WO2010037916A1

WO2010037916A1 - Equipment synchronization independent of the standard of the synchronization signal

Info

Publication number: WO2010037916A1
Application number: PCT/FR2008/051797
Authority: WO
Inventors: Thierry Tapie; Serge Defrance; Claude Chapel
Original assignee: Thomson Licensing
Priority date: 2008-10-03
Filing date: 2008-10-03
Publication date: 2010-04-08

Abstract

The present invention relates to the field of video equipment. It relates more particularly to a sending device able to send packets in a packet communication network which comprises means for: - receiving image pips; initializing an image counter on the basis of the image pips; initializing a counter CPT_PCR every “m” zero-crossings of the image counter; sampling the counter CPT_PCR every period T_ech, where T_ech arises from a time base synchronized with all the stations of said network; and - sending packets containing the samples PCR_e of the counter CPT_PCR in the network. According to the invention it furthermore comprises means for: evaluating a value of excursion Video_Modulus of the image counter; evaluating a value of excursion PCR_Modulus of the counter CPT_PCR; sending the excursion Video_Modulus and the excursion PCR_Modulus in the network.

Description

SYNCHRONIZATION OF EQUIPMENT INDEPENDENT OF THE STANDARD

SYNCHRONIZATION SIGNAL

Field of the invention

The present invention relates to the field of video equipment. The present invention relates more particularly to a transmitting device and to a receiving device for transmitting a synchronization signal, for example of the "Genlock" type.

(synchronization lock), on a packet-switched communication network, for example of IP type (acronym for the English expression "Internet

Protocol "), whether the network is wired (for example

Ethernet (IEEE802.3)) or non-wired, (eg IEEE

802.16 D-2004). State of the art

Advances in the ability of IP networks to carry any type of signal (data or video) make it possible to use such networks as a backbone architecture for video studios. A major interest of this evolution is to have then a unique infrastructure for the transport of data. While in the past, multiple media were needed to carry different types of signals between devices, the multiplexing properties offered by the IP layer can reduce to one the number of media required: an IP network that connects the different devices.

In the state of the art, the synchronization of video equipment (cameras, etc.) in a studio is achieved by the transmission of a synchronization signal commonly called "Genlock" or "black burst". For example, the Genlock signal is composed of two synchronization signals, one is repeated every 40 ms and indicates the beginning of the video frame, the other is repeated every 64 μs (for a standard format and less for an HD format) and indicates the beginning of the lines in the video frame. The waveform of the synchronization signals is a function of the format of the image transmitted over the network. For example, for a high-definition image, the synchronization signal has a shape called tri-level (-30OmV, OV, +300 mV).

Known disadvantages presented by an IP / Ethernet network come from the fact that it introduces on the one hand a strong jitter, on the other hand a latency, in a signal transmission, and in particular for the transmission of a synchronization signal . The jitter is reflected by temporal fluctuations in the duration with which the information carried by the synchronization signal reaches the equipment, latency induces a delay, generally fixed in time in the synchronization of equipment.

In the prior art, devices are known for reconstructing at each camera, a clock clock specific to this camera to overcome a jitter induced by the transmission on the network. The principle of these devices is based on a strong attenuation of the amplitude of the jitter of the synchronization signal at the reception. It can thus be guaranteed that an image generated by a camera is strictly in phase with all the images generated by the neighboring cameras connected to the same network. Examples of such devices are described in the international application PCT FR2007 / 050918, they act on digital signals called counter (or PCR which is the acronym for the English expression "Program Clock Reference"), which are representative very precise clock signals of reference. These digital signals are provided to cameras through a network so that they can locally reconstruct clock signals in phase with the reference clock. The international patent application PCT FR2007 / 050918 thus proposes a transmission device and a reception device making it possible to mitigate the effects of the jitter. In this international patent application, there is described a transmission device capable of transmitting packets in a packet communication network comprising at least two stations and a reception device able to receive packets of a similar network.

A disadvantage of the devices of the prior art is that their operation is linked to a video format defined a priori.

Thus, in case of evolution of the standard of the images conveyed. By standard we mean essentially the duration of an image. For example, we want to put ourselves in a situation in which the image period is reduced from 40 ms to 33 ms, these devices must be adapted to each other: the new value of the period must be communicated to all transmitters and transmitters. reception desk installed.

Another unfortunate consequence of this image-format dependence is that it does not make it possible to use on the network equipment producing images of different formats, for example a switch between a first source producing NTSC format images and a second source producing images in the NTSC format. source producing PAL images.

One of the aims of the present invention is to make the transmission and reception devices of the prior art independent of the image format and therefore independent of the format of the synchronization signal.

Presentation of the invention

The technical problem that the present invention proposes to solve is the transmission of a synchronization signal through a switched network. in packets, with a non-constant transmission duration, without prior knowledge of the format of the synchronization signal.

For this purpose, the present invention relates, according to a first aspect, to a transmission device capable of transmitting packets in a packet communication network comprising at least two stations, said device comprising:

means for extracting image tops from a synchronization signal, a top image being uniquely identified by an index i, the top image i being chronologically followed by the top image i + 1, the top index image; i occurring at a date top _^; means for initializing an image counter from said image tops; means for initializing a counter CPT_PCR all "m" zero crossings of the image counter, the counter CPT_PCR being clocked by a clock produced by the image counter; means for sampling the counter

CPT_PCR all periods T _ech , where T _ech is issued from a synchronized time base on all the stations of said network; and means for transmitting packets containing PCR samples _e of the CPT_PCR counter in the network. According to the invention, it comprises:

means for evaluating a Video_Modulus excursion value of the image counter;

means for evaluating a PCR_Modulus excursion value of the counter CPT_PCR from the excursion value Video_Modulus; means for transmitting the Video_Modulus excursion and the PCR_Modulus excursion in the network.

According to a second aspect, the present invention relates to a reception device able to receive packets in a packet communication network. comprising at least two stations, said device comprising: means for receiving packets containing PCR samples _{r of} said network, said PCRr samples coming from data sampled all the Tech periods, where Tech comes from a synchronized time base on all stations of said network; means for regenerating a CSR-PCR ₁ counting ramp using a PLL ₁ phase-locked loop receiving the PCR samples _r and additionally delivering local samples PCR-IoC ₁ all the Tech periods and a reconstituted clock CLK-OUt ₁ ;

means for initializing, at all the zero crossings of the counting ramp CSR-PCR ₁ , a picture counter CPT which is clocked by the reconstituted clock CLK-out1;

means for generating image tops all the zero crossings of said image counter CPT; and

means for reconstituting a synchronization signal from said image tops.

According to the invention, it further comprises: means for receiving packets containing a Video_Modulus excursion value of the image counter and a PCR-Modulus excursion value of the CPT-PCR counter; means for initializing the CSR-PCR ₁ counting ramp as soon as it reaches the PCR-Modulus-1 excursion value;

means for initializing the image counter CPT as soon as it reaches the excursion value video_Modulus-1.

An advantage presented by the invention is to ensure synchronization of equipment from a synchronization signal whose format is not known a priori.

Brief description of the drawings The invention will be better understood by means of the description, given below for purely explanatory purposes, of one embodiment of the invention, with reference to the appended figures: FIG. 1 illustrates the transmission of a Genlock information between two cameras connected by an IP / Ethernet network;

- Figure 2 illustrates the interfacing between an analog domain and an IP / Ethernet network, according to the prior art;

FIG. 3 illustrates the regeneration of the Genlock signal on the reception side according to the prior art;

FIG. 4 illustrates the operation of a phase-locked loop used on the reception side according to the prior art;

- Figure 5 shows the interface between an analog domain and an IP / Ethernet network, according to the invention;

FIG. 6 illustrates the regeneration of the Genlock signal on the reception side according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The current analogue world is interface to the IP / Ethernet network on the sending side, and the IP / Ethernet network is interface to the analogue world on the receiving side, as shown in Figure 1.

In this same figure, the emission side consists of a "Genlock master" (or "Genlock master")

MGE that is connected to an Analog / IP I_AIP interface. The Genlock MGE Master produces a Genlock SGO signal for I_AIP interfaces.

The reception side consists of two cameras (CAM1, CAM2) each connected to an interface

IP / Analogue I_IPA. The I_IPA interfaces that will eventually be included in the cameras themselves have the task of reconstructing Genlock signals SGl, SG2 to destination CAMl CAM2 cameras. The cameras CAM1, CAM2 each produce a video signal SV1, SV2 that is desired to synchronize perfectly.

The transmitting and receiving sides are interconnected by a packet-switched network which causes jitter appearing on the Genlock SGO signal.

A sampling top, at the time T _ech , is generated from a first synchronization layer, for example IEEE1588, is addressed to the transmission and reception sides. Indeed, the PTP protocol (acronym for Precision Time Protocol) based on IEEE1588 makes it possible to obtain synchronization between the devices connected on an Ethernet network of the order of a microsecond. In other words, all time bases of all devices evolve at the same time to an accuracy close to the microsecond. These time bases can be used in this case to generate each their own sample at the time T _ech . The use of the IEEE1588 layer is not a must. Any system for providing sampling tops at the T _ech period on different devices connected on a network may be suitable. For example, a 5ms sampling sample from a physical wireless transmission layer may be used.

In Figure 2, we detail the processing of Genlock SGO signal from MGE, within the I_AIP interface. First, an EXS module extracts timing information from the SGO signal to retrieve a timing video clock (denoted CIk video in Figure 2). More precisely, the EXS module is in charge of generating a top image at each image start. In addition, the EXS module includes an image counter, for example a 40 ms counter, which is not shown in FIG. 2. The output of this counter image evolves according to a counting ramp passing through 0 at each image period, that is to say every 40 ms if we consider the image counter cited above as an example.

The image counter delivers a signal in "stair steps". The steps have a unit height (or increment count). The signal excursion, that is to say the height corresponding to the level difference between the highest steps and the lowest steps is equal to 40ms. F _out , where F _out is the frequency of the CIk video clock. The counter CPT successively delivers all the integer values from 0 to 40 ms.F _out -1.

The Clk_video timing video clock is used to clock a CPT_PCR counter. The output of the counter CPT_PCR is a counting ramp CSE-PCR ₁ , whose period is worth m image periods. All "m" images, the counter CPT_PCR is reset, that is to say that the counting ramp CSE_PCR is reset to 0.

The term "counting ramp" designates a "stair step" signal whose steps have a unit height (or counting increment). The signal excursion, that is the height corresponding to the level difference between the highest of the steps and the lowest of the steps, is equal to m.40ms.F _out . _CPT-1 PCR counter supplies successively all integer values from 0 to -1 M.40 ms.F _out.

Subsequently, an LCH module samples the counting ramp CSE_PCR is all T _ech periods to produce PCR _e samples. These PCR _e samples are sent over the network and reach the reception side through a network interface (INTE block).

Figure 3 shows the receiving side according to the prior art. The I_IPA interface retrieves PCR _e samples that have been sent over the network. These PCR _e samples are received through a network interface (INTR module) with a delay related to the transport between the transmitting device and the receiving device: the INTR module produces PCR samples _r . _E PCR samples which were produced at regular intervals T _ech transmission side, arrive at irregular intervals receiving side: this is due mainly to the jitter introduced during transmission over the network. The PCR samples _r are taken into account at regular intervals T _ech and because of this, the majority of the jitter introduced during the transport of packets is eliminated.

The inaccuracy between the sampling instants realized on the emission side and those realized on the reception side is absorbed by a PLL phase lock loop ₁ whose bandwidth is appropriate. The characteristics of the PLL ₁ loop guarantee CLK-OUt ₁ regenerated clock generation with reduced jitter.

The phase locked loop PLL ₁ behaves as a system receiving PCR samples _r and delivering:

- a reconstituted clock CLK-OUt ₁ ,

a counting ramp CSR-PCR ₁ and,

- Local samples PCR-IoC ₁ .

When the PLL loop ₁ operates in steady state mode, the PCR samples _r are substantially equal to the PCR-IoC ₁ samples.

The reconstituted clock CLK-OUt ₁ clock a CPT image counter similar to the emission-side image counter, for example a 40 ms counter. The CPT image counter is reset each time the counting ramp CSR-PCR ₁ passes by 0. Between two successive initializations, the image counter CPT evolves freely and produces a top image which feeds a local generator of

Genlock, GEG to produce a reconstructed Genlock signal SG1, SG2 for synchronizing the cameras CAM1, CAM2.

The reconstructed Genlock signal SG1, SG2, which is generated from the counting ramp CSR-PCR ₁ and reconstituted clock CLK-OUt ₁ is in phase with the Genlock SGO signal on the transmitting side, at the clock stroke. PLL ₁ provides filtering which can be modeled as a 2nd order low pass filter. FIG. 4 illustrates a mode of operation of a PLL phase locked loop ₁ employed in an I_IPA interface. As represented in FIG. 4, the phase-locked loop PLL ₁ comprises: a corrector / comparator CORCMP ₁ which receives the PCR samples _{r as} well as the local samples PCR-IoC ₁ . CORCMP ₁ delivers a result of comparing the samples;

a parameterizable oscillator VCO ₁ which is controlled by a sample comparison result and which delivers a reconstituted clock CLK-OUt ₁ ;

a CPT-PCR counter ₁ producing a counting ramp CSR-PCR ₁ at a rate printed by the reconstituted clock CLK-OUt ₁ ; a value maintenance system LATCH ₁ , which generates local samples PCR-IoC ₁ from the values of the counting ramp CSR-PCR ₁ at times T _ech ;

Figure 5 illustrates the generation of PCR samples _e listed emitting device according to the invention. This invention is of particular interest when the format of the synchronization signal is unknown a priori or is modified over time, that is to say when two consecutive image tops are separated by an unknown duration a priori or by a duration likely to vary over time.

One uniquely identifies a top image by an index i. The top image i being chronologically followed by the top image i + 1, the top index image i occurs at a top _^ date. The image counter is reset at each image top. When a duration At ₁ separating the top image of index i + 1 from the top image of index i is fixed in the time, we can speak of an excursion of the image counter and an excursion of the counter CPT_PCR.

In what follows, the duration At ₁ is unknown a priori or is likely to vary over time, it is defined by "excursion of the image counter" the last value at the output of the image counter before the reset made on the date top _{i +1} .

The analog / IP interface I_AIP further includes an EXC excursion evaluation module. The EXC module includes:

means for evaluating a Video_Modulus excursion value of the image counter;

means for evaluating a PCR_Modulus excursion value of the counter CPT_PCR from the excursion value Video_Modulus;

means for transmitting in the network the excursion values Video_Modulus andPCR_Modulus.

Advantageously, for example, the excursion value PCR_Modulus is evaluated by a relation of the type PCR_Modulus = m. Video_Modulus.

The EXC module initializes the Video_Modulus excursion value when the transmitting device is turned on.

Subsequently, the Video_Modulus and PCR_Modulus excursion values are reset when a change in the timing signal format is detected, i.e., when a change in the duration At ₁ is detected. Resetting tour values includes the following steps:

- a suppression of the current Video_Modulus and PCR_Modulus excursion values;

- an evaluation of a new Video_Modulus excursion value; an initialization of the image counter; an evaluation of a new PCR_Modulus excursion value; an initialization of the counter CPT_PCR. After each evaluation of the Video_Modulus and PCR_Modulus excursion values, these excursion values are transmitted over the network. Advantageously, the transmission device further comprises means for comparing, at each top image i, a duration At ₁ separating the top image i and the top image i-1 at the duration At _1-1 .

Advantageously, the comparison means make a comparison between a value of a counter sample CPT_PCR made at the top _i date and a theoretical value determined from the excursion value Video_Modulus.

Advantageously, the transmitting device further comprises means for resetting the Video_Modulus excursion value and the excursion value.

PCR_Modulus when a difference between the duration At ₁ and the duration At _1-1 is detected.

FIG. 6 illustrates the regeneration of the Genlock signal by an IP / Analog interface I_IPA according to the invention. The IP / analog interface I_IPA further comprises means for:

receiving packets containing a Video_Modulus excursion value of the image counter and a PCR_Modulus excursion value of the counter CPT_PCR;

- initialize the counting ramp CSR-PCR _{1 as} soon as it reaches the excursion value PCR_Modulus-1;

- initialize the image counter CPT as soon as it reaches the excursion value video_Modulus-l;

The invention is described in the foregoing by way of example. It is understood that the skilled person is able to realize different variants of the invention without departing from the scope of the patent.

Claims

A transmission device capable of transmitting packets in a packet communication network comprising at least two stations, said device comprising:

- means for receiving image tops, a top image being uniquely identified by an index i, i being the top picture chronologically followed by the top image i + 1, the top image of index i occurring at a date top _^ ;

means for initializing an image counter from said image tops;

means for initializing a counter (CPT_PCR) all the "m" zero crossings of the image counter, the counter (CPT_PCR) being clocked by a clock produced by the image counter;

means for sampling the counter (CPT_PCR) all the periods T _ech , where T _ech originates from a synchronized time base on all the stations of said network; and means for transmitting packets containing the samples (PCR _e ) of the counter (CPT_PCR) in the network, characterized in that it comprises: - means for evaluating an excursion value

Video_Modulus of the image counter;

means for evaluating a PCR_Modulus excursion value of the counter (CPT_PCR) from the Video_Modulus excursion value; means for transmitting the excursion values Video_Modulus and PCR_Modulus into the network.

2. Transmission device according to claim 1, characterized in that it further comprises means for comparing, at each top image i, a duration At ₁ separating the top image i from the top image i-1 to the duration At _1-1 .

3. Transmission device according to claim 2, characterized in that the comparison means make a comparison between a value of a counter sample (CPT_PCR) made at the top _i date and a theoretical value determined from the value Video_Modulus tour guide.

4. Transmitting device according to claim 3, characterized in that it further comprises means for modifying the excursion value Video_Modulus and the excursion value PCR_Modulus when a difference between the duration At ₁ and the duration At _1-1 is detected.

Receiving device capable of receiving packets in a packet communication network comprising at least two stations, said device comprising:

means for receiving packets of said network containing samples (PCR _r ) coming from data sampled at all times T _ech , where T _ech originates from a synchronized time base on all the stations of said network; means for regenerating a counting ramp

(CSR-PCR ₁ ) using a phase locked loop (PLL ₁ ) receiving the samples (PCR _r ) and further delivering local samples (PCR-IoC ₁ ) all periods T _ech and a clock reconstituted (CLK-OUt ₁ );

means for initializing, at all zero crossings of the counting ramp (CSR-PCR ₁ ), an image counter (CPT) which is clocked by the reconstituted clock (CLK-OUt ₁ ); means for generating image ticks at all zero crossings of said image counter CPT; and characterized in that it further comprises:

means for receiving packets containing a Video_Modulus excursion value of the image counter and a PCR_Modulus excursion value of the counter (CPT_PCR);

means for initializing the counting ramp (CSR-PCR ₁ ) as soon as it reaches the value PCR_Modulus-1;

means for initializing the image counter (CPT) as soon as it reaches the video_Modulus-1 value.