Annex 6 to AVC-496R

Support Aspects Required for "Layered" Video Coding Algorithms

The term "layered" is intended here to cover data partitioning, frequency scalability and spatial scalability schemes in which the coded representation comprises more than one bit stream.  They are logically and physically separated for storage/transmission to provide features such as error resilience (priority treatment of more important bit stream) and scalability (intentional discarding of some of the bit stream). 

IDENTIFICATIONS

-	Video decoder needs to know which  layering scheme the encoder has used.
-	Video decoder needs to know which data is for which layer.

The MPEG System is an appropriate way to convey both of these, as proposed in Document AVC-495.

ALIGNMENT

Decoder needs information to align the layers.  Both spatial and temporal "addressing" are required for every "layer".  Spatial can be accomplished with macroblock address, slice header, etc.  At least two methods are possible for the temporal aspect.

-	PTS present for every layer to absolutely time each one.
-	Use Temporal References to align layers relative to each other.  Insert/use PTS in one (bottom layer?) to determine absolute timing of all decoded layers.

ERROR RESILIENCE

-	Need to ensure that mechanism in data partitioning for "switching" between "transmission layers" to concatenate data is not totally dependent on error free situation.  (Otherwise error resilience of partitioning is compromised.)
-	Need to ensure that alignment information and decoder techniques are sufficient for adequately rapid re-establishment of alignment after errors (cell losses).  It may be commonly assumed that higher layers are lost until end of slice and that normal quality decoder can restart with next slice.  This may not be true if extent of lost data is comparable to one frame because there is uncertainty about which picture a slice belongs to if the picture header has been lost.

ERROR FLAGGING

In general there is potential benefit if the video decoder can be informed of the presence and location of errors/cell losses.  For fully integrated terminals, mechanisms to pass such information need not be standardised.  However, in the case of concatenated networks, a standardised method for carrying information about errors in a prior network would be useful.  (The second "network" may actually be a storage device.)

PARTLY FILLED CELLS

Issues of stuffing or partly filled cells may become more important with multi-layer (multi-stream) because bit rate per stream becomes lower.

PACKET RATE LIMITATION

For multi-stream cases the packet rate may need to be higher.  The MPEG-1 limit of 300 packets per second is probably too low even for main profile main level.