1. Field of the Invention
The invention relates to the field of the synchronization of data delivery onto equipments connected over a packet switching network, in particular when the equipments are devoted to video and audio data processing and are connected over a local area network for forming a video/audio production studio. More precisely the invention concerns a data inserter device for inserting a timestamp in packets, and a delay compensating device for extracting said timestamp and compensate the delivery delay and in particular the jitter, the network delay and the packet generation duration.
2. Description of the Prior Art
Future production environment will move forward using Ethernet technology. This architecture may provide enhancements in the current data stream alignment mechanism which are crucial for video production applications.
This architecture is based on a Local Area Network, which implements only Ethernet switches IEEE1588 compliant Certain problems must be solved to ensure the alignment of Audio/Video/Data streams in an Ethernet/IP production environment.
The production environment shall offer synchronization capabilities based on a reference signal transported and regenerated from a reference counter. The main assumption is to ensure real time transport over an Ethernet/IP network, the data is encapsulated in a UDP/RTP packet.
The solution should be considered as additional to the prior ones which are IEEE1588 layer and Reference Synchronization layer as presented in FIG. 1. These prior works are disclosed in European Patent application 07731734.5.
The IEEE1588 layer, shown on FIG. 1 provides a functional means to produce an accurate system clock (second, nanosecond) over the entire production environment. This IEEE1588 layer implementation ensures the delivery of a System Ticker (CLKech) with a precision of 1 μs.
The System layer provides a functional means to identify different timing zones (also named “Genlock Planes”) in the production environment and to regenerate faithfully the genlock OUT signal with the correct phase on the devices which interface Ethernet-less equipment to the packet switched network. This layer provides information in the extension section of the reference message about the deployed system, such as a studio for audio/video data transport or an automotive environment for car manufacturing.
This System layer provides also a means to give the Time Code information to all the devices connected on the packet switched network. This Time Code information is necessary to do timeless processing in order to restore time relationship between streams which have been stored. These System layer is already disclosed in European Patent applications 08300197.4.
Alignment means that the different delays between the reference and all the sources processed by the devices deployed on the system are compensated for. This ensures stream recombination without issues.
Based on System, Reference and IEEE1588 layer implementation, the main requirements to fulfill stream alignment with the proposed solution are:                Compensation for network jitter and network delay;        Delivery of flows on the appropriate Genlock Plane;        Consideration and compensation of the processing delay linked to the way the flow is encapsulated and inserted in the network;        Guarantee of post-production processing on flows issued from different means and places;        Consideration and Compensation of additional delay linked to pre-processing performed by external devices or additional delay provided by the “users”        Handling clean switching of streams in the IP domain.        
FIG. 2, shows he different paths taken by the audio and video flows before recombining and delivery.
The interface devices are represented by a Video Ethernet Inserter (VIN), Audio Ethernet Inserter (AIN), Video Ethernet Extractor (VEX), Audio Ethernet Extractor (AEX).
Incoming signals, feeding the Inserter devices, are delivered at the same instant on the Ethernet/IP network. The problem is how to recombine these signals at the extractor devices level in the presence of the following disruptive elements: network jitter and the different delays of different data paths. The jitter disrupts the continuity of the stream at the point of reconstruction. It must be canceled.
There is also a disparity of the path delays, even a maximal delay defined in the standard 802.1 AVB is 2 ms for a complex infrastructure, and the limits of current processing equipment (SDI or AES switcher, SDI or AES router) leads to alignment issues which have an impact on the recombination (Audio/Video lip sync).
The apparatus REF is a module which transmits the synchronization message. Management of the System and Reference layers is ensured by this synchronization message.
One of the goals of the present invention is to propose a solution for realizing an accurate alignment between streams based on the IEEE1588 and the Reference layers.
The invention consists of associating to each data packet (video, audio or data), synchronously with the studio infrastructure reference time base (reference layer), a temporal marking “Time Stamp” on the transmitter side.
On the receiver side, this Time Stamp is extracted. The assumption is that the maximal latency LAN_MaxDly (related to the maximum delay and the maximum jitter) of the deployed network is known and so added to the extracted Time Stamp marker. This information provides the instant, synchronous to the studio infrastructure reference time base (reference layer), when the received packet must be output from the latency buffer. This solution does not care of receiving instant as it is generally done with solutions using Time Stamping. In this way, distinct sources with the same type and format that are aligned at the emission side, are perfectly restituted whatever their data paths.