While that still holds true -- the tuner being in the TV -- the tuner within the TV is not always used when watching TV. This is a fine point that is easy to overlook.
Digital signals from cable and satellite companies are scrambled and require a tuner provided by the cable or satellite company in order to see them. In contrast, digital TV signals from broadcast TV stations are not encrypted and can be processed by your TV tuner.
The reason of this difference in signal delivery by cable, satellite and broadcast TV is because the cable and satellite industry is not required to follow government broadcast policy. Cable and satellite is in the private sector.
Where Is the Digital TV Tuner?
When watching broadcast digital TV signals on an analog TV the digital TV tuner is in the DTV converter box.
When watching broadcast digital TV signals on a digital or high definition television (HDTV) then the digital tuner is inside the TV. The only exception would be if your digital TV was actually a digital monitor -- there is a difference.
For cable and satellite subscribers, the digital TV tuner is the set-top box that your provider gave you unless you are one of the few people that use a CableCard. Then the tuner is the CableCard.