FR2920942A1 - Genlock type synchronizing signal packet transmission device for e.g. Internet Protocol type network, has transmission unit for transmitting video modulus deviation value and program clock reference modulus deviation value to network - Google Patents

Abstract

The device has a receiving unit for receiving image tops, and an initializing unit for initializing a corel photopaint image (CPT) image counter from the image tops. A measurement unit measures a video modulus deviation value of the image counter and measures a program clock reference modulus (PCR Modulus) deviation value of a corel photopaint image and program clock reference counter from the video modulus deviation value. A transmission unit transmits the video modulus deviation value and the program clock reference modulus deviation value to packet switching communication network. An independent claim is also included for a device for receiving a packet in a packet switching communication network.

Description

BACKGROUND 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 receiver device for transmitting a synchronization signal, for example of Genlock type 10 (synchronization lock), over a packet-switched communication network, for example of type IP (acronym for the English expression Internet Protocol), whether the network is wired (for example Ethernet (IEEE802.3)) or non-wired, (for example IEEE 15 802.16 D-2004). STATE OF THE ART Advances in the ability of IP networks to carry all types of signals (data or video) make it possible to use such networks as backbone architecture for video studios. A major interest of this evolution is to have then a unique infrastructure for the transport of data. Whereas in the past, several media were needed to carry different types of signals between the devices, the multiplexing properties offered by the IP layer can reduce to one the number of necessary media: 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 else 2 black burst. For example, the Genlock signal is composed of two synchronization signals, one is repeated every 40 ms and indicates the start of the video frame, the other is repeated every 64 s (for a standard format and less for a HD format) and indicates the beginning of the lines in the video frame. The waveforms of the synchronization signals are a function of the format of the image transmitted on the network. For example, for a high-definition image, the synchronization signal has a shape called tri-level (-300mV, 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, there are known devices 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 jitter attenuation devices are described in PCT international application FR2007 / 050918, they act on so-called digital counter signals (or PCR which is the acronym for the English expression Program Clock Reference ), which are representative of very precise reference clock signals. 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 jitter mitigation devices of the prior art is that their operation is related 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 transmission devices and 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 format of the synchronization signal.

DISCLOSURE OF THE INVENTION The technical problem that the present invention proposes to solve is the transmission of a synchronization signal through a packet switched network, 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 picture tops to 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 top date. ; -initializing an image counter from said image tops; - Initialize a counter CPT_PCR every m zero crossings of the image counter, the counter CPT_PCR being clocked by a clock produced by the image counter; - Sampling the counter CPT_PCR all Teh periods, where Teh is from a synchronized time base on all stations of said network; and transmitting packets containing the PCR samples of the counter CPT_PCR in the network, characterized in that it comprises means for: evaluating a Video_Modulus excursion value of the image counter; - evaluate a Modulus PCR excursion value of the CPT_PCR counter from the VideoModulus excursion value; - Issue the Video Modulus excursion and the Modulus PCR 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 PCRr samples of said network, said PCRr samples from data sampled at all Tech times, where Tech is derived from a synchronized time base on all stations of said network; regenerating a CSR_PCR1 counting ramp using a PLL1 phase-locked loop receiving the PCRr samples and delivering in addition local samples PCR_loc1 for all the Tech periods and a reconstituted clock CLK_out,; - Initialize, at all zero crossings of the counting ramp CSR_PCR1, a picture counter CPT which is clocked by the reconstituted clock CLK_outl; Image-generating all zero crossings of said image counter CPT; and - reconstituting a synchronization signal from said image tops; 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; - initialize the counting ramp CSR_PCR1 35 as soon as it reaches the excursion value PCR_Modulus-1; - initialize the image counter CPT as soon as it reaches the value of excursion videoModulus-1; An advantage presented by the invention is to ensure equipment synchronization from a synchronization signal whose format is not known a priori.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood from the following description, given purely for explanatory purposes, of one embodiment of the invention, with reference to the appended figures: FIG. transmission of 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; Figure 4 illustrates the operation of a phase-locked loop employed on the receiving side according to the prior art; - Figure 5 interfacing between an analog domain and an IP / Ethernet network, according to the invention; - Figure 6 the regeneration of Genlock signal reception side according to the invention. DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION The present analog world is interfaced to the transmission-side IP / Ethernet network, and the IP / Ethernet network is interfaced to the receiving-side analog world, as illustrated in FIG. the same figure, the emission side consists of a Genlock Master (or Genlock master) MGE which is connected to an Analog / IP interface I_AIP. The Genlock MGE Master produces a Genlock SGO signal for AIP I interfaces. 7 The reception side consists of two cameras (CAM1, CAM2) each connected to an IP / Analog interface I_IPA. The I_IPA interfaces that will ultimately be included in the cameras themselves are responsible for reconstructing Genlock SG1, SG2 signals for cameras CAM1, CAM2. The cameras CAM1, CAM2 each produce a video signal SV1, SV2 which one wishes 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 period Teh, 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 one microsecond. In other words, all the time bases of each device evolve at the same time to an accuracy close to the microsecond. These timebases can be used in this case to generate each their own sample at the Teh period. The use of the IEEE1588 layer is not a must. Any system for providing sample rates at the Teh period on the 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 processing of the Genlock SGO signal from MGE, within the interface I_AIP. First, an EXS module extracts timing information from the SGO signal to retrieve a timing video clock (denoted Clk 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 comprises an image counter, for example a 40 msec counter, which is not shown in FIG. 2. The output of this image counter changes 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 sawtooth signal composed of a succession of steps. The steps have a unit height. The total excursion of the signal, that is to say the height corresponding to the difference in level between the lowest high of the steps and the highest of the steps is equal to 40ms.Fout, where Fout is the frequency of the Clk_video clock. The counter CPT successively delivers all integer values from 0 to 40 ms. Fout-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_PCR1, whose period is worth m image periods. All images, the counter CPT_PCR is reset, that is to say that the counting ramp CSE_PCR is reset to 0. By counting ramp is also designated a sawtooth signal composed of a succession of steps of staircase. The steps have a unit height. The total excursion of the counting ramp CSE_PCR, that is to say the height corresponding to the level difference between the highest of its steps and the lowest of its steps is equal here to m.40ms. FOAT. The counter CPT_PCR successively delivers all the integer values from 0 to m.40 ms.Fout-1.

Subsequently, an LCH module samples the counting ramp CSE_PCR every Teh period to produce PCRe samples. These PCR samples are sent on the network and flow to the receiving side through a network interface (INTE block).

Figure 3 shows the receiving side according to the prior art. The I_IPA interface retrieves PCRe samples that have been sent over the network. These PCR 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 PCRr samples. The PCR samples that have been produced at regular intervals on the transmit side, arrive at irregular intervals on the receiving side: this is due mainly to the jitter introduced during transport on the network.

PCRr samples are taken into account at regular Tech intervals and as a result, most of the jitter introduced during packet transport is eliminated. The inaccuracy between the sampling instants made on the emission side and those made on the receiving side is absorbed by a PLL1 phase-locked loop whose bandwidth is appropriate. The characteristics of the PLL1 loop guarantee CLK_out reconstituted clock generation with reduced jitter. The phase locked loop PLL1 behaves as a system receiving PCRr samples and delivering: - a reconstituted clock CLK_out1, - a counting ramp CSR_PCR1 and - local samples PCR_loc1. When the PLL1 loop is operating in steady state, the PCRr samples are substantially equal to the PCRloc1 samples. The reconstituted clock CLK_out, clock a CPT image counter similar to the emission-side image counter, for example a 40 ms counter. The image counter CPT is reset each time the count ramp CSR_PCR1 passes by 0. Between two successive initializations, the image counter CPT evolves freely and produces a top image which feeds a local Genlock generator, GEG to produce a reconstructed Genlock signal SG1, SG2 intended to synchronize the CAM1, CAM2 cameras. The reconstructed Genlock signal SG1, SG2, which is generated from the counting ramp CSR_PCR1 and reconstituted clock CLK_out, is in phase with the Genlock SGO signal on the sending side, at the clock stroke. The PLL1 loop provides filtering that can be modeled as a 2nd order low pass filter.

FIG. 4 illustrates a mode of operation of a PLL1 phase-locked loop used in an I_IPA interface. As represented in FIG. 4, the PLL1 phase-locked loop comprises: a CORCMP corrector / comparator, which receives the PCRr samples as well as the local PCR loci samples. CORCMP, delivers a sample comparison result; a parameterizable oscillator VCO1 which is controlled by a sample comparison result and which delivers a reconstituted clock CLK_out, a counter CPT_PCR1 producing a counting ramp CSR_PCR1 according to a rate printed by the reconstituted clock CLK_out; a value maintenance system LATCH ,, which generates local samples PCR_loc1 from the values of the counting ramp CSR_PCR1 at times Teh;

FIG. 5 illustrates the generation of PCR samples on the emission side according to the invention. This invention is of particular interest when the format of the synchronization signal is unknown a priori or is changed over time, that is to say when two consecutive image tops are separated by an unknown duration a priori or else by a duration that may 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 top image too, when a duration At, separating the top image from i + 1 index of the index image i is fixed in time, we can speak of a total excursion of the image counter and a total excursion of the counter CPT_PCR. In the following, the duration At, is unknown a priori or is likely to vary over time, it is defined by the displacement of the image counter the last value of the output of the image counter before the reset performed at the date top. ,. The analog / IP interface I_AIP further comprises an EXC excursion evaluation module. The EXC module includes means for: evaluating a Video_Modulus excursion value of the image counter; - evaluate a Modulus PCR excursion value 25 of the CPT_PCR counter from the VideoModulus excursion value; to issue the Video Modulus excursion and the Modulus PCR excursion in the network.

Advantageously, for example, the PCR_Modulus excursion value is evaluated by a PCR type relationship Modulus = m. Video Modulus. The EXC module initializes a Video Modulus excursion value when the transmitting device is turned on. Subsequently, the Video Modulus and PCR Modulus excursion values are reset 12 when a change in the timing signal format is detected, i.e., when a change in the duration At, is detected. A reset of the excursion values has the following 5 steps. - a suppression of the current VideoModulus and PCRModulus excursion values; - an evaluation of a new VideoModulus excursion value; An initialization of the image counter; - an evaluation of a new PCRModulus excursion value; an initialization of the counter CPT_PCR.

After each Video_Modulus and PCR_Modulus excursion value evaluation, the Video Modulus and PCR Modulus excursion values are transmitted over the network. Advantageously, the transmission device 20 further comprises means for comparing, at each top image i, a duration At, separating the top image i from the top image i-1 to the duration Ati_1. Advantageously, the comparison means make a comparison between a value of a sample of the CPT_PCR counter made on the tops date and a theoretical value determined from the Video Modulus excursion value. Advantageously, the transmitting device further comprises means for resetting the Video Modulus excursion value and the PCR_Modulus excursion value when a difference between the duration At, and the duration Ati_1 is detected.

Figure 6 illustrates the reception of the Genlock signal according to the invention. The IPA / analog interface 1 IPA further comprises means for: 13 -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_PCR1 as soon as it reaches the excursion value PCR_Modulus-1; - initialize the image counter CPT as soon as it reaches the value of excursion videoModulus-1; 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. 10 15 20 25 30

Claims (5)

A transmission device capable of transmitting packets in a packet communication network comprising at least two stations, said device comprising means for: receiving image signals, 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 top date. ; - initialize an image counter from said image tops; - Initialize a counter CPT_PCR every m zero crossings of the image counter, the counter CPT_PCR being clocked by a clock produced by the image counter; sampling the counter CPT_PCR all the periods Teh, where Teh comes from a synchronized time base on all the stations of said network; and - issuing packets containing PCRe samples of the counter CPT_PCR in the network, characterized in that it comprises means for: - evaluating a Video Modulus excursion value of the image counter; evaluate a Modulus PCR excursion value of the CPT_PCR counter from the VideoModulus excursion value; - issue the Video Modulus excursion and the Modulus PCR excursion in 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, separating the top image i from the top image i-1 to the duration Ati_1. Transmitting device according to Claim 2, characterized in that the comparison means make a comparison between a value of a sample of the counter CPT_PCR carried out on the top date, and a theoretical value determined from the value of Video Modulus tour. 4. Transmitting device according to claim 3, characterized in that it further comprises means for resetting the Video Modulus excursion value and the excursion value PCR_Modulus when a difference between the duration At, and the duration Ati_1 is detected. 5. Receiving device capable of receiving packets in a packet communication network comprising at least two stations, said device comprising means for: receiving packets containing PCRr samples of said network, said PCRr samples coming from sampled data all the periods Teh, where Teh is derived from a synchronized time base on all stations of said network; regenerating a CSR_PCR1 counting ramp using a PLL1 phase locked loop receiving the PCR samples and further delivering PCRlac local samples, all Tech periods and a reconstituted clock CLK_out,; initializing, at all zero crossings of the counting ramp CSR_PCR1, a picture counter CPT which is clocked by the reconstituted clock CLK_out 1; generating all the zero crossings of said image counter CPT; and characterized in that it further comprises means for. receive packets containing a Video_Modulus excursion value of the image counter and a PCR_Modulus excursion value of the counter CPT_PCR; initialize counting ramp CSR_PCR1 as soon as it reaches the excursion value PCRModulus-1; initialize the CPT image counter as soon as it reaches the Modulus-1 video excursion value.