Digital video is a bit unspecified. Assume an existing MPEG file that you want to convert to a constant rate bit-stream, TS, and up-convert to a frequency suitable for transmitting. Is that what you want to do?
Should it also include a live stream such as from a camera and some kind of compressing format will it be a bit more complex. There is no such thing as a single chip solution, such as for a FM sound transmitter.
Digital coded high resolution pictures at high frame rate cost enormous bandwidth if transmitted in uncompressed format.
To reduce the need for bandwidth is both compression protocol and more complex modulation methods selected compared to analog video. Composite video/PAL/NTSC is much simpler to transmit, all that is needed is a RF oscillator that can be modulated with with the analog video stream. At least in its simplest form.
A cheap digital alternative is to buy
this USB dongle. It is still a lot more complex to use then corresponding TV-receiver to handle.
If it is a low resolution camera or if it is a very slow frame rate is maybe a compressing protocol not needed to keep bandwidth reasonable narrow.
Check if amateur DTV formats or slow scan TV fits your need better, is it rather easy to find schematics at amateur homepages.
A bit different solution for transmitting digital video is
this, as it can use an existing PC as only required hardware. It is a rather old example, with a modern PC/graphic card (x10 better performance) is it probably possible send several channels of live video.