Scanbox samples, transfers and processes one frame at a time. Frame processing involves correcting for non-uniform spatial sampling along the resonant axis, motion stabilization, real-time extraction of signals, displaying the images, streaming to disk, and so on. This overhead in frame processing can limit the maximum frame rate achievable if the computer cannot keep up with the data. One way to circumvent this issue is to “fold” a number of images within a single frame. By folding, I mean repeated sampling of the same area.
For example, in the picture below, the frame has 512 lines but the number of “fold lines” was set to 32. This means that Scanbox will acquire 512/32 = 16 subframes within an image. Each of the stripes in the image, from top to bottom, represent scans of the same area. The effective subframe rate in this case would be 15.6 fps * 16 = 248 subframes/sec. In bidirectional mode, that will result in near 500 subframes/sec. The computer is still processing 15.5 frames per second, so the overhead per frame is kept independent of the number of sub-frames defined. Note if you want an integer number of sub-frames you need the total number of lines (512 in this case) to be divisible by the number of fold lines (32 in this case).