Video Signals And Timing

From OSDev.wiki
Revision as of 22:25, 3 April 2009 by Combuster (talk | contribs) (Initial dump of Brendan's posts)
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to navigation Jump to search

Work in progress. Is currently a dump of Brendan's worked out formulae

Brendan's Posts

>Any problems if I wikify this? None at all - IMHO this information is required just to use VBE reliably; along with EDID (but you can find version 1.1 of the EDID data structures on Wikipedia) and details for the VBE DDC function (which VESA will let you have for free).

However, I should point out that this information is incomplete - the value of certain constants are missing from my original post. The default values for these constants are: CELL_GRAN_RND = 8, MARGIN_PRECENT = 1.8, MIN_PORCH_RND = 1, MIN_V_SYNC_AND_BACK_PORCH = 550, C_PRIME = 40 and M_PRIME = 600. I'd also mention that for my version of the formulas REFRESH_RATE_REQUIRED, V_FIELD_RATE_REQUIRED, H_FREQ_REQUIRED, PIXEL_FREQ_REQUIRED, V_FIELD_RATE, H_FREQ, PIXEL_FREQ are all in Hz (and not a mixture of Hz, KHz and MHz), which helps to simplify the formulas.

There's also some strange things that can happen with these formulas. For example, if any of the calculations give a negative result then it indicates invalid input (e.g. if the PIXEL_FREQ_REQUIRED input parameter is too low, then you can end up with horizontal blanking that takes "negative N us"). In practice, some of these calculations probably have an allowed range (e.g. things like "horizontal blanking must be >= 20% of the horizontal total") but I've been unable to find anything indicating what the allowed ranges for anything is. :(

This is for my own reference, and in case anyone is wondering what they look like... The GTF formulas.

There's actually 3 separate calculations here - finding all timing parameters from X & Y resolution and desired refresh rate, finding all timing parameters from X & Y resolution and desired horizontal frequency, and finding all timing parameters from X & Y resolution and desired pixel clock frequency.

GTF Formulas For Using Desired Refresh Rate

H_PIXELS_RND = ( ROUND ( H_PIXELS / CELL_GRAN_RND ) ) * CELL_GRAN_RND

IF ( INTERLACE_REQUIRED == "y") {
    V_LINES_RND = ROUND ( V_LINES / 2 )
    V_FIELD_RATE_REQUIRED = REFRESH_RATE_REQUIRED * 2
    INTERLACE = 0.5
  } else {
    V_LINES_RND = ROUND ( V_LINES) )
    V_FIELD_RATE_REQUIRED = REFRESH_RATE_REQUIRED
    INTERLACE = 0
  }

IF ( MARGINS_REQUIRED == "y" ) {
    TOP_MARGIN_LINES = ROUND ( MARGIN_PRECENT / 100 * V_LINES_RND )
    BOTTOM_MARGIN_LINES = ROUND ( MARGIN_PRECENT / 100 * V_LINES_RND )
    LEFT_MARGIN_PIXELS = ( ROUND ( ( H_PIXELS_RND * MARGIN_PRECENT / 100 / CELL_GRAN_RND ) , 0 ) ) * CELL_GRAN_RND
    RIGHT_MARGIN_PIXELS = ( ROUND ( ( H_PIXELS_RND * MARGIN_PRECENT / 100 / CELL_GRAN_RND ) , 0 ) ) * CELL_GRAN_RND
  } else {
    TOP_MARGIN_LINES = 0
    BOTTOM_MARGIN_LINES = 0
    LEFT_MARGIN_PIXELS = 0
    RIGHT_MARGIN_PIXELS = 0
  }

H_PERIOD_ESTIMATE = ( 1 / V_FIELD_RATE_REQUIRED - MIN_V_SYNC_AND_BACK_PORCH / 1000000 ) / ( V_LINES_RND + 2 * TOP_MARGIN_LINES + MIN_PORCH_RND + INTERLACE ) * 1000000

V_SYNC_AND_BACK_PORCH = ROUND ( MIN_V_SYNC_AND_BACK_PORCH / H_PERIOD_ESTIMATE )

V_BACK_PORCH = V_SYNC_AND_BACK_PORCH - V_SYNC_RND

TOTAL_V_LINES = V_LINES_RND + TOP_MARGIN_LINES + BOTTOM_MARGIN_LINES + V_SYNC_AND_BACK_PORCH + INTERLACE + MIN_PORCH_RND

V_FIELD_RATE_ESTIMATE = 1000000 / H_PERIOD_ESTIMATE / TOTAL_V_LINES

H_PERIOD = H_PERIOD_ESTIMATE * V_FIELD_RATE_ESTIMATE / V_FIELD_RATE_REQUIRED

V_FIELD_RATE = 1000000 / H_PERIOD / TOTAL_V_LINES

IF ( INTERLACE_REQUIRED == "y") {
    V_FRAME_RATE = V_FIELD_RATE / 2
  } else {
    V_FRAME_RATE = V_FIELD_RATE
  }

TOTAL_ACTIVE_PIXELS = H_PIXELS_RND + LEFT_MARGIN_PIXELS + RIGHT_MARGIN_PIXELS

IDEAL_DUTY_CYCLE = C_PRIME - M_PRIME * H_PERIOD / 1000

H_BLANK_PIXELS = ( ROUND ( ( TOTAL_ACTIVE_PIXELS * IDEAL_DUTY_CYCLE / ( 100 - IDEAL_DUTY_CYCLE ) / ( 2 * CELL_GRAN_RND ) ) ) ) * 2 * CELL_GRAN_RND

TOTAL_PIXELS = TOTAL_ACTIVE_PIXELS + H_BLANK_PIXELS

PIXEL_FREQ = TOTAL_PIXELS / H_PERIOD * 1000000

H_FREQ = 1 / H_PERIOD


GTF Formulas For Using Horizontal Frequency

H_PIXELS_RND = ( ROUND ( H_PIXELS / CELL_GRAN_RND ) ) * CELL_GRAN_RND

IF ( INTERLACE_REQUIRED == "y") {
    V_LINES_RND = ROUND ( V_LINES / 2 )
    INTERLACE = 0.5
  } else {
    V_LINES_RND = ROUND ( V_LINES) )
    INTERLACE = 0
  }

IF ( MARGINS_REQUIRED == "y" ) {
    TOP_MARGIN_LINES = ROUND ( MARGIN_PRECENT / 100 * V_LINES_RND )
    BOTTOM_MARGIN_LINES = ROUND ( MARGIN_PRECENT / 100 * V_LINES_RND )
    LEFT_MARGIN_PIXELS = ( ROUND ( ( H_PIXELS_RND * MARGIN_PRECENT / 100 / CELL_GRAN_RND ) , 0 ) ) * CELL_GRAN_RND
    RIGHT_MARGIN_PIXELS = ( ROUND ( ( H_PIXELS_RND * MARGIN_PRECENT / 100 / CELL_GRAN_RND ) , 0 ) ) * CELL_GRAN_RND
  } else {
    TOP_MARGIN_LINES = 0
    BOTTOM_MARGIN_LINES = 0
    LEFT_MARGIN_PIXELS = 0
    RIGHT_MARGIN_PIXELS = 0
  }

H_FREQ = H_FREQ_REQUIRED

V_SYNC_AND_BACK_PORCH = ROUND ( MIN_V_SYNC_AND_BACK_PORCH * H_FREQ / 1000000 )

V_BACK_PORCH = V_SYNC_AND_BACK_PORCH - V_SYNC_RND

TOTAL_V_LINES = V_LINES_RND + TOP_MARGIN_LINES + BOTTOM_MARGIN_LINES + INTERLACE + V_SYNC_AND_BACK_PORCH + MIN_PORCH_RND

V_FIELD_RATE = H_FREQ / TOTAL_V_LINES

IF ( INTERLACE_REQUIRED == "y") {
    V_FRAME_RATE = V_FIELD_RATE / 2
  } else {
    V_FRAME_RATE = V_FIELD_RATE
  }

TOTAL_ACTIVE_PIXELS = H_PIXELS_RND + RIGHT_MARGIN_PIXELS + LEFT_MARGIN_PIXELS

IDEAL_DUTY_CYCLE = C_PRIME - ( M_PRIME / H_FREQ )

H_BLANK_PIXELS = ( ROUND ( TOTAL_ACTIVE_PIXELS * IDEAL_DUTY_CYCLE / ( 100 - IDEAL_DUTY_CYCLE ) / ( 2 * CELL_GRAN_RND ) ) ) * 2 * CELL_GRAN_RND

TOTAL_PIXELS = TOTAL_ACTIVE_PIXELS + H_BLANK_PIXELS

H_PERIOD = 1 / H_FREQ

PIXEL_FREQ = TOTAL_PIXELS * H_FREQ

GTF Formulas For Using Pixel Clock Frequency

H_PIXELS_RND = ( ROUND ( H_PIXELS / CELL_GRAN_RND ) ) * CELL_GRAN_RND

IF ( INTERLACE_REQUIRED == "y") {
    V_LINES_RND = ROUND ( V_LINES / 2 )
    INTERLACE = 0.5
  } else {
    V_LINES_RND = ROUND ( V_LINES) )
    INTERLACE = 0
  }

PIXEL_FREQ = PIXEL_FREQ_REQUIRED

IF ( MARGINS_REQUIRED == "y" ) {
    TOP_MARGIN_LINES = ROUND ( MARGIN_PRECENT / 100 * V_LINES_RND )
    BOTTOM_MARGIN_LINES = ROUND ( MARGIN_PRECENT / 100 * V_LINES_RND )
    LEFT_MARGIN_PIXELS = ( ROUND ( ( H_PIXELS_RND * MARGIN_PRECENT / 100 / CELL_GRAN_RND ) , 0 ) ) * CELL_GRAN_RND
    RIGHT_MARGIN_PIXELS = ( ROUND ( ( H_PIXELS_RND * MARGIN_PRECENT / 100 / CELL_GRAN_RND ) , 0 ) ) * CELL_GRAN_RND
  } else {
    TOP_MARGIN_LINES = 0
    BOTTOM_MARGIN_LINES = 0
    LEFT_MARGIN_PIXELS = 0
    RIGHT_MARGIN_PIXELS = 0
  }

TOTAL_ACTIVE_PIXELS = H_PIXELS_RND + RIGHT_MARGIN_PIXELS + LEFT_MARGIN_PIXELS

IDEAL_H_PERIOD = ( ( C_PRIME - 100 ) + ( SQRT ( ( ( 100 - C_PRIME ) ^ 2 ) + ( 0.4 * M_PRIME * ( TOTAL_ACTIVE_PIXELS + RIGHT_MARGIN_PIXELS + LEFT_MARGIN_PIXELS ) / PIXEL_FREQ / 1000000 ) ) ) ) / 2 / M_PRIME * 1000

IDEAL_DUTY_CYCLE = C_PRIME - ( M_PRIME * IDEAL_H_PERIOD / 1000 )

H_BLANK_PIXELS = ( ROUND ( TOTAL_ACTIVE_PIXELS * IDEAL_DUTY_CYCLE / ( 100 - IDEAL_DUTY_CYCLE ) / ( 2 * CELL_GRAN_RND ) ) ) * 2 * CELL_GRAN_RND

TOTAL_PIXELS = TOTAL_ACTIVE_PIXELS + H_BLANK_PIXELS

H_FREQ = PIXEL_FREQ / TOTAL_PIXELS

H_PERIOD = 1 / H_FREQ

V_SYNC_AND_BACK_PORCH = ROUND ( MIN_V_SYNC_AND_BACK_PORCH * H_FREQ / 1000000 )

V_BACK_PORCH = V_SYNC_AND_BACK_PORCH - V_SYNC_RND

TOTAL_V_LINES = V_LINES_RND + TOP_MARGIN_LINES + BOTTOM_MARGIN_LINES + INTERLACE + V_SYNC_AND_BACK_PORCH + MIN_PORCH_RND

V_FIELD_RATE = H_FREQ / TOTAL_V_LINES

IF ( INTERLACE_REQUIRED == "y") {
    V_FRAME_RATE = V_FIELD_RATE / 2
  } else {
    V_FRAME_RATE = V_FIELD_RATE
  }

Note: The "round()" function here rounds to the nearest integer (it doesn't round down or round up).

Cheers,

Brendan

Retrieved from "https://osdev.wiki/wiki/Video_Signals_And_Timing?oldid=7749"