Video data is typically transmitted in contiguous bursts. Each burst comprises active pixel data. This data is transmitted in contiguous clock cycles which can be followed by clock cycles of no active data. These cycles of “no data” are called blanking periods. There are horizontal blanking periods which occur during each between video lines, and vertical blanking periods that equate to full video lines with no active pixel data at all.
To a memory subsystem, this translates to periods of bursts of video data the size of the active video frame size followed by burst gaps the length of the video blanking period. Therefore, for a given video frame, there are periods that require a certain peak bandwidth, or BW peak , followed by quiescent periods of no data transmittal. This equates to a peak bandwidth requirement, or BW peak .
BW peak is calculated from the data width, or bits-per-pixel (bpp), and from the video pixel clock frequency, Fvid. F vid can be calculated from the video frame rate (F frame ) measured in frames-per-second, the number of lines-per-frame (including blanking lines) and the number of pixel clock-cycles-per-line (including blanking clock cycles), shown in This Equation .
Equation 2-1 F vid = F frame * N full lines * N pixels
The BW peak is calculated by multiplying the Video Pixel clock frequency by the number of bits-per pixel, shown in This Equation .
Equation 2-2 BW peak = F vid * bpp
The average bandwidth requirement is defined as the overall number of bits within a frame over a one entire times the frame rate video frame period (not just during the bursts). This is the average bandwidth and is always lower than the peak bandwidth requirement. For a given video frame period, the average bandwidth is B Wave . This is shown in This Equation .
Equation 2-3 F ave = F frame * N active lines * N active pixels
The B Wave is calculated the same as BW peak by multiplying the Video Pixel clock frequency by the number of bits-per pixel. This is shown in This Equation .
Equation 2-4 BW ave = F ave * bpp
It is important to keep the BW peak and B Wave in mind when designing video subsystems, as these numbers define the clock frequencies and data width of the video IP core(s) and of the memory subsystem.