Classic Black & White TV

Previous: Understanding NTSC


At the start of broadcast TV in the U.S., broadcasts were entirely in black and white (or, if you prefer, greyscale (or, if you prefer, grayscale)).

A Scanline

Here is a diagram of a single visible scanline from a black & white signal:

FP HSync BP Visible FP

Coarsely, a scanline can be considered as having two sections: the blanking interval (the first three regions in the diagram) and the visible portion of the scanline, which is the large portion conspicuously labeled "Visible."

The Blanking Interval

The first region (labeled FP in the diagram) is the front porch. This is the start of the blanking interval, and it rests at the blanking level (the upper voltage of the blanking interval) for a short bit to let voltage levels settle to a known value after the visible portion ends before the sync truly begins.

That occurs at the horizontal sync pulse (HSync): a drop in voltage telling the CRT to send the electron beam sailing back to the left side of the screen to start the scanline.

After the sync pulse ends is the back porch (BP), a return to the blanking level to once again allow voltage to stabilize, plus give the electron beam enough time to complete its transit from right to left so it's ready when the visible portion of the scanline arrives.

The Visible Portion

After the blanking interval ends, the visible portion of the scanline begins: a part of the picture that you see on-screen.

In black & white TV, this is merely a luminosity value ranging from the black level to the white level (canonically the black level is ~53mV above the blanking level and the white level is ~714mV above that). Basically, the voltage at any given point along this span corresponds directly to the brightness that is visible on-screen at that position, starting on the left and ending on the right. Lower voltages are darker, higher voltages are brighter.

Note that this is a purely analog signal - there are no pixels along a scanline (hadn't been invented yet). Instead, it's a smooth analog signal that changes between bright and dark as needed to display the intended picture.

When the visible portion of the scanline ends, the signal drops back down to the blanking level and the front porch of the next scanline begins.

Fields and Frames

Eventually, the field runs out of visible scanlines (it reached the bottom of the picture), and the visible portion of the field ends. At this point, the next field begins with a series of non-visible scanlines called the vertical blanking interval.

Next: Vertical Sync and Interlacing