Measurements on the MPEG-2 Transport Stream - Digital Video and Audio Broadcasting Technology - page 190

Although real PTS errors occur only rarely, a perceptible lack of lip
sync between video and audio happens quite frequently. In practice, the
causes of this are difficult to detect and identify during a broadcast and can
be attributable both to older MPEG-2 chips and to faulty MPEG-2 decod-
ers. The direct measurement of lip synchronism would be an important test
parameter.

11.11 The Conditional Access Table Error (CAT_Error)

An MPEG-2 transport stream packet can contain scrambled data but only
the payload part must be scrambled and never the header or the adaptation
field. A scrambled payload part is flagged by two special bits in the TS
header, the Transport Scrambling Control bits. If both bits are set to zero,
there is no scrambling. If one of the two is not zero, the payload part is
scrambled and a Conditional Access Table (CAT) is needed to descramble
it. If this is missing or only rarely there, a CAT_error occurs. The CAT has
a 1 as PID and also a 1 as table ID. Apart from the EIT in the case of the
transmission of a program guide, all DVB tables must be unscrambled.

188 byte

4 byte
header

184 byte
payload

Optional
adaptation
field CAT

PID=1
Table ID =1
Conditional
Access
Table

Header

Transport
error
indicator

Payload
unit start
indicator

Transport
scrambling
control

Adaptation
field
control

Sync
byte

Transport
priority

Continuity
counter

PID

81 1 1 3 2 2 4 i t

Value
(binary)

Description

00 No encrypted data contained in

the packet

01, 10, 11 Defined by user

Fig. 11.9. CAT_error

A CAT_error (Fig. 11.9.) occurs when

• a scrambled TS packet has been found but no CAT is being

transmitted,