Measurements on the MPEG-2 Transport Stream - Digital Video and Audio Broadcasting Technology - page 185

A “PID_error” (Fig. 11.3.) occurs when

• transport stream packets with a PID referred to in a PMT are not
contained in the transport stream or
• if their repetition rate exceeds a user-definable limit which is
usually of the order of magnitude of 500 ms.

188 byte

4 byte
header

184 byte
payload

Optional
adaptation
field

Header

Transport
error
indicator

Payload
unit start
indicator

Transport
scrambling
control

Adaptation
field
control

Sync
byte

Transport
priority

Continuity
counter

PID

8 1 1 1 1 3 2 2 4 B i t

Fig. 11.4. Continuity_count_error

11.6 The Continuity_Count_Error

Each MPEG-2 transport stream packet contains in the 4-byte-long header a
4-bit counter which continuously counts from 0 to 15 and then begins at
zero again after an overflow (modulo 16 counter). However, each transport
stream packet of each PID has its own continuity counter, i.e. packets with
a PID=100, e.g., have a different counter, as do packets with a PID=200. It
is the purpose of this counter to enable one to recognize missing or re-
peated transport stream packets of the same PID in order to draw attention
to any multiplexer problems.
Such problems can also arise as a result of errored remultiplexing or
sporadically due to bit errors on the transmission link. Although MPEG-2
allows discontinuities in the transport stream, they must be indicated in the
adaptation field, e.g. after a switch-over (discontinuity indicator=1). In the
case of zero packets (PID=0x1FF), on the other hand, discontinuities are
allowed and is not checked, therefore.