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I understand that the raspberry pi can record from USB webcams. My question is, if I use a powered hub to get extra USB ports, what kind of frame rate will the raspberry pi be able to record video at using 4 cameras?

I'm assuming it won't be able to record 1080p from all 4 cameras each at 60fps?

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Let's do the math. Uncompressed 1080p RGB frame is about ~6MB big. USB 2.0 can transfer up to 60 MB/s but it works in half duplex so one-way data stream won't exceed 30 MB/s.

It means typical single USB 1080p webcam can provide you 5fps.

There are webcams with hardware H.264 or MJPEG encoders which could provide ~10x more frames, but a) 60fps would be still hard to get even from single webcam and b) it is an open question, whether such hardware can be handled by Pi (http://www.raspberrypi.org/phpBB3/viewtopic.php?f=31&t=28887, https://superuser.com/questions/639703/using-hardware-compression-for-webcam-in-linux).

And even if you manage to get 60fps HD stream from webcam, you probably won't be able to save it anywhere. LAN is too slow (100mbit is about 12 MB/s), class 10 SD card is ~10 MB/s, USB is already saturated in opposite direction, so you won't achive the same speed the other way.

60fps in HD is still domain of dedicated hardware, like GoPro sport cameras.

  • To help me find a sweet spot, how did you get the ~6MB value for an uncompressed 1080p RGB frame? – oshirowanen Jan 7 '14 at 22:15
  • 1920x1080 is (roughly) 2 million pixels. Given 1 byte per color channel per pixel, we will get 6 million bytes in RGB representation or 8 million bytes in RGBA representation. My suggestion: determine your needs, then choose adequate hardware. If you need to record HD video, maybe device like Mobius ActionCam ($70) will be better than Raspberry Pi with USB webcams. – tomash Jan 8 '14 at 9:03
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No it will not be able to.

I've been told many times that if you have more extension cords, it will not give as much power. A usb hub works like an extension cord and takes up some energy too. They will work but not like you hoped.

  • powered hub has own power supply and provides energy for downstream devices – tomash Dec 4 '13 at 14:34
  • it still wont work like you said because the raspberry pi only has so much ram. It wont get as many fps as u want. – psp Dec 5 '13 at 22:13

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