When did the digital revolution begin?
It finds its roots in the 1960s but did not truly blossom until the 1990s.
The concept of digital comes from binary math, a system whereby all forms of math are executed using only 1s and 0s. The first computer goes back to the late 1940s and used binary math, hence digital. Those were known as analog computers because they AC current rising and falling as a basis for computation.
In the 1950s the transistor was invented but that was still the analog universe.
The first ICs (integrated circuits - chips) were invented in the late 1960s and Hewlett Packard used one in its first calculator. This was a giant leap from the slide rule which had been what engineers had all used prior to that first calculator.
The true digital age came in the late 1970s when the Motorola 6800 microprocessor was invented. Apple seized upon it to invent its Apple Computer. But it was not alone, Zenith created the Z-80 chip to compete. The chip was used in both Zenith and Radio Shack computers. Texas Instruments also invented a microprocessor the TMS-8800. All of these predated the Intel 8080.
When powerhouse IBM took the Intel 8080 on as its processor the game was basically over for all other computer companies save Apple. The 8080 was slower than Apple's 6800 and less powerful but the difference came in "architecture." Apple had proprietary software which ran its CPU and would not allow developers access to it. Conversely, Intel and IBM developed an "open architecture" which gave rise to most, if not all of today's software.
And so the simple answer is, 1981, when IBM introduced its PC-XT the war was on.