Enable synchronization through tighter rate control

[wilaw]

There are many solutions coming to market which use synchronized playback between clients as a differentiating factor. For betting and gambling applications, having tight sync between clients is a legal requirements to avoid giving an advantage to a subset of users who may be ahead of others. We would like dash.js to be able to offer a simple syncrhonization feature

Goal: synch clients to within 250ms of each other

Approach: The current 2.8.0 release features a catch-up feature, in which playback rate can be accelerated up to 20% faster than real-time in order to return the playhead to a target latency. This behavior seems to have some tolerance (perhaps 500ms) before it activates. By tightening this tolerance and raising the maximum catch up rate to 50%, as well as developing an algorithm to both accelerate and retard the playhead around the target latency, we feel that a simple level of synchronization can be achieved.

Proposed algorithm:

1. Define a target latency in seconds of T. This is set by the application using setLiveDelay()
2. Define M = latency tolerance in seconds at which max distortion is applied (1s)
3. On a fast loop during playback
   var L = getCurrentLiveLatency(),
   var D = (L - T)*5/M
   var S= sigmoid function of D = 1/(1+e^-D)
   playback rate R = 0.5 + S

You may prefer a linear function as opposed to this sigmoid one. Irrespective of the algorithm used, goal is for the latency to rapidly & stably converge on the target. You may also wish to define a small window of allowable drift before the rate is corrected away from 1, as I am not sure of the long-term effects of playing back a stream at a rate != 1.