3 ms·

Introduced latency on the capture side makes latency tuning your entire production pretty difficult. For non-real time usage, sure, latency in the 100-200ms ran

Introduced latency on the capture side makes latency tuning your entire production pretty difficult. For non-real time usage, sure, latency in the 100-200ms range is more than acceptable (assuming it's deterministic, as you pointed out), but in the real-time world? Keeping things within a frame is pretty much required, and with the popularity of software-driven studio workflows across both amateur streaming and professional production, it's been real hard to get reliable performance out of USB hardware that didn't add frustrating amounts latency due to pre-ingest compression or seemingly random amounts of delay due to protocol or CPU time starvation.

justinlloyd 4y ago
Yep, there's a reason I have Blackmagic quad 4K capture cards in my workstation. Syncing multiple video streams with USB capture cards would be nigh impossible even if you put them on separate USB controllers. Ingest over USB is fine (though slow) but pretty much every USB capture card does its own internal compression, as you point out, and then involves the CPU to decompress it and get it into VRAM or DRAM.
- dsr_ 4y ago
  Realtime video production is definitely an outlier. You probably want a workstation-class system anyway, with a full TB of RAM so you know that's never an issue.
Dylan16807 4y ago
USB latency should be under a millisecond as far as I know.
- pclmulqdq 4y ago
  This is both only true for small transfers (not bulk/asynchronous transfers) and for the ~99th percentile. Large transfers have some buffer management and handshaking, so they tend to have highly variable latency that has a very fat tail. The latency degradation for large transfers is so noticeable that most audio DACs (not just ones for gullible audiophiles, also for the pro market) use custom drivers and USB protocols. For 1/1000000th the data rate that a capture card would need.
- boondaburrah 4y ago
  but is it /consistent/?

justinlloyd 4y ago

Yep, there's a reason I have Blackmagic quad 4K capture cards in my workstation. Syncing multiple video streams with USB capture cards would be nigh impossible even if you put them on separate USB controllers. Ingest over USB is fine (though slow) but pretty much every USB capture card does its own internal compression, as you point out, and then involves the CPU to decompress it and get it into VRAM or DRAM.

dsr_ 4y ago
Realtime video production is definitely an outlier. You probably want a workstation-class system anyway, with a full TB of RAM so you know that's never an issue.

Dylan16807 4y ago

USB latency should be under a millisecond as far as I know.

pclmulqdq 4y ago
This is both only true for small transfers (not bulk/asynchronous transfers) and for the ~99th percentile. Large transfers have some buffer management and handshaking, so they tend to have highly variable latency that has a very fat tail. The latency degradation for large transfers is so noticeable that most audio DACs (not just ones for gullible audiophiles, also for the pro market) use custom drivers and USB protocols. For 1/1000000th the data rate that a capture card would need.
boondaburrah 4y ago
but is it /consistent/?