Cirrus Logic CobraNet - Manual (Page 10)

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Copyright 2006 Cirrus Logic, Inc. DS651PM25
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
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