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General Interest USB Communications Sensitive to PC Performance
General Interest

Information in this article applies to:

  • uScope Navigator (All Versions)

Article ID: NGN2313 — Created: 20 Aug 2020 — Reviewed: 22 Aug 2020

Question

Why does camera data transmitted over USB sometimes get dropped?

Answer

The uScopeMXII, uScopeHXII, uScopeDX, and uScopeGX connect to your PC using a USB 2.0 interface. The maximum bandwidth of USB 2.0 is 480Mb/sec which is about 60MBytes per second.

The camera in the uScope sends data to the PC at the camera's scan rate which is up to 30FPS (frames per second). This means that the PC must be able to receive a 2 megapixel image (which is actually 6MB (megabytes) in size) every 33ms. That's a total bandwidth of 180MB per second (3 times more than USB 2.0 provides).

Modern cameras use compression techniques to send data efficiently. With 3:1 compression (which is typical) the required bandwidth is 60MB per second. This leaves little bandwidth for other USB peripherals (like external backup drives).

To exacerbate the problem, image data is produced too fast to be buffered in the uScope. So, data that is dropped by the PC cannot be re-transmitted—the whole image must be re-captured and re-transmitted. If this re-capture/re-transmit process continues to fail, uScope Navigator eventually gives up and uses the best image it can.

More Information

Using a dedicated USB 2.0 port (not a hub), that is not shared with other USB devices, can mitigate most performance problems.

The worst case scenario would be to save scanned images to an external USB drive. This requires receiving the image from the uScope and saving it to the drive all over the same USB interface. Effectively, halving the USB bandwidth.

Resolution

If you experience USB data drops, refer to Too Many Errors Attempting to Process Camera Data for things to try which can resolve the problem.

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