The Video Buffer Verifier (VBV) is a model hypothetical decoder buffer that will not overflow or underflow when fed a conforming MPEG bit stream.  Thus, part of the definition of a compliant stream is that it does not cause underflow or overflow of this model buffer.  The quantities that MPEG transmits to specify decoder buffer action according to this model are explained fully below. However, decoders do not have to use these quantities, but instead can rely on redundant information provided by time stamps. A thorough explanation is in the section on Buffer Synchronization and Startup .

Operation of the VBV is tied to two transmitted values,  vbv_buffer_size_value, the maximum buffer fullness, and vbv_delay, the delay between storing a picture start code in the buffer and starting the decoding of that picture.  vbv_buffer_size is transmitted in every sequence header. vbv_delay is transmitted in every picture header.   Note that the sequence header and vbv_buffer_size may not be available when tuning to a program in progress.  However, proper operation of the decoder buffer can be obtained by use of the System Time Clock and various time stamps and an a priori knowledge of the maximum buffer size that may be required, so that vbv_delay and vbv_buffer size are redundant.

In the ATSC standard A/53 Annex A, the constraint on buffer size is:

maximum video buffer = B = 7995392 bits,

which is specified by transmitting:

vbv_buffer_size_value <= 488 (lower 10 bits of vbv_buffer_size)

vbv_buffer_size_extension = 0 (upper 8 bits of vbv_buffer_size)

where B = 16*1024*vbv_buffer_size.

The ATSC constraint on vbv_delay is

vbv_delay <= 45000

vbv_delay is a 16 bit unsigned integer representing  the number of 90kHz clock periods between entering a picture's start code into the buffer and removing and decoding the entire picture.

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