What Is the Horizontal Sync for 640x480 60Hz VGA?

The default horizontal synchronization (H-sync) frequency for an industry-standard 640x480 Video Graphics Array (VGA) signal running at 60 Hz is 31.469 kHz (commonly rounded to 31.5 kHz). This timing standard, established by IBM and formalized by VESA, dictates how rapidly an analog display beam sweeps across an individual scanline to produce a stable progressive image.

Understanding standard VGA timing requires examining the scanline structure beyond just the 640 visible pixels. A single horizontal scanline is divided into four distinct operational phases: the active video area, the front porch, the sync pulse, and the back porch.

For the classic 640x480 at 59.94 Hz (standard 60 Hz nominal) mode, the pixel clock operates at 25.175 MHz. The complete horizontal line takes 800 pixel clock cycles to complete:

• Active Display: 640 pixels (25.42 µs)
• Front Porch: 16 pixels (0.64 µs)
• Sync Pulse (H-Sync): 96 pixels (3.81 µs)
• Back Porch: 48 pixels (1.91 µs)

Totaling 800 pixel clocks per line at 25.175 MHz yields the horizontal scan rate calculation:

\[f_H = \frac{25.175\text{ MHz}}{800\text{ pixels}} \approx 31.46875\text{ kHz}\]

A complete frame consists of 525 total lines (480 visible active lines, a 10-line front porch, a 2-line vertical sync pulse, and a 33-line back porch). Dividing the 31.46875 kHz horizontal rate by 525 lines per frame results in the vertical refresh rate:

\[f_V = \frac{31468.75\text{ Hz}}{525\text{ lines}} \approx 59.94\text{ Hz}\]

Both the horizontal and vertical synchronization pulses for this specific resolution use negative polarity, meaning the pulse signals drop to a logic low state to trigger beam retrace. This 31.469 kHz frequency remains the fundamental baseline reference for vintage computing hardware, FPGA video generation, and analog CRT monitor compatibility.