Scrambling and descrambling method for the transport of MPEG2 audio video data streams
View Patent ↗A scrambling and descrambling method that includes reading of original PID and TSC bits in the PID and TSC fields of a packet header of an MPEG2 stream which is at least partially scrambled. A temporary PID may be written in a PID field in place of the original PID, and may be a function of the original PID value and TSC bits according to a correspondence table of an original PID and TSC bits with the temporary PIDs. The value zero may be written in the TSC field in place of the original value, and scrambling of the stream with modified fields may occur. Applications include the satellite transmission of partially scrambled MPEG2 digital signals.
1. Scrambling method for an over-scrambling of a MPEG2 stream at least partially scrambled, the stream being structured in packets, a packet header comprising a packet identifier (PID) field defining the service to which the packet is affected, a transport scrambling control (TSC) field, the TSC field having bits being relative to the scrambling of the packet, a continuity counter field, comprising the following steps:
reading of an original PID and TSC bits in the PID and TSC fields of a packet header of said stream,
writing of a temporary PID in a PID field in place of the original PID, as a function of the original PID value and TSC bits according to a correspondence table of an original PID and TSC bits with the temporary PIDs, and
overscrambling of the stream with modified fields.
2. Method according to claim 1 , wherein the choice of temporary PID is made from a predefined correspondence table attributing temporary PIDs according to the original PIDs in the stream and the value of TSC bits.
3. Method according to claim 1 , wherein the choice of temporary PID is made from a correspondence table automatically calculated according to the signal received.
4. Method according to claim 1 , comprising a reading step of the continuity counter value and wherein the temporary PID is also a function of the continuity counter value, a different PID being attributed, for a same value of TSC bits, according to whether the continuity counter is at zero or other than zero.
5. Descrambling method for descrambling of a MPEG2 stream at least partially over-scrambled, the over-scrambled stream being structured in packets, a packet header comprising a packet identifier (PID) field defining the service to which the packet is affected, a transport scrambling control (TSC) field, the TSC bits being relative to the scrambling of the packet, a continuity counter field, comprising the following steps:
descrambling of an over-scrambled stream to provide a scrambled stream, reading of a temporary PID in a PID field of a packet header of said scrambled stream,
writing of an original PID and TSC bits in the PID field and the TSC field of said scrambled stream, in place of the temporary PID and the TSC bits, the values being a function of a correspondence table of an original PID and TSC bits with temporary PIDs.
6. Method according to claim 5 , wherein the choice of original PID is carried out according to a predefined correspondence table attributing for a temporary PID, values of the original PID and TSC bits.
7. Method according to claim 5 , wherein the choice of original PID is made according to a correspondence table transmitted in the data stream, attributing, for a temporary PID, values of the original PID and TSC bits.
8. Method according to claim 5 , wherein the temporary PID is also a function of the value of the continuity counter, also comprising a writing step of a continuity counter value in the corresponding field, the value being zero when this temporary PID corresponds to a PID indicating a zero value of the counter in the original signal, then incremented according to the arrival of packets relating to the same original PID.
9. Over-scrambling device, comprising a MPEG2 stream scrambler, a second scrambled stream scrambler implementing the method of claim 1 .
10. Descrambling device comprising a MPEG2 stream descrambler implementing the method of claim 5 , a descrambled stream descrambler.
11. The Method according to claim 1 , further comprising the step of writing of the value zero in the TSC field in place of the original value.