nal_hrd setting, what is it used for?

[pstnf1]

Hi,

I see this setting often accompanies bufsize & maxrate, I thought it was a bluray compliance setting but then I saw that even Netflix & Max seem to use it. I'm just wondering when it should be used, and if it affects compatibility at all? I really couldn't find much at all on it online.

thanks

[cubicibo]

HRD: Hypothetical Reference Decoder.

NAL HRD models a basic decoder at the output of the encoder. This decoder consumes data packets stored in a buffer (vbv-bufsize) filled at some bitrate (vbv-maxrate). The HRD consumes frames from the said buffer as time goes: the encoder must shape the datastream to guarantee sufficient data at decoding timestamps. Furthermore the incoming data shall never exceed the buffer size. The encoder must distribute bits appropriately to meet these constraints. Those streaming companies target a wide range of device with various memory size and decoder implementation. HRD constraints guarantee the decoder does not need more RAM and bandwidth than what has been specified at the time of encoding.