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If VGA (video graphics array) is analog, then how is it compatible with a digital monitor? When I say "digital monitor", I am referring to TFT LCD display monitor.
My guess is that maybe it carries an analog-modulated digital signal.
If not, then how is it analog? If you search on google it always says VGA is analog.
The actual video pins on a VGA connector are red, green, blue, horizontal sync, and vertical sync (there are some digital monitor ID pins, but they don't matter here). Sync is pulses (per line and per field) and RGB are simply analog voltage levels, 0 for black through 0.7V for full intensity. There's no bits being modulated onto anything.
An analog monitor uses those signals directly. A digital monitor looks at the timing of the sync signals to figure out what resolution it's dealing with, then sets up some analog-to-digital converters (ADCs) to sample the voltages of the RGB lines once per pixel. That digital pixel data gets sent to the display controller (LCD or whatever).
You may have noticed that sometimes with those monitors, things are a little bit fuzzy or "pixelly", or wiggle around for a few seconds after turning on, and that you can improve the quality using width or "phase" adjustments on the monitor — this is making slight corrections to the sample timing of the ADCs so that they sample the "best part" of each pixel instead of sampling during the transition from one pixel to another.
VGA signals are analog RGB. They can be digitized in the monitor and processed digitally, then presented to the display, or used directly on a CRT.
What monitors digitize the signal?
Later-generation CRT television sets digitize the composite signal and process it into RGB for the display. If they also support RGB input, that tends to bypass the TV decoding as analog.
All OLED, LCD, plasma TVs and monitors digitize their VGA input and process in digital to the display.
All DLP projectors digitize VGA input and work in digital internally (PWM signals modulate the micro mirrors.)
That leaves only CRT monitors, which use VGA analog with minimal processing to drive the display guns.
Are there CRT monitors that digitize? I’ve never encountered one, though they might exist. DVI supplanted VGA about the same time flat panels supplanted CRT (late 2001 - early 2002), so there wasn’t significant adoption of DVI in CRTs. (Note some versions of DVI also support analog VGA.)
If VGA (video graphics array) is analog, then how is it compatible with a digital monitor?
It's not "compatible" by any meaning of the word. (I'd even say: a signal is never compatible with a device; a device might be able to deal with a signal). The digital monitor has an sampling input converter (an ADC) that converts the 3 analog pixel brightness signals (R, G and B) to 3 digitized sample streams, these get combined into pixels, which then are used to fill a sample buffer that is used to address the TFT matrix.
My guess is that maybe it carries an analog-modulated digital signal.
no, not in the way you mean it: there's no "image bits" exchanged between the graphics card and the screen, but three analog intensity signals. It's very archaic.
That's why VGA was basically a legacy solution since the late 1990s. Converting a digital data stream into the intensities in the device that actually contains the display, be it a cathode ray tube or an LCD screen, a plasma screen or an OLED matrix, makes more sense, but was prohibitively expensive for consumer screens until the early / mid 1990s. It's really a surprise you still find VGA, although rarely, on modern new devices, and a testament how formative the 1980s were for computing and displays.
