Re: [libusb] less data length in async bulk transfer with 2 cameras

[Tim R. <ti...@pr...>]

Zheng Rong wrote:
>
> Yes, I tried two off-the-shelf USB cameras simultaneously and get 
> 60fps+60fps performance. And, these cameras use non-compressed image 
> format and have the same size as my design. They also use BULK 
> endpoint for data transfer. Actually this is the reason why I chose to 
> use bulk xfer type in my design, I thought I could be able to get the 
> similar performance. But at the current  point I can not get that. Any 
> clue to explain this?

How much buffering do you have in your design?  If you were delayed for 
a millisecond, would you lose data?


> I will try isochronous xfer mode in my design as you suggested. You 
> said the isochronous xfer can guarantee the bandwidth for 2 cameras. 
> Why? Is it because isochronous xfer won't do retry when it get NACK?

No.  When you select an alternate setting for an isochronous endpoint, 
the system reserves a piece of each frame specifically for that 
endpoint, whether it uses it or not.  You are guaranteed that your 
device will have a time slot when your turn comes around.  With two 
isoch devices, assuming there is room for both, each gets its own 
guaranteed time slot.  There is no competition.  With a bulk pipe, there 
is no such reservation.  You are competing with all the other bulk 
devices for whatever bandwidth is left over.


> One more question: My computer's USB is version 3.0, and cameras are 
> USB2.0. If I connect multiple USB2.0 cameras to the computer, the 
> bandwidth for upstream is 480Mbps or 5Gbps? For a USB2.0 hub, the 
> upstream bandwidth is certainly 480Mbps. What is the total upstream 
> bandwidth for a USB3.0 hub connected with USB2.0 devices?

The USB 2.0 devices are all sharing 480Mbps.  The USB 3 host controller 
ends up exposing an xHCI controller that runs at USB 3 speeds, and an 
EHCI controller that runs at USB 2 speeds.


> I ask this question because I found: if 2 off-the-shelf cameras are 
> connected to PC through a USB2.0 hub, the frame rate can only get 
> about 45fps+45fps; But if I connect the cameras directly to PC, the 
> frame rate can be 60fps+60fps even more.

The frame rate is not important.  What's important is bytes per second.  
Adding a hub always affects the timing by introducing delays.  It's 
possible that the hub's communication is causing your retries to fall 
across some threshhold so that you lose a packet time.

-- 
Tim Roberts, ti...@pr...
Providenza & Boekelheide, Inc.