Video Signal Timing Calculation Reference

Timing Parameter Table

Format	Pixel Clock (MHz)	BT656 Clock (MHz)	Hactive (pixel)	Vactive	Htotal (pixel)	Hblank (µs)
720P25_36M (AHD)	36	72	1280	720	1920	17.78
720P30_36M (AHD)	36	72	1280	720	1600	8.89
720P25_37.125M (AHD/TVI)	37.125	74.25	1280	720	1980	18.86
720P30_37.125M (AHD/TVI)	37.125	74.25	1280	720	1650	9.97
1080P25 (AHD/TVI)	74.25	148.5	1920	1080	2640	9.70
1080P30 (AHD/TVI)	74.25	148.5	1920	1080	2200	3.77
CVBS PAL	17.734	—	720	576	864	8.12
CVBS NTSC	14.318	—	720	480	858	9.64

BT656 Clock = 2× Pixel Clock（DDR 采样，每个像素时钟沿采 Cb 或 Cr）

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Calculation Method

Key Parameters

Parameter	Symbol	Unit
Pixel Clock	f_pix	MHz
Frame Rate	fps	frames/s
Horizontal Active Pixels	Hactive	pixels
Vertical Active Lines	Vactive	lines
Vertical Blank	Vblank	lines
Vertical Total	Vtotal = Vactive + Vblank	lines
Horizontal Total	Htotal	pixels
Horizontal Blank Time	Hblank	µs

Calculation Steps

Step 1 — Determine Vtotal

Vtotal = Vactive + Vblank

Vblank 是标准规定值（标清/高清格式固定），不能自行更改。
例：720P → Vblank = 30 lines → Vtotal = 720 + 30 = 750 lines

Step 2 — Calculate Line Period (time per line)

T_line = 1 / (Vtotal × fps)   [seconds]
       = 1,000,000 / (Vtotal × fps)   [µs]

Step 3 — Calculate Htotal

Htotal = T_line × f_pix   [pixels]
       = f_pix (MHz) × T_line (µs)

Step 4 — Calculate Hblank

Hblank_pixels = Htotal − Hactive
Hblank_time   = Hblank_pixels / f_pix   [µs]

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Worked Example：720P25 @ 36 MHz Pixel Clock

Vactive  = 720 lines
Vblank   = 30 lines  (standard)
Vtotal   = 720 + 30  = 750 lines

Total lines/s = 750 × 25 = 18,750 lines/s
T_line        = 1 / 18,750 = 53.33 µs

Htotal        = 53.33 µs × 36 MHz = 1920 pixels
Hblank        = 1920 − 1280 = 640 pixels
Hblank_time   = 640 / 36 MHz = 17.78 µs  ✓

Same Resolution with 37.125 MHz Pixel Clock

Htotal        = 53.33 µs × 37.125 MHz = 1980 pixels
Hblank        = 1980 − 1280 = 700 pixels
Hblank_time   = 700 / 37.125 MHz = 18.86 µs  ✓

注意：AHD 标准 Pixel Clock 为 36 MHz；TVI 为 37.125 MHz。
MDR-742 使用的 ARK7118 SoC 输出 37.125 MHz，而部分 AHD 接收端期望 36 MHz，
导致 PLL 锁定失败（pixel clock mismatch 根因）。

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Standard Pixel Clocks

Standard	Pixel Clock	BT656 Clock	Applicable Formats
AHD	36 MHz	72 MHz	720P25 / 720P30
AHD / TVI	37.125 MHz	74.25 MHz	720P25 / 720P30 / 1080P25 / 1080P30
CVBS PAL	17.734 MHz	—	576i
CVBS NTSC	14.318 MHz	—	480i

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Quick Formula Summary

Vtotal      = Vactive + Vblank
T_line [µs] = 1,000,000 / (Vtotal × fps)
Htotal      = T_line [µs] × f_pix [MHz]
Hblank [µs] = (Htotal − Hactive) / f_pix [MHz]