CODEWITHSUNDEEP
hdmi
Tai Chau
Tai Chau

Reputation: 33

Why 24.5 MHz pixel clock needs to achieve 60 frames per seconds?

When I read one HDMI document for FPGA one fpga4fun website: https://www.fpga4fun.com/HDMI.html

I see this paragraph:

Let's create a 640x480 RGB 24bpp @ 60Hz video signal. That's 307200 pixels per frame, and since each pixel has 24 bits (8 bits for red, green and blue), at 60Hz, the HDMI link transports 0.44Gbps of "useful" data.

But video signals usually also have an "off-screen" area, which is used by the HDMI receiver (TV or monitor) for housekeeping. Our 640x480 frame is actually sent as an 800x525 frame.

With that in mind, we need a 24.5MHz pixel clock to achieve 60 frames per seconds, but HDMI specifies a 25MHz minimum pixel clock, so that's we use (which gets us a 61Hz frame rate).

In this paragraph, it said that "We need a 24.5MHz pixel clock to achieve 60 frames per second". Why 60 frames are considered for HDMI, but not any other frames, and how 24.5MHz can be calculated?

Thank you

Upvotes: -3

Views: 496

Answers (1)

gatecat
gatecat

Reputation: 1186

I'm not sure where 24.5MHz comes from either...

60fps is the framerate of the video standard considered here. Anything else might not be accepted by the sink device.

Likewise, the paragraph mentions that the frame size with blanking and sync is 800x525.

Therefore the pixel clock is 60×800×525 = 25200000 = 25.2MHz...

Upvotes: 4

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