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:
* 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 | modeRateControl |
|---|---|
| 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 |
