First we need to cover a bit of background on what an LNB actually does. LNB stands for Low Noise Block Downconvertor and sometimes they are just called Downconvertors. The word downconvertor conveys what an LNB actually does - it converts high frequencies down to lower frequencies. This 'downconversion' is required to enable a satellite receiver to decode the signal into video and audio. The frequencies transmitted by a satellite are too high to be decoded by a set-top box.
Satellite receivers are capable of receiving a number of different signal bands and a 'universal' LNB has the ability to receive all of these. It is normal for a universal LNB to receive the following bands:
The quad LNB provides four outputs, each capable of providing each of the four frequency bands and polarisations. The LNB outputs are switched by the satellite receiver, as explained above, with a 22Hz tone and a switching voltage. This is like a universal LNB with four outputs.
In a simple residential satellite TV installation it is normal to install a quad LNB. In a Sky installation, often two of the outputs from a quad LNB will be used to feed a Sky+ receiver (it has two feeds so you can record while watching live TV), with either the other two feeding a second Sky+ receiver or just a single output feeding a regular Sky box.
A quad LNB is ideal where a maximum of four satellite feeds is required and there is not much possibility of expansion.
Attention: Even though the QUAD LNB has 4 outputs one will be used to track the satellite, so you will pratically have 3 outputs to connect to a receiver.
The quattro LNB provides four outputs, each with a single band/polarization specific to that output. The band/polarization available at each output is fixed at the time of manufacture. It is not possible to switch the band/polarization with a tone or switching voltage.
A quattro LNB is normally used to feed a large distribution system. A single dish with a quattro LNB fitted and a multiswitch (or IRS) is able to feed anything between 4 and 100 locations. The multiswitch (or IRS) is able to provide any of the input signals at each output, depending on a tone/voltage sent up the cable by the receiver at each location. It's as though the receiver at each location has it's own dedicated LNB.
The outputs on a multiswitch (or IRS) are able to be controlled with a 22Hz tone and switching voltages in the same way that the outputs from normal universal LNB can be, i.e. to change band and polarisation. The satellite receivers in each of the locations behave as though they are connected to their own dedicated dish and LNB, but in reality are just connected to a multiswitch (or IRS) output.
Quattro LNBs are considered to have a better lifetime than quad LNBs as they are not being continually switched.