Something relaxing - The fan settings tab

The first thing to understand is that RBE will display one of four different boxes here, depending on what fan controller your card uses. RBE will choose the right fan controller type automatically.
It's not so clear what cards use what controller. There is a pattern, but you'll never know what ideas video card vendors come up with. One thing is certain: If you are using a video card with a non-reference design with a non-reference cooler, it is most likely that this cooler is not controlled by the card's fan controller chip any more. Many of those coolers aren't controlled at all or control themselves using an internal heat sensor. Those cards will contain a fan controller chip anyway, because the controller is integrated into the GPU, mostly. However, changing settings here won't influence the cooler's behaviour, of course. Spoken for myself, I care of buying reference design cards only. Here I'm sure to have a not so bad cooler mostly which can be controlled by RBE.

Okay, back to the four fan control boxes mentioned. Let's discuss them one after the other:

1. This controller, the LM63 from national semiconductors, was used mostly for R600 based cards:
   
   
   
   It is fairly easy: You can enter (or just click and drag inside the graph) eight temperature thresholds and a fan duty for each threshold. If the temperature exceeds a threshold temperature, the fan duty for that threshold will take effect. If the temperature drops below a threshold minus hysteresis, the lower fan duty will kick in.
   
   That's what hysteresis is used for: Imagine the temperature switches between 69°C and 70°C constantly in this example. Without hysteresis, the fan duty would switch constantly as well, being very annoying. So hysteresis tries to avoid that and therefore, is a psychoacoustic feature. Be sure to use it.
   
   The graph can be switched to step function mode. This is only optical as it is more precise because the continuous graph suggests a continuous fan duty regulation which is not the case, as you've already read.
2. This controller is built in into all RV6xx GPUs. So it is used for 2400 XT, 2600 XT, 3850, 3870, 4870, also most 4850 cards and many other cards like 3650 and so on:
   
   
   
   It's a little more complicated, but not much. At first you have the choice of using the old look up table mode similar to the one known from the LM63.
   But this is not recommended because using the transfer function mode is way more powerful. You simply enter (or click and drag inside the graph) a Tmin value. Until this temperature is reached, the fan runs at fan duty cylce min only, which can also be changed.
   Beginning from Tmin, the fan duty rises continuously. The controller calculates for each temperature a fan duty dependent on a linear function. If the temperature exceeds Tmax, the fan will run at 100%.
   You can move the mouse cursor over the graph and see which temperature will result in which fan duty.
   
   Note that Tmin and Tmax impact the function's slope. Setting a higher Tmax or lower Tmin will result in a smaller slope and thus, a lower fan duty for a certain temperature (and vice versa).
   
   Hysteresis works similar to the one for the LM63, but it's measured not in °C any more but in %. The Tmin hysteresis is still measured in °C, but as the name suggests, it takes the job for the fan duty entry (the Tmin section) only.
   
   PWM ramp is another new powerful psychoacoustic feature. It will make the fan duty switching smooth and not as abrupt as for the LM63 any more.
   
   If you don't know what to do here, just use the "set all fan settings to recommended values" button and be happy!
3. This controller can be found inside some 4850 cards and seems to be a derivative of the controller described above in section 3.4.2:
   
   
   
   This controller works exactly as the one above, but only has no look up table mode any more.
4. The 4870X2 has a completely new fan controller. I suppose ATI will use it for future cards also. It's the ADT7473 from Analog Devices, which has proven reliable on recent NVIDIA cards already:
   
   
   
   It works similar to the controller described in section 3.4.2, but has no look up table mode any more but some additional features.
   The dynamic mode will try to hold the temperature at (or below) a certain target temperature, which can be changed using RBE. You can disable dynamic mode and if you do, the fan will behave just like the one described at section 3.4.2.
   
   The controller also has a manual mode which will make the fan run at the same duty for all times. You shouldn't use that except you do extreme overclocking and want to run the fan at 100% all the time.