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Comment by nrp | Hacker News Reader

USB cameras are interfaced over USB 2.0 because USB 3.0 is extremely difficult to make work from an EMI perspective running through a laptop’s hinges and lid. USB 2.0 means heavy compression to get 1080p60, and on pretty mediocre ISPs in those low cost camera controllers.

MIPI is easier from an EMI perspective and lets you use a much better ISP on the processor.

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USB-2.0 is up to 480mbps which I believe is several multiples higher bit rate than even the highest quality 4k Blu-ray disks. I don't see why "heavy" compression would be required.

Edit: doing some quick math, it seems like transmitting a raw/uncompressed signal at even 720p60 far exceeds the bandwidth of USB-2.

So if the goal is to shift all transcoding to the CPU, MIPI does make sense.

Cameras use isochronous mode, which maxes out at less than half the theoretical max USB 2.0 bandwidth, and they use MJPEG compression on each frame rather than something like H.265 that can compress much more by leveraging P/B-frames.

> because USB 3.0 is extremely difficult to make work from an EMI perspective running through a laptop’s hinges and lid

Is that an issue with USB 3.0 causing interference on the 2.4GHz band and thus interfering with Wifi/BT antennas mounted in the lid? It can't just be a signal frequency issue on the USB side, since eDP signals for the display itself are much higher bandwidth.

> MIPI is easier from an EMI perspective

In the "MIPI family" (e.g. FPD-link, GMSL) there are solutions for longer lengths via coax, but MIPI CSI itself seems pretty focused on short distance links (as expected, considering its phone industry roots). It seems like the distance from processor to hinge to top-of-screen could easily get close to the nominal maximum length of MIPI CSI. Or are laptops already using MIPI-over-coax to simplify the cabling through the hinge?

At least for the Lenovo ThinkPad X1 Carbon Gen10 I know that they use a hybrid FFP/micro coax design. FFP to fit behind the display panel as the cable runs up to the sensor assembly and micro coax for fitting through the hinge. I don't think you'll see a lot of purely coax based designs, given the trend to thin displays with small bezels. Other laptops like the Dell XPS 13 Plus run straight FFP from the mainboard all the way to the sensor.

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