HSYNC
The horizontal synchronization signal incorporated in a video transmission, such as for a TV broadcast.
HSYNC: the pulse that keeps raster lines in step
HSYNC is a timing pulse embedded in analog video signals that tells a display receiver when to start drawing each horizontal line of the image. In a television broadcast or composite video feed, HSYNC occurs at the end of each active video line and triggers the electron beam (or modern pixel clock) to return to the left edge of the screen and begin the next line. Without HSYNC, the image would drift horizontally and become unusable.
The signal itself is a low-voltage pulse, typically around 0.3 volts in amplitude, that sits in the blanking interval between active video lines. HSYNC must occur at precise intervals: in NTSC video, the horizontal line rate is 15,734 Hz (one pulse every 63.5 microseconds), while PAL runs at 15,625 Hz. These rates are locked to the vertical refresh frequency to maintain frame stability.
HSYNC works alongside VSYNC, the vertical synchronization pulse that triggers a return to the top of the screen. Together, they form the composite timing reference that governs raster-scan displays. In modern digital video interfaces like HDMI or DisplayPort, HSYNC information is embedded differently (often as part of timing metadata rather than a separate analog pulse), but the functional requirement remains identical.
Problems with HSYNC manifest as horizontal drift or skew in the displayed image. A weak, noisy, or misaligned HSYNC signal causes the receiver to lose lock, particularly in old CRT monitors or broadcast equipment. Poor cable shielding, ground loops, and long transmission distances over analog lines can degrade HSYNC integrity. Test equipment like waveform monitors display HSYNC as a discrete pulse in the blanking region, allowing technicians to verify signal quality.
The term originates from the mechanical requirement of scanning electron beams across a phosphor screen at regular, synchronized intervals. Although CRT technology has largely been replaced, HSYNC terminology persists in video standards documents and equipment specifications, and the underlying synchronization principle remains fundamental to all raster displays.