Firewire lower latency
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Contents |
Introduction
In different discussions the questions has arised why SSL does not use Firewire like FS2 does, being able to reduce the latency even more.
Start
USB 1.1 bandwidth: 1.5Mbps/12Mbps USB 2.0 bandwidth: 480Mbps
SSL uses 4*stereo channels + 1 mono channel (mic) = 9 mono channels, each at 44.1kHz/16Bit, that makes a total bandwidth consumption of 6350400 Bits/s which is about 52.92% of the available bandwidth on USB1.1.
Firewire indeed offers a lower latency, about 0.375ms, while the lowest USB (V1.1 or V2) buffers give a latency of about 1ms.
SSL's minimum latency is 8ms; 3ms total latency due to the USB in, 1ms processing, 3ms USB out. Various buffers from the audio signal to the PC memory need to be filled which cause the 3ms latency on the input and on the output.
On the other hand the latency of FS2 is at least 21ms, although it is using Firewire. So assuming you also need 3 buffers (translate into 3 timeslots, 1 timeslot is 0.125ms) on the firewire side, that will make:
- 3 timeslots FW in latency
- At least 1 timeslot processing
- 3 timeslots FW out latency
Thats a total of 7 timeslots, so the latency is 7 * 0.125ms = 0.875ms at best.
Links
- On the SSL board Discussion and suggestion.
- | Technical introduction to Firewire
- Firewire whitepaper on audio equipment
Acknowledgements
Credits for this article go out to:
Todos
- Unfinished and most probably not right.
