Low Latency Audio Support

Implementation

Low latency modes will be supported on CM-1 and reference design hardware. No hardware changes will be required to support low latency modes. Low latency modes will be supported on future hardware platforms i.e. CobraNet chip. Low latency modes will be made available with a firmware upgrade. The default mode of operation will remain 5-1/3ms latency at 48KHz sample rate.

Running in low latency mode requires more DSP resource at the receiving and transmitting nodes. This implies there will be a tradeoff between amount of traffic supported and reduction of latency. Some Reference Design based products will need to operate at reduced channel count to support the lower latency. CM-1 based devices will face some limitations in channel capacity and number of transmitters and receivers and multiple unicast transmissions supported.

The following table shows CM-1 channel capacity for several latency and sample rate operating modes. 8 channel bundles unicast to a single destination or multicast with 20-bit resolution is assumed.

Latency Sample Rate CM-1 Channel Capacity
5-1/3ms 48KHz 64
5-1/3ms 96KHz 48 (4 channel bundles)
2-2/3ms 48KHz 64
2-2/3ms 96KHz 32
1-1/3ms 48KHz 32
1-1/3ms 96KHz 16

Low latency modes will also put additional demands on network performance. Specifically, in order to achieve the desired latency, forwarding delay across the network need to be reduced by approximately the same factor that that audio latency is being reduced. These requirements will bring into play new network design rules which will be incorporated into an updated version of CobraCAD.

Lower latency is achieved by transmitting audio packets at a higher rate. Since the amount of data remains constant, this implies that each packet is smaller for the low latency case than for the high latency case. A restriction on the number of audio channels allowed in a bundle is due to a restriction on the maximum sized Ethernet packet. Lower latency modes thus have lessened restrictions. Audio channel count restrictions are summarized below.

Channels Per Bundle 16-bit, 48K 20-bit, 48K 24-bit, 48K 16-bit, 96K 20-bit, 96K 24-bit, 96K
5-1/3ms 8 8 7 4 4 3
2-2/3ms 8 8 8 8 8 7
1-1/3ms 8 8 8 8 8 8

A CobraNet interface will operate at a single latency and sample rate mode as specified by the modeRateControl variable. This latency mode will apply to all incoming and outgoing audio at the interface. 96KHz and low-latency are orthogonal enhancements. 96KHz is supported in the new low-latency modes of operation

With the introduction of low latency modes, the following orange data formats will be introduced as options for txSubFormat

txSubFormat Value Resolution Sample Rate Latency
0 No signal
0x044000 6-bit 48KHz 5-1/3ms
0x054000 20-bit 48KHz 5-1/3ms
0x064000 24-bit 48KHz 5-1/3ms
0x148000 16-bit 96KHz 5-1/3ms
0x158000 20-bit 96KHz 5-1/3ms
0x168000 24-bit 96KHz 5-1/3ms
0x042000 16-bit 48KHz 2-2/3ms
0x052000 20-bit 48KHz 2-2/3ms
0x062000 24-bit 48KHz 2-2/3ms
0x142000 16-bit 96KHz 2-2/3ms
0x152000 20-bit 96KHz 2-2/3ms
0x162000 24-bit 96KHz 2-2/3ms
0x041000 16-bit 48KHz 1-1/3ms
0x051000 20-bit 48KHz 1-1/3ms
0x061000 24-bit 48KHz 1-1/3ms
0x141000 16-bit 96KHz 1-1/3ms
0x151000 20-bit 96KHz 1-1/3ms
0x161000 24-bit 96KHz 1-1/3ms

rxSubFormat will appropriately indicate the type of audio being received. As documented, the LS bit of this value is used to indicate whether the data in the sub channel is being decoded. An interface can only receive audio from a device operating at the sample rate and latency mode. In the case of a mode mismatch between transmitter and receiver, the LS bit in rxSubFormat will be clear.

HMI Variables

 

Name modeRateControl
Description

Selects latency and sample rate operating mode for the interface. The following modes are supported:

  • 0x701 - 5-1/3ms latency, 96Khz sample rate
  • 0x700 - 10-2/3ms latency, 48Khz sample rate*
  • 0x600 - 5-1/3ms latency, 48Khz sample rate
  • 0x601 - 2-2/3ms latency, 96Khz sample rate
  • 0x500 - 2-2/3ms latency, 48Khz sample rate
  • 0x501 - 1-1/3ms latency, 96Khz sample rate
  • 0x400 - 1-1/3ms latency, 48Khz sample rate
  • 0x401 - 2/3ms latency, 96Khz sample rate*
  • 0x300 - 2/3ms latency, 48Khz sample rate*
  • 0x301 - 1/3ms latency, 96Khz sample rate*
  • 0x200 - 1/3ms latency, 48Khz sample rate*
  • 0x201 - 1/6ms latency, 96Khz sample rate*

* These modes are not supported

Host Address 0x2100
SNMP Object ID Not available via SNMP
Type Integer24
Attributes Read/Write - Persistent
Implemented Version 2.9.0

 

 

Name modeRateStatus
Description Indicates the latency and sample rate operating mode currently in effect for the interface. modeRateStatus and modeRateControl will differ if an unsupported mode value is written to modeRateControl.
Host Address 0x2005
SNMP Object ID Not available via SNMP
Type Integer24
Attributes Read Only
Implemented Version 2.9.0