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7.2.2. De-Interleaving Process

7.2.2. De-Interleaving Process​

There are two buffering states in the receiver: initial buffering and buffering while playing. Initial buffering occurs when the RTP session is initialized. After initial buffering, decoding and playback are started, and the buffering-while-playing mode is used.

Regardless of the buffering state, the receiver stores incoming NAL units, in reception order, in the de-interleaving buffer as follows. NAL units of aggregation packets are stored in the de-interleaving buffer individually. The value of DON is calculated and stored for each NAL unit.

The receiver operation is described below with the help of the following functions and constants:

  • Function AbsDON is specified in Section 8.1.

  • Function don_diff is specified in Section 5.5.

  • Constant N is the value of the OPTIONAL sprop-interleaving-depth media type parameter (see Section 8.1) incremented by 1.

Initial buffering lasts until one of the following conditions is fulfilled:

  • There are N or more VCL NAL units in the de-interleaving buffer.

  • If sprop-max-don-diff is present, don_diff(m,n) is greater than the value of sprop-max-don-diff, in which n corresponds to the NAL unit having the greatest value of AbsDON among the received NAL units and m corresponds to the NAL unit having the smallest value of AbsDON among the received NAL units.

  • Initial buffering has lasted for the duration equal to or greater than the value of the OPTIONAL sprop-init-buf-time media type parameter.

The NAL units to be removed from the de-interleaving buffer are determined as follows:

  • If the de-interleaving buffer contains at least N VCL NAL units, NAL units are removed from the de-interleaving buffer and passed to the decoder in the order specified below until the buffer contains N-1 VCL NAL units.

  • If sprop-max-don-diff is present, all NAL units m for which don_diff(m,n) is greater than sprop-max-don-diff are removed from the de-interleaving buffer and passed to the decoder in the order specified below. Herein, n corresponds to the NAL unit having the greatest value of AbsDON among the NAL units in the de-interleaving buffer.

The order in which NAL units are passed to the decoder is specified as follows:

  • Let PDON be a variable that is initialized to 0 at the beginning of the RTP session.

  • For each NAL unit associated with a value of DON, a DON distance is calculated as follows. If the value of DON of the NAL unit is larger than the value of PDON, the DON distance is equal to DON - PDON. Otherwise, the DON distance is equal to 65535 - PDON + DON + 1.

  • NAL units are delivered to the decoder in ascending order of DON distance. If several NAL units share the same value of DON distance, they can be passed to the decoder in any order.

  • When a desired number of NAL units have been passed to the decoder, the value of PDON is set to the value of DON for the last NAL unit passed to the decoder.

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