Measurements on the MPEG-2 Transport Stream - Digital Video and Audio Broadcasting Technology - page 181

11.1 Loss of Synchronisation (TS_sync_loss)

The MPEG-2 transport stream consists of 188-byte-long data packets
composed of 4 bytes of header and 184 bytes of payload. The first byte of
the header is the synchronisation or sync byte which always has the value
0x47 and occurs at constant intervals of 188 bytes. In special cases a spac-
ing of 204 or 208 bytes is also possible, namely when the data frame with
Reed Solomon error protection according to DVB or ATSC is similar. The
additional 16 or 20 bytes are then dummy bytes and can be simply ignored.
At any rate, there is no useful information present since it is not the Reed
Solomon coder and decoder which represent the first or, respectively, last
element of the transmission link but the energy dispersal unit, and thus any
Reed Solomon error protection bytes present would not fit in with the ac-
tual transport stream packet. According to DVB, synchronism is achieved
after 5 successive sync bytes have been received at correct intervals and
with the correct content. When 3 successive sync bytes or transport stream
packets have been lost, the MPEG-2 decoder or the corresponding trans-
mission device drops lock again.

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. 11.1. TS_sync_loss

The state of loss of transport stream synchronization, which may occur
either because of severe interference or simply because of a break in a line,
is called “TS_sync_loss” (Fig. 11.1.).
“TS_sync_loss” occurs when

• the content of the sync bytes of at least 3 successful transport
stream packets is not equal to 0x47.