Cirrus-logic CobraNet User Manual download pdf (Page 10)

CobraNet Programmer’s Reference

Overview

1.3 Timing and Performance

CobraNet provides real-time audio delivery and requires real-time performance from the

network on which it is deployed. The best means of insuring a network will deliver the

performance required by CobraNet is to verify the design using Cirrus Logic’s CobraCAD

CobraNet modeling software (available for download at www.cirrus.com). The design

check feature in CobraCAD assures that the performance requirements shown in Table 1

are met and that the network is capable of delivering the bandwidth required to support

the modeled application.

Table 1. Network Performance Requirements

Parameter Maximum Comments

Beat Packet Delay Variation 250µs

If delivery of beat packets periodically varies from the nominal delay by

more than this value, then the Receivers may loose sample lock or fail to

meet clock delivery specifications.

Forwarding Delay,

5-1/3ms latency

500µs

Forwarding delay is the sum of store forward, queuing and propagation

delays. Forwarding delay includes delay variation - i.e. 150µs forwarding

delay + 250µs delay variation = 400µs. Thus tolerance of forwarding

delay is reduced in the presence of delay variation. When forwarding

specification is exceeded, audio is delivered reliably with additional

latency. rxDelay and rxMinDelay can be used to observe and control this

adaptation to forwarding delay.

Forwarding Delay,

2-2/3ms latency

250µs

Forwarding Delay,

1-1/3ms latency

a. Store-forward delay on a 100Mbit Ethernet connection is 121us (assuming maximum length packets). This forwarding

delay specification is only achievable on an audio-only dedicated network. The lowest latency achievable with CobraNet on

a non-dedicated network is 1-2/3ms (using the 1-1/3ms latency mode with an rxMinDelay setting of 0x40 to make

receivers tolerant to queuing delays introduced by non-audio traffic).

125µs

Maximum Forwarding Delay 5000µs Audio cannot be delivered at any latency with extreme forwarding delays.

Maximum Forwarding Delay Variation,

5-1/3ms latency

1000µs

Delay variation exceeding these specifications will result in unreliable

audio transport due unstable rxDelay determination. In some cases this

may be addressed through manual rxMinDelay setting.

Maximum Forwarding Delay Variation,

2-2/3ms latency

500µs

Maximum Forwarding Delay Variation,

1-1/3ms latency

250µs

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