The base unit is basically a 4 channel IP camera (like an encoder) and the camera heads are (my assumption) serially streaming raw data from the imaging head to the ISP/DSP. Maybe I2C?
HDcctv is designed such that third party encoders / recorders can work with different cameras / imagers.
Axis F series is Axis closed / only. You cannot attach the 'heads' / imagers' to anything else but Axis.
Tedor- Just as a technical comparison, Hikvision has a dual head covert camera with separate Main and Sensor modules. Ours have a 2m and 8m cable options. I asked my R&D if we used I2C (I Squared C) serial bus between sensor and main unit. It is not I2C but a different type of serial data bus. i suspect Axis is the same but I will defer to an Axis rep at this point.
I don't think this unit in comparable to the hdcctv style products at all.
Almost every camera has some cable between the image sensor and the processing/encoding board. For most cameras, this cable is very short because the desire is to keep all the parts inside of a single small enclosure that can be sealed from dust, elements, etc.
For this camera, Axis targets a small niche, customers who want to mount camera sensors in unique enclosures or areas. This could be for covert purposes, for aesthtic purposes, or other reasons. But for those cases the desire is to have an image sensor in the smallest possible form factor.
Hdcctc products generally exist to reuse coax cables (though there are options for twisted pair), and they all try to emphasize the max cable length they can support. They also usually connect back to a DVR, which handles encoding, viewing, storage and so forth. The Axis camera base really would need a VMS in most cases (yes, I'm sure it has a simple web interface and maybe an SD card slot, but it wouldn't be a "full" system for most uses).
Interesting enough this may not be a merely academic question of "what's in the wire" as whether it's analog or digital might impact how many licenses one must buy.