96KHz Sample Rate Support
Interoperability
With the introduction of 96KHz there remains a single network clock. The sample clock used for 96KHz audio is exactly 2x the existing 48KHz network clock generated by the conductor. 96KHz and 48KHz audio may coexist on the same network.
A CobraNet interface may operate at either 48KHz or 96KHz but not both rates simultaneously.
A device operating at 48KHz will not be able to receive audio from device operating at 96KHz. The converse is also true.
Implementation
96KHz will only be supported on CM-1 and derivative hardware. 96KHz will be supported on future hardware platforms i.e. CobraNet chip.
No hardware changes will be required to support the increased sample rate. A CM-1 firmware upgrade will reconfigure the synchronous serial ports to operate at the doubled word and bit rates required for 96KHz audio (see Hardware section below for details on these changes).
A CobraNet interface will operate at either 48KHz or 96KHz sample rate. Sample rate is selected by the modeRateControl variable which selects sample rate and audio latency mode.
96KHz and low-latency are orthogonal enhancements. 96KHz will be supported in the new low-latency modes of operation.
There are no new MI variables associated with 96KHz mode. The following orange data formats will be introduced as options for txSubFormat
| txSubFormatValue | Resolution | Sample Rate | Latency |
|---|---|---|---|
| 0 | No signal | ||
| 0x044000 | 16-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 |
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 operating at 48KHz cannot decode 96KHz audio. An interface operating at 96KHz cannot decode 48KHz audio. In either of these cases, the LS bit in rxSubFormat will be clear.
96KHz audio requires twice the bandwidth and thus twice the amount of space in an Ethernet packet as its 48KHz counterpart. Due to the additional payload size, at the standard 5-1/3ms latency mode, only 4 96KHz audio channels can fit into a bundle at 16 and 20 bit resolutions and only 3 96KHz channels at 24 bit resolution. Lower latency modes feaure relaxed restrictions on the number of audio channels allowed in a bundle.
Hardware interface
When operating in 96KHz mode the Master Clock remains at the same 24.576MHz rate present in 48KHz mode. 96KHz mode, the Sample Clock Output (FS1) will produce a 96KHz signal. If a Sample Clock Cascade and or Reference Clock input is supplied, this signal may be either 48KHz or 96KHz in 96KHz mode but must be 48KHz in 48KHz mode. With these exceptions, synchronous serial timing will continue to operate according to the specifications found in the CobraNet Technology Datasheet (p.18).
| Synchronous Serial Port Operating Mode | 48KHz Audio Bit Clock (SCK) rate | 96KHz Audio Bit Clock (SCK) rate |
|---|---|---|
| 64Fs (2 channels x 4 interfaces) | 3.072MHz | 6.144MHz |
| 128Fs (4 channels x 4 interfaces) | 6.144MHz | 12.288MHz |
| 256Fs (8 channels x 4 interfaces) | 12.288MHz | 24.576MHz |