interlace

Interlace is a video transmission method that splits each complete image into two fields: one containing all odd-numbered horizontal lines, the other containing all even-numbered lines. These fields are sent sequentially at twice the line rate, so a viewer sees a complete frame reformed on their screen. For a 50 Hz broadcast standard, the receiver actually draws 50 odd fields and 50 even fields per second, creating the illusion of a full 25 Hz interlaced picture.

The core advantage is bandwidth efficiency. A 576-line interlaced signal (PAL standard) transmits half the data per field compared to a progressive 576-line signal at the same frame rate. This made interlace essential when broadcast channels were constrained and CRT phosphor persistence could actually exploit the technique: the glow from each field remained visible long enough for the human eye to perceive a flicker-free image at lower refresh rates than progressive scanning would require.

Interlace was the broadcast standard for television from the 1930s onward, embedded in NTSC (525 lines, 59.94 Hz), PAL (625 lines, 50 Hz), and SECAM systems. CRT monitors and televisions were built around this timing; the electron beam naturally scanned odd lines, then even lines, in a single pass down the screen. Analog videotape formats, composite video signals, and cable transmission all relied on interlaced fields as the fundamental unit.

Interlace artifacts and modern rejection

Interlace creates visible defects when video contains motion or fine detail. A moving object appears to have horizontal serration or combing because the odd and even fields were captured at different moments in time. Diagonal lines and thin horizontal patterns can flicker or alias. These artefacts became unacceptable once storage and bandwidth constraints eased and LCD, plasma, and digital projection displays replaced CRTs, since these devices require a complete progressive frame to function properly.

Modern standards, HDTV, streaming protocols, and digital cinema, abandoned interlace almost entirely in favor of progressive scanning. Conversion from interlaced source material to progressive video (deinterlacing) requires motion estimation or interpolation and remains a standard post-production step in broadcast-to-digital workflows. Some regions still transmit interlaced signals for backward compatibility with older receivers, but new video infrastructure is uniformly progressive.