Transfer Latency

The transfer latency is the time interval between the time when the last camera pixel of the image enters the frame grabber and the same processed pixel is stored in the host PC memory.

Case #1: Camera pixel rate < max pixel processing or pixel delivery rate

Case #2: Camera pixel rate > max pixel processing or pixel delivery rate

For both cases

The camera delivers the image starting at time t₀ and ending at time t₂. For the simplicity, the line blanking intervals are not shown.
The on-board pixel processing and DMA transfer to the host PC memory begins at time t₁ and ends at time t₃.
The pixel processing requires that a minimum of pixel data (P₁ pixels) is available into the buffer. Consequently, the processing of the first pixel is retarded until the minimum amount of pixel is available. The time interval (t₁ - t₀) is the "initial latency"; it depends only on the camera pixel data rate and P₁.
The time interval (t₃ - t₂) is the "latency".

For case #1

After t₁, the effective pixel processing rate is equal to the camera pixel rate; the processing is eventually halted at line boundaries until enough data is available from the camera to resume it.
When the camera has delivered the last pixel of the image, the processing continues at the max pixel processing (or the maximum delivery rate) until the last pixel of the image. The latency depends only on P₁ and the maximum processing/delivery rate.

For case #2

After t₁, the effective pixel processing rate is the maximum pixel processing/delivery rate until the last pixel of the image. The time interval (t₃ - t₂) is the "latency"; it depends only on P₁ and on the maximum processing/delivery rate.

For the estimation of the latency

The following board characteristics are be considered:

P₁ is the amount of pixel that must be received from the camera before initiating the pixel processing. This amount is equal to the number of pixels contain into:
- Two lines of the image if the Bayer CFA decoder is used.
- One line of the image if the Bayer CFA decoder is not used.
The maximum processing/delivery pixel rate is the smallest value of :
- The pixel processing rate.
- The pixel delivery rate over the PCIe interface.

For the pixel processing rate, the following processing functions may limit the processing rate if they are used:
- For the Bayer CFA decoder performances, refer to Bayer CFA to RGB Conversion for more details
- For LUT processor performances, refer to Look-up Table Transformation for more details.
- For the pixel delivery rate, refer to Image Transfer for an estimation of the data transfer rate and divide by the number of bytes/pixel to obtain an evaluation in pixels/s.