USB Cheat Sheet

Post by lxgr » Thu, May 05, 2022, 7:39 PM UTC

fabiensanglard wrote: post #110

masklinn wrote: post #10

Some of the entries seem incorrect: "USB 3.2 (USB 3.2 Gen 2x2) and "USB 4" (USB 4 USB4 Gen 2×2) should have the same nominal data rate of 2500MB/s, they're 2 lanes (x2) of 10GB/s. Though they are apparently coded differently electrically, so they're distinct protocols. The tables would benefit from mentioning the coding (8/10 or 128/132) as IMO it's one of the most confusing bits when you see the effective data rates…

Thank you for noticing these issues, I have updated the table. I would be happy to improve it and add encoding. I am surprised by some of the summary entries on Wikipedia ( https://en.wikipedia.org/wiki/USB4 ). Looks like USB4 "reverted" to 128b/132b. It is accurate?

Post by lxgr » Thu, May 05, 2022, 7:43 PM UTC

martinmunk wrote: post #117

masklinn wrote: post #86

Earlier quoted context omitted.

Surprised they don't use a hardware video encoder, is it because the well and efficiently supported formats are all MPEG, and thus have fairly high licensing cost on top of the hardware? Or because even efficient HVEs use more resources than webcams can afford? Or because inter-frame coding requires more storage, which (again) means higher costs, which (again) eats into the margin, which cheap webcam manufacturers co…

My older Logitech C920 has an on-board H.264 encoder. Newer revisions of the same model does not. I haven't figured out why they chose to remove it, but your point about licensing cost combined with them not advertising it much as a feature, and most of their competitors not including "proper" video encoding might explain it. Edit: Found an official explanation here: https://www.logitech.com/en-us/video-collaboration…

Post by Dylan16807 » Thu, May 05, 2022, 7:54 PM UTC

grishka wrote: post #87

IvanK_net wrote: post #74

Fun fact: USB 2.0 webcams have been existing for over 10 years. USB 2.0 is 60 MB/s. A pixel of an image is 3 Bytes. A 1920x1080 FullHD image is 6.2 MB. At 30 frames per second, second of a FullHD video is 186 MB. How did they do that? Answer: frames are transferred as JPEG files. Even a cheap $15 webcam is a tiny computer (with a CPU, RAM, etc), which runs a JPEG encoder program.

Hm. But then wouldn't it make more sense to just stream the raw sensor data, which is 1 byte per pixel (or up to 12 bits if you want to get fancy), and then demosaic it on the host? Full HD at 30 fps would be 59.33 MB/s, barely but still fitting into that limit. But then also I think some webcams use H264? I remember reading that somewhere.

Post by nybble41 » Thu, May 05, 2022, 8:22 PM UTC

BayAreaEscapee wrote: post #95

grishka wrote: post #87

Earlier quoted context omitted.

Hm. But then wouldn't it make more sense to just stream the raw sensor data, which is 1 byte per pixel (or up to 12 bits if you want to get fancy), and then demosaic it on the host? Full HD at 30 fps would be 59.33 MB/s, barely but still fitting into that limit. But then also I think some webcams use H264? I remember reading that somewhere.

I don't know where you get "1 byte per pixel" from. At minimum, raw 4:2:0 video would be two bytes per pixel, and RGB would be three bytes per pixel with 8-bit color depth.

Post by Dylan16807 » Thu, May 05, 2022, 8:30 PM UTC

dual_dingo wrote: post #34

CoastalCoder wrote: post #27

Earlier quoted context omitted.

Not my area of expertise, but maybe some (unrealistic) options include using fiber optics for the data lines, or adding more data lines.

I guess at some point optical will be the only way forward. Having more data lines in a serial bus is interesting, as the whole reasoning to go from parallel lines (e.g. Centronics, ATA/SCSI or ISA/PCI buses) to serial (SATA/SAS, PCIe, USB) was that coordinating multiple data lines got impossible due to physical limitations where e.g. minimal differences in cable lengths started to matter).