HRD hypothetical reference decoder
Hypothetical reference decoder (HRD) - Specifies a buffering model, including operations for both the coded picture buffer (CPB) and decoded picture buffer (DPB). While the DPB is a memory buffer that holds uncompressed pictures for referencing or output reordering after the pictures have been decoded, the CPB is a buffer of compressed bits that temporarily holds the coded data coming from the bitstream until it is removed from the CPB for decoding. Through the parameters and the operations, the HRD directly imposes constraints on different timing, buffer sizes, and bit rate values, which indirectly imposes constraints on bitstream characteristics and statistics. For example, the level definitions rely on the HRD process. There are five basic HRD parameters, namely the initial CPB removal delay, CPB size, bit rate, initial DPB output delay, and DPB size. Bitstream conformance and decoder conformances (output order decoder conformance and timing decoder conformance) are also specified as part of the HRD specification. Although the name includes the word “decoder”, the HRD is typically implemented along with the encoder, to guarantee that the generated bitstream is conforming.