Video device and method for synchronising time bases of video devices

1. A method for synchronising time bases of a first video device which transmits a video signal comprising image information and synchronisation information and of a second video device which receives said composite video signal, comprising the steps of;

a) extracting synchronisation information from the video signal received by the second video device and from the time base of the second video device;

b) determining a phase difference between the video signal received by the second video device and the time base of the second video device based on said extracted synchronisation information;

c) transmitting control information of a first type representative of the amount of said phase difference to said first device;

d) switching, in said first device, the phase of the video signal by a phase angle represented by said first type control information,

wherein the first type control information is a binary data word having a predefined number n≧2 of bits;

e) determining the first type control information and a second type control information such that the sum of phase differences represented by said first and second type control information is the phase difference determined in step b);

f) transmitting the second type control information to said first device; and

g) applying a phase shift as represented by said second type control information to the video signal in said first device.

2. The method as claimed in claim 1 , wherein steps a) to g) are cyclically repeated.

3. The method of claim 1 , wherein step g) is carried out by increasing or decreasing the frequency of the pixel clock signal of the time base of said first device according to the sign of the phase shift represented by said second type control information.

4. The method of claim 1 , wherein step g) is carried out by increasing or decreasing the frequency of the pixel clock signal of the time base of said first device according to the amplitude of the phase shift represented by said second type control information.

5. The method as claimed in claim 1 , wherein steps c) and d) are carried out only if the phase difference represented by said first type control information is different from zero.

6. The method as claimed in claim 1 , wherein a time for carrying out step d) is chosen according to the phase difference determined in step b) such that after carrying out step d) the video signal at the first device assumes a predefined phase.

7. The method as claimed in claim 5 , wherein the predefined phase is the beginning of an image in the video signal.

8. The method as claimed in claim 1 , wherein the image information comprised in the video signal is formed of a sequence of images, each image comprising a plurality of lines, and the first type control information is representative of a number of lines by which the compositevideo signal received from the first video device is phase shifted with respect to the time base of the second device.

9. The method as claimed in claim 1 , wherein the video signal is transmitted in digital form as a sequence of data packets, each data packet comprising image data and at least some of the data packets comprising a sending time data derived from the time base of the first device and representative of a sending time of the packet in which it is comprised, wherein each sending time data, upon receipt by the second device, is compared to receiving time data derived from the time base of the second device, and the second type control information is derived from a difference between said sending and receiving time data.

10. The method as claimed in claim 8 , wherein said first device comprises a first counter operating based on a pixel frequency derived from the time base of said first device and the second device comprises a second counter operating based on a pixel frequency derived from the time base of said second device, and wherein a counting value of said first counter is included in a data packet as sending time data, and is compared to a counting value of said second counter as receiving time data.

11. The method as claimed in claim 9 , wherein when the difference between counting values of said two counters exceeds a given threshold, the second counter is set to the value received from the first counter.