gimbal: expand introduction section

en/advanced-gimbal-control.md

@@ -1,22 +1,30 @@
 # Gimbal Control Setup
 
+If you want to control a gimbal with a camera (or any other payload) attached to
+the vehicle, you need to configure how you want to control it and how PX4 can
+command it. This page explains the setup.
+
 PX4 contains a generic mount/gimbal control driver with different input and
-output methods. Any input method can be selected to drive any output.
+output methods. The input defines how you control the gimbal: via RC or via
+MAVLink commands (for example in missions or surveys). The output defines how
+the gimbal is connected: some support MAVLink commands, others use PWM
+(described as AUX output in the following). Any input method can be selected to
+drive any output. Both have to be configured via parameters.
 
 ## Parameters
-The parameters are described [here](parameter_reference.md#mount). The most
+[These parameters](parameter_reference.md#mount) are used to setup the mount
+driver. The most
 important ones are the input (`MNT_MODE_IN`) and the output (`MNT_MODE_OUT`)
-mode. By default, the input is disabled. After selecting the input mode, reboot
-the vehicle so that the mount driver starts. Any input method can be selected to
-drive any of the available outputs.
+mode. By default, the input is disabled and the driver does not run. After
+selecting the input mode, reboot the vehicle so that the mount driver starts.
 
 If the input mode is set to `AUTO`, the mode will automatically be
 switched based on the latest input. To switch from mavlink to RC, a large stick
 motion is required.
 
 ## Configure the gimbal mixer for AUX output
 
-If the `MNT_MODE_OUT` parameter is set to `AUX`, a mixer has to be configured.
+If the `MNT_MODE_OUT` parameter is set to `AUX`, a mixer file has to be configured.
 The gimbal uses the control group #2 (see [Mixing and Actuators](concept-mixing.md)).
 This is the basic mixer configuration:
 
@@ -39,7 +47,7 @@ S: 2 2  10000  10000      0 -10000  10000
 
 Add those you need to your [main or auxiliary mixer](advanced-system-startup.md#starting-a-custom-mixer).
 
-There is also a generic quad airframe config which includes a mount mixer:
+There is also a generic quad airframe config which includes a [mount mixer](https://github.com/PX4/Firmware/blob/master/ROMFS/px4fmu_common/mixers/mount.aux.mix):
 [Generic Quadrotor X config with mount](airframe_reference.md#quadrotor-x).