Apple Inc. (Cupertino, CA)

Methods and apparatus to synchronize media playback in wireless networks. The present invention is aimed towards the precise synchronization of media playback within a wireless system. It is based on repeated measurements and a common time reference. One of the examples uses a Time Synchronization Function of a Wireless Local Area Network. Another exemplary embodiment of this disclosure is that the modem processor and the application processor determine the pulse width and alter the playback of media.

1. Technological Field

The present disclosure relates generally to the field of processing of multimedia. The focus of the present disclosure is specifically synchronizing multimedia processing in a wireless network.

2. Description of related Technology

As is well known within the design of electronic circuits high-speed integrated circuit (IC) design usually demands a consistent and precise distribution of the time reference. A precise reference to time is required to ensure that data processing is consistent and precise across the integrated circuit. Media processing is a specific segment of processing that can cause problems with timing that are easily discernible by human. For example audio playback that has an unstable or unreliable clock could cause audible artifacts or distortions. Additionally, playing back video with low-quality clocks could cause visible freezing, “chunking”, etc. The most common video glitches are noticeable for a couple of milliseconds, while audio errors can be detected within a matter of milliseconds. Human hearing is more sensitive than the video since it is able to detect irregularities within just a few milliseconds.

Consider, for instance, two or more wireless audio devices set to give an “stereo” and/or “surround” audio experience. If the audio sources on wireless are not correctly synchronized, then the audio played back by one speaker will beslightly offset in time in comparison to the other speaker(s). Human brains interpret volume and time differences between wireless audio devices as caused by virtual locations of the sound sources in relation to human. So an offset of a small amount in time may be seen as an incorrectly reproduced stereo image. Specifically, the resulting “inter-temporal distortion” is seen as a stereo image that is not properly centered, and is slightly off towards one direction. Time differences can be detected by the human brain in as little as 10 microseconds (e.g. half an audio file).

Wireless networks need to be able to handle changing radio environments, unknown propagation delays, and other. Most wireless devices employ an internal oscillator free-running in order to generate internal clocking. Because of the manufacturing variations and tolerances in oscillator components wireless networks have to be aware that each device is an imperfect time reference. There are many synchronization methods and standards that can be used to sync time across wireless devices. Common methods for synchronizing networks frequently provide information on time across the wireless network; each device in the network decides on the appropriate adjustment to its local free-running clock. Existing wireless synchronizationtechniques are accurate to only within a few milliseconds (which is magnitudes greater than the human brain’s audio acuity).

Wireless systems can sync their internal clocks to one standard time reference for modem operations. However, most media processing takes place via an independent processor (e.g. an application processor). Accordingly,distortion-free media playback also requires that the media sub-system can receive a high quality time reference from the modem chip.

This disclosure offers improved methods and equipment to precisely coordinate the processing of media within wireless networks.

A method for precisely synchronizing media processing within an internet network is disclosed. In one embodiment, the method includes receiving a common time stamp from a common reference to time; receiving media data associated with the common timereference and deriving a local copy of the common reference for playback of media using the common time stamp and playing the media according with the derived reference time.

A variant of the network consists of a wireless local network (WLAN) as well as the reference used is usually a time-synchronization-function value (TSF) received via a beacon. In certain variants the local version of the common time reference is clocking from a free operating clock oscillator. In certain implementations, determining the local equivalent of the common time reference is based on determining a mismatch between the local version of the common reference and the common timereference. In other implementations, the determination of rate mismatch is made based on the analysis of a variety of time stamps that are observed, and an ideal timeline that is derived from the plurality of observed time stamps.

The invention reveals an approach to interrupt-driven time synchronization in a device. In one example the method comprises the following steps: creating the common event between a first processor and a second processor; monitoring the first number of clock cycles for the first processor during the shared event; observing a second amount of clock cycles of the second processor during the same event; determining a rate mismatch using the first and second number and adjusting one or more media samples to playback on the first processor based on the rate mismatch that is determined; and in cases where media samples are provided by the second processor.

Certain variations include a typical event that is a pulse width generated when the processor in the first set an enumeration latch, and the subsequent processor reacts by resetting it. The method may also include receiving one or more time stamps via the main link. In these cases, the received one or more time stamps may be correlated with at least one edge of the pulse width.

It is disclosed a wireless media device that can receive media via a wireless network. One particular embodiment of the wireless media device includes two processors, a first clock that is associated with the first processor, and a second clock for the second processor; and a non-transitory computer-readable media containing some or all instructions. In one example, the instructions are programmed to cause the first processor to after execution, request an event for time synchronization; watch a first number of first clock cycles for the first clock in an event that is common to all; receive a second number of second clock cycles from the second clock in the event common to all participants; determine a rate mismatch using the first and second numbers. Then, rectify one or more media samples to playback based on the identified rate mismatch.

One variant comprises an application processor and wireless modem. One exemplary variant of instructions is for the first processor to create an initial latch. The second processor then will be capable of resetting the latch. Determine the pulse width produced by the latch. Certain variants also feature audio data streaming on the wireless network.

In other variations the wireless modem is time-synchronized with the wireless network. In some implementations one or more of the instructions may be programmed to cause the first processor to receive a time-stamp through an out-of-band link to a second processor. This time stamp is associated with the event that is common.

In some variations in some variants, the one or more media samples are streaming audio samples.

Another variant selects the second clock to be oscillating at a rate that is substantially like that of the wireless network’s radio frequency. In some implementations the first clock is chosen in accordance with one or more processor capabilities.

Certain variants get the second number of a second clock cycle through an out of band link to the second processor.

Other variations are based on historical measurements.

Other advantages and features of the disclosed invention are immediately apparent to persons who are skilled in the field by referring to the accompanying drawings and detailed description of some examples as shown below.

Click here to view the patent on USPTO website.


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