Video Buffer Verifier

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.

Video buffering verifier

From Wikipedia, the free encyclopedia

The Video Buffering Verifier (VBV) is a theoretical MPEG video buffer model, used to ensure that an encoded video stream can be correctly buffered, and played back at the decoder device.

By definition, the VBV shall not overflow nor underflow when its input is a compliant stream, (except in the case of low_delay). It is therefore important when encoding such a stream that it comply with the VBV requirements.

One way to think of the VBV is to consider both a maximum bitrate and a maximum buffer size. You'll need to know how quickly the video data is coming into the buffer. Keep in mind that video data is always changing the bitrate so there is no constant number to note how fast the data is arriving. The larger question is how long before the buffer overflowings. A larger buffer size simply means that the decoder will tolerate high bitrates for longer periods of time, but no buffer is infinite, so eventually even a large buffer will overflow.

Operation Modes

There are two operational modes of VBV: Constant Bit-rate (CBR) and Variable Bit-rate (VBR).

CBR - The decoder's buffer is filled over time at a constant data rate in the case of CBR. VBR - and at a non-constant rate in the case of VBR.

In both cases, data is removed from the buffer in varying chunks, depending on the actual size of the coded frames.

Standards