routing Primer Rev. 1.