How to determine if a video signal is interlaced

Question

I know what the difference between interlaced and progressive scan is: in progressive video modes, the monitor displays all the lines of a video field sequentially. In interlaced, it displays them on alternate lines, then displays a second video field offset downwards slightly on the other alternate lines.

The question is, how does the display circuitry know which approach to use? What characteristic of the signal says "interlaced"? Is this carried on the sync pulses somehow?

The context may be VGA display modes, or computer video output using composite or RGB signals to a CRT monitor.

Answer 1

This comes up with digital working - flat panels, frame grabbers and the like. These systems that deal with interlace have to do the right thing with the scan line data to reconstruct full frames. On the other hand, analog CRTs just 'figure it out' (by design actually) and make the correct raster for interlace.

How?

Say you're tasked with capturing video and so have to worry about this. The vertical sync vs. horizontal sync timing gives a clue to the capture system how to deal with the input.

Succinctly,

• interlace: the 'second' field vsync starts in the middle of a line
• progressive: vsync always aligns with or near h-sync

1080i50 example:

From here: https://e2e.ti.com/support/processors-group/processors/f/processors-forum/430417/dm8148-1080i-interlace-video-capture-with-vps_vip_fid_detect_mode_vsync-mode

Analog video with embedded sync (composite, component or sync-on-green RGB) requires a sync separator block that teases out H / V sync and blanking give you digital H and V signals as well as blank. V sync specifically is detected by looking at the composite sync equalization / serration pulses.

VGA with TTL sync already sends H and V separately, but nonetheless can also support interlace or progressive in the same way as embedded sync, by modifying V sync timing on the second field.

Either way, once you have the digital H, V and H blank signals, you test for V sync changing state during the active (non-blank) interval. A very simple way to do this is to clock the H blank signal period with the leading edge of V sync. If interlace is in use, first field will clock low, second field will clock high. If the field flip-flop never toggles (that is, V sync always falls during blank), congratulations, you have progressive.

A now-obsolete chip, the LM1881, did this for you. Link: https://www.ti.com/lit/ds/symlink/lm1881.pdf In addition to H and V sync, it outputs a toggling field signal, that won't toggle if progressive is in use. The circuit diagram shows the basic clocked-flop principle for field detect.

Answer 2

The “magic” about interlacing is that it doesn't need any additional circuit.

In a CRT, the horizontal lines aren't really displayed horizontally. They are slanted so that the right end of a scanline is at the same vertical position as the left end of the next scanline. During the horizontal backporch, only the horizontal position is reset. The vertical position stays the same.

Now the trick. During the display of the last line, the vertical sync kicks in. Roughly in the middle of the last line. During the vertical backporch, only the vertical position is reset. The horizontal position stays the same.

Because the horizontal lines are slanted a bit, that means the first line of the odd field starts in the middle of a horizontal line, and also at a vertical position that is half a line height offset from the even field.

That's all. No additional circuit. Just timing. If you want progressive scan instead, do the vertical sync at the beginning of the last line instead. That's within the range of the acceptable vertical sync.

Answer 3

The sync signals tell the monitor how to scan the signal, so the CRT monitor does not really know or care if it is an interlaced signal, it just scans the signal as it comes in.

In an interlaced signal, the frame, or two interlaced fields, is an odd integer amount of lines, which means that each field does not have an integer amount of lines as there is additional one half of a line per field.

In other words, every other field starts at the same time as the horizontal line and every other field starts in the middle of a horizontal line.

What this means in practice is that when the CRT beam is deflected from up to down at constant velocity for each field, after the lines of first field have been drawn, the drawing of second field starts half a line later in time, so the lines of the second field are drawn half a line down, between the lines of the first field.

The above describes the typical 2:1 interlacing scenario, but nothing prevents from using other interlacing scenarios such as 3:1 but they are just not practical.

In a progressive signal, each field is integer amount of lines, so the field always starts at the same time as the horizontal line.

• SDTV 480i, 525 lines per frame, 262.5 lines per field
• VGA 480p, 525 lines per frame, 525 lines per field
• SDTV 576i, 625 lines per frame, 312.5 lines per field
• HDTV 1080i, 1125 lines per frame, 562.5 lines per field
• HDTV 1080p, 1125 lines per frame, 1125 lines per field
• XGA 1024x768i, 817 lines per frame, 408.5 lines per field