Add PTS and DTS sections

Author: Noarkhh

guides/useful_concepts/timestamps.md

@@ -41,3 +41,56 @@ example video frames in raw video streams or RTP packets in RTP streams.
 These units finally can have timestamps assigned to them - and in
 most cases they do. For example, a PTS assigned to a buffer containing a
 raw video frame determines then the frame should be displayed.
+
+## Presentation Time Stamps
+
+As previously mentioned, PTSs are used to tell when a piece of media should be
+presented to the user. It can mean either displaying a video frame, or playing a
+chunk of audio.
+
+### Realtimer
+
+[Realtimer](https://hexdocs.pm/membrane_realtimer_plugin/Membrane.Realtimer.html)
+is an element from
+[membrane_realtimer_plugin](https://hex.pm/packages/membrane_realtimer_plugin).
+It takes in a stream and limits it's flow according to it's PTSs. For example, if
+it receives three buffers with `:pts` of 0ms, 200ms and 400ms, then it will send
+the first buffer, the second buffer after 200ms pass, and the third one after
+another 200ms pass.
+
+This element is useful if we have non-realtime input, and realtime output, for
+example we want to stream the contents of a MP4 file with WebRTC. If we didn't
+use Realtimer, then we would read contents of the file as fast as possible and
+send them as fast as possible, which is not what we want, we want the receiver
+to receive the stream in realtime, so that they can display it as it comes.
+
+## Decoding Time Stamps
+
+The purpose of DTSs is to tell a decoder when a given media chunk should be
+decoded. In a lot of codecs the media can be decoded as it comes, but in some
+cases, like in [H264](../membrane_tutorials/h264/1_Introduction.md), it's
+not that simple. In a nutshell, in H264 some frames are encoded based on
+information from other frames. There are three main types of frames:
+
+* I-frame (Intra-coded picture) - A frame of this type is encoded without the
+  information from any other frames.
+* P-frame (Predicted picture) - A frame of this type is encoded with the usage
+  of information of previous frames. If we have a static shot, then it takes
+  much less space to encode a frame by using the fact that it's almost the
+  same as the previous one.
+* B-frame (Bidirectional predicted picture) - A frame of this type is similar
+  to a P-frame, as it uses information from other frames for it's encoding.
+  However, it not only depends on previous pictures, but also on future ones.
+  That's where the DTSs come in, because to decode a B-frame we also need to
+  decode all frames it's encoding is based on, including the future ones.
+  For example, let's assume that we have a slice of a stream consisting of
+  three frames where frame 2 is a B-frame that's encoded based on the
+  frames 1 and 3. If a decoder receives these frames with the following
+  timestamps:
+
+  1) pts: 0ms, dts: 0ms
+  2) pts: 200ms, dts: 400ms
+  3) pts: 400ms, dts: 200ms
+
+  It will first decode the frames in order (1, 3, 2). If it hadn't decoded
+  frames 1 and 3 first, it couldn't decode frame 2.