Reference synchronisation

1. A method at a first device for synchronising a first clock of the first device to a second clock of a second device, the method comprising:

receiving a first message comprising an identifier from a third device;

generating a first timestamp in dependence on the time at which the first message is received at the first device according to the first clock;

receiving a second message from the second device comprising the identifier and a second timestamp, the second timestamp having been generated in dependence on the time at which the second device received the first message from the third device according to the second clock; and

adjusting the first clock in dependence on a time difference between a time indicated by the first timestamp and a time indicated by the second timestamp.

2. The method as claimed in claim 1 , further comprising the steps of:

receiving a plurality of identifiers and storing each of the identifiers in memory;

for each of the plurality of identifiers, storing in memory an associated timestamp indicating a time at which that identifier was received at the first device according to the first clock;

in response to receiving the second message, searching the memory so as to identify a stored identifier that corresponds to the identifier in the second message and retrieving a timestamp stored in association with the stored identifier, wherein the retrieved timestamp is the first timestamp.

3. The method as claimed in claim 1 , wherein the first message is broadcasted by the third device.

4. The method as claimed in claim 1 , wherein the first and second clocks are clocks for timing of media playback.

5. The method as claimed in claim 1 , further comprising:

receiving media and a media playback time from the second device; and

playing out the media in dependence on the media playback time according to the adjusted first clock.

6. The method as claimed in claim 5 , wherein the media is received via the third device.

7. The method as claimed in claim 1 , wherein the second message is transmitted to the second device via the third device.

8. The method as claimed in claim 1 , wherein the identifier is generated in dependence on a third clock at the third device.

9. The method as claimed in claim 8 , wherein the first clock is configured to operate to a first clock resolution and the third clock is configured to operate to a second resolution lower than the first resolution.

10. A method, at a first device comprising a first clock, of enabling synchronisation of a second clock at a second device with the first clock, the method comprising:

receiving a first message comprising an identifier from a third device;

generating a timestamp in dependence on the time at which the first message is received at the first device according to the first clock;

generating a second message comprising the identifier and the generated timestamp; and

transmitting the second message to the second device.

11. The method as claimed in claim 10 , further comprising:

determining a playback time for playing media in dependence on a time according to the first clock and an offset period of time; and

transmitting the determined playback time to the second device.

12. The method as claimed in claim 11 , wherein the playback time is transmitted to the second device via the third device.

13. The method as claimed in claim 10 , wherein the second message is transmitted to the second device via the third device.

14. The method as claimed in claim 10 , wherein the identifier is generated in dependence on a third clock at the third device.

15. The method as claimed in claim 14 , wherein the first clock is configured to operate to a first clock resolution and the third clock is configured to operate to a second resolution lower than the first resolution.

16. The method as claimed in claim 10 , wherein the third clock is a wireless interface clock for timing of wireless communications.

17. The method as claimed in claim 10 , wherein the third device is an access point.

18. The method as claimed in claim 10 , wherein the first device is configured to operate according to a WLAN standard and the identifier is a timing synchronisation function.

19. The method as claimed in claim 10 , wherein the first device is a wireless speaker.

20. A system comprising a master device, a slave device and a third device, wherein the master device comprises:

a first clock,

a first communications interface configured to receive a first message comprising an identifier from the third device and generate a first timestamp in dependence on the time at which the first message is received at the master device according to the first clock, and

a first controller configured to generate a second message comprising the identifier and the generated first timestamp, wherein the communications interface is further configured to transmit the second message to the slave device; and

wherein the slave device comprises:

a second clock,

a second communication interface configured to:

receive the first message comprising the identifier from the third device and generate a second timestamp in dependence on the time at which the first message is received at the slave device according to the second clock; and

receive the second message from the master device, and

a second controller configured to adjust the second clock in dependence on a time difference between a time indicated by the first timestamp and a time indicated by the second timestamp.