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====== StatMux Priorities ======
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This information comes from an Envivio document, there is no listed author so I am unable to credit or reference anyone.
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In a Stat Mux system, all channels get the same priority, there is no way from the GUI to change the priority of a channel (to give it more bits ultimately). There is an Advanced Parameter that can help, but it is quite limited.
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==== Context ====
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In the Statmux context, channel’s priority should help preserving the video quality of several channels. This user story aims at implementing and testing a short term version of the statmux priorities, i.e. with a limited number of priorities and without supporting advanced features like mix of codec.
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According to [1], priorities can be implemented easily without completely modified the source code. Indeed, just by weighting channel complexity, we could provide a simple priority management.
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==== Specifications ====
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In this first version, the priority will be set only by mpeg4.cfg by specifying for each channel the prioritized level (1 or 0), using  the following line:
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<color red> <serviceName>.<profileName>.vid<videoId>.statmuxPriority=1. </color> With <videoId> starting from 0.
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Example:
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{{:wiki:statmuxpriorities01.png?500|}}
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To set a priority to this channel:
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<color red>Service1.Profile 1-1.vid0.statmuxPriority=1</color>
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By default, all channels will set to un-prioritized.
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The expected behaviour is that a prioritized channel will have a better average objective video quality than without being prioritized. It could be checked by comparing in the same pool the same channel with and without priority. 
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The improved VQ will be achieved by allocating more bitrate to prioritized channels compared to others. However, a prioritized channel could have in particular cases, a lower bitrate and so a lower VQ compared to non-prioritized channel.
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The maximal increase of bitrate, i.e. by comparing a channel bitrate with a VERY HIGH PRIORITY and the same channel with a normal priority, could be changed by mpeg4.cfg using the following parameter: 
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<color red>statmux.m_smxPriorityRateFactor=0.5.</color>
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The default value is 0.2. The overridden value is clipped between 0.05 and 1.
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==== 3	Priorities design ====
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Five priority levels have been planned. However, in the current state, only two priorities are used (VERY_HIGH and NORMAL):
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  * 	SMX_PRIORITY_VERY_HIGH		= 5
  * 	SMX_PRIORITY_HIGH		= 4
  * 	SMX_PRIORITY_NORMAL		= 3
  * 	SMX_PRIORITY_LOW		= 2
  * 	SMX_PRIORITY_VERY_LOW		= 1
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According to the maximal increase of bitrate  , priorities factor are:
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{{:wiki:statmuxpriorities02.png?500|}}
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Once the priority factor is computed, the statmux algorithm weights the complexity (STC) with each priority in order to balance the channel bit rate. Priorities are computed each time a channel is added or removed from the pool.
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==== 4	Priorities and distortion control ====
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The distortion control aims at weighting channel’s complexity according to their MSE. Bigger the channel’s MSE, bigger the distortion factor is. Factors are computed every four statmux iterations (around twice a second). Factors are clipped between 0.5 and 2.
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However, when priorities are enabled, the distortion control could in some cases nullify the priority.  Indeed, as distortion factors can inferior to 1, it could nullify the impact of prioritizing a channel.
To avoid such cases, factors are clipped according to the channel’s complexity:
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  * 	1.00 < F < 2.0 with a very high priority
  * 	0.75 < F < 2.0 with a high complexity
  * 	0.50 < F < 2.0 with a normal priority
  * 	0.50 < F < 1.5 with a low priority
  * 	0.50 < F < 1.0 with a very low complexity.
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