BROADCAST VIDEO DESYNCHRONISER

Abstract: An arrangement for desynchronising video signals broadcast over a synchronous network comprises a feedback loop incorporating a FIFO store (21) and a tunable oscillator (25) adapted to provide a video line clock from demapped video information. The oscillator frequency is controlled such that the rate of information flow around the loop is substantially constant. Video bytes are written into the store and are then read at a rate determined by the oscillator frequency. This substantially eliminates phase disturbances caused by the arrival of video pointers in the synchronous signal.

Description: BROADCAST VIDEO DESYNCHRONISER This invention relates to the provision of television or video services on a synchronous network, and in particular to the desynchronisation of such services at a user station BACKGROUND OF THE INVENTION The provision of 34Mbιts television services on the synchronous network requires stringent control of low frequency phase at the desynchroniser As 34M video codecs linearly encode the colour sub carrier of the PAL signal, any phase disturbances introduced by the synchronous transport (e.g SDH) network propagate through the codec and into the reconstituted video There are two bounding parameters that the video network providers consider to be important to deliver an acceptable quality of service to customers First the phase transients in the bit stream must be such that the colour sub carrier, at 4 43MHz will have no greater than a 0 1 Hz per second rate of change Second the delay budget in a telephony link must be no more than 1 00 iLr' se onds The final parameter of interest in bounding the desynchi o iser system is that the network requires a minimum of 30s to set up a connection This is very much a worst case scenario and moie common switch times are in the order of up to 30 minutes and more The most difficult problem is that of eliminating phase disturbances arising from the arrival of the video pointers which are used to bring the net flow of information back to the original input rate A number of techniques have been pioposed to address this problem, but none has been entirely successful The object of the invention is to m inim ise or to overcome this disadvantage SUMMARY OF THE INVENTION Accordmg to one aspect of the invention there is provided an arrangement for desynchronising video signals broadcast over a synchronous network, the arrangement including a feedback loop incorporating a tunable oscillator adapted to provide a video line clock from demapped video information, there being means for controlling the oscillator frequency such that the rate of information flow around the loop is substantially constant According to another aspect of the invention there is provided an arrangement for desynchronising video signals broadcast over a synchronous network, the arrangement including a feedback loop incorporating a buffer store for demapped video information, atunable oscillator adapted to provide a video line clock from the demapped video information there being means for controlling the oscillator frequency and for reading the video information from the store at a rate determined by the oscillator frequency such that the rate of information flow around the loop is substantially constant According to a further aspect of the invention there is provided a method of desynchronising video signals broadcast over a synchronous network the method including demapping a video information stream from the broadcast signal, passing the demapped information around a feedback loop incorporating a buffer store for the information and tunable oscillator providing a video line clock controlling the frequency of the oscillator and reading information f rom the store at a rate determ ined by the oscillator frequency such that the rate of information flow around the loop is maintained substantially constant Reference is here directed to our copending application No 2 283,885 which relates to a method of controlling the leakage of a pointer for a pointer justification event (PJE) at the exit from a synchronous transport network to a piesiochronous tributary when video services are carried on the network In that application the leak rate of a primary ouffer is modulated in a manner whereby the rate of change leak rate is dependent on the rate of arrival of PJE's BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention will now be described with reference to the accompanying drawings in which - Figure 1 is a schematic diagram of a synchronous network provided with video broadcast facilities, and Figure 2 is a schematic diagram of a desynchroniser arrangement for use in the network of figure 1 DESCRIPTION OF PREFERRED EMBODIMENT Referring to figure 1 , video information from a transmitter station 10 is mapped into virtual containers via a codec 1 1 for transmission across the synchronous network 12 At the receiving station or user terminal 13, the video information in the arriving containers is demapped via a codec 1 4 and is then fed to the user terminal for display The process for recover ing the video infoi mation is described with refei ence to figure 2 below Referring to figure 2, low frequency phase control is provided by an arrangement based on a control loop This loop uses the incoming C3 demapped information stream to control the frequency of a voltage controlled oscillator, e g a crystal oscillator (VCXO) which in turn provides the 34M line clock We have found that th is a rrangement operates effectively as the phase noise due to poιnte r and other activities in the information flow into the control loop is outside the bandwidth of the loop Thus incoming phase noise appears on the output as a ripple The loop is therefore designed to maintain this ripple within the video parameters The loop comprises a FIFO buffer 21 which accepts bytes arriving at input port 20 from the C3 demapping function and provides a control signal via an amplifier 22 and a transfer function circuit 23 to tne oscillator 25 The oscillator output is fed back via an integrator 26 to a second input of the FIFO buffer The fill depth of the buffer is the error reference used by the control loop This error is filtered and goes through a digital to analogue conversion to provide the input drive to the oscillator 25 . This oscillator is used to read the bytes from the FIFO and provides the reconstituted line clock at output port 27. The principle of operation of the loop is that the rate of information flow is substantially constant. That is to say that the original input to the synchronous network is from a serial 34M bit stream and the bit rate of this is effectively encoded in the justification pattern of the incoming C3 information stream and in the pointer activity. Thus when an SDH frequency offset comes into play, the arriving pointers operate to bring the net flow of information back to the original input rate. For example, if the receiving node is receiving an AU4 at +3ppm, the rate of arrival will be too high. The node will re transmit with negative pointer justifications keeping the average information flow rate constant. The pointer activity may thus be viewed as phase disturbance on the 34M serial bit stream. The amplitude of the phase disturbances has two parts. One part is consequential to the SDH framing and is entirely predictable. The other is the pointer amplitude which is, in the worst case, 8 bits. As the time constant of the control loop is relatively long, a sampled digital system may have a low sampling interval, and for validity, the sampling should occur at a point where the input is stable. This occurs during the section overhead part of the frame. The maximum sampling rate possible in this scheme is thus on a per row basis giving 72kHz. The loop may be described by the phase transfer function Φ n sGτ Φ, (1 ) ,v : + τs + Gτ Assuming that the worst step height will be 16 bits, this function may be used to design the loop. The worst case of 16 bits is based on twice that which would be expected from a pointer. The response to a step is given by multiplying through by K/s and transferring back into the time domain. There are two roots to the denominator. These roots are given as -τ±Vτ 2 - AGτ (2) ln a critically, or over damped system the loop transfer function in the form sω,r (3) ,v 2 + 2ζω„s+ ω, l gives the roots as The optimal system will be one where the response is critically damped. From equation 3 above, the step response will be where K is the step height. This equation is of the form 1 . 1 ,»-! -nl ,ps {s + a)" (n - iy. e With n = 2 and a = ω,, . Thus the time domain response to a step input is The parameter of interest in the desynchroniser for video use, is the rate of change of phase. This can be found from differentiating equation (7) with respect to time giving Φ,> = Kω» l -ω,,t e" Λ " + e ' " M ) (8) The maximum of the modulus of Φ»will occur when t = 0. Therefore Φonu = Kω,r (9) Going back to the video specification of 0.1 Hz per second at 4.43MHz, this can be expressed as 0.0225ppm/s and applied to any carrier rate. At the 34.368Mbit carrier rate, and using 0.0223ppm/s for margin, this corresponds to 0.76bits, or 0.76UI per second. This is Φomax. So in response to a 16 bit step, K = 16 and ω.< is derived as 0.22rad/s. Using the format of equation 5 in equation 3 the values of the time constant and gain follow as 0.44 and 0.11 respectively. For final implementation of the circuit, some boundaries may be defined. The VCXO should preferably have a gain typically of 50ppm/V. Thus the maximum swing may be + or - 2.0V about a 2.5V mean. The overall VCXO gain includes a scalar for the centre frequency, as the deviation is a few parts per million of the centre frequency. Therefore the gain may be split into three components, G the calculated gain, G v the VCXO gain in ppm/V and Gf the centre frequency component. Thus 0.109 = G G v Gf. Solving for G-|, we have G, = °- 109 = 6. x Kr 5 (10) 50 x 34.368 When in use the loop will effectively be driven by a phase ramp. The VCXO responds to the difference between the information flow rate into the FIFO from the C3, and its own centre frequency. Therefore, in the steady state, a static error is present in the buffer representing the drive voltage to maintain the offset. The step input to the system is given as amplitude 16bits. The rate of change of frequency is detected by the du/dt term and the output response gives an initial rate of change of frequency of 0.76. The frequency performance of the loop may be determined by providing an input ramp. In this case, the interesting activity is the response of the system to a step change in input frequency. Such conditions can occur in the network if a user switches the 34M line from one source to another. This may give a theoretical maximum step change of 40ppm. The initial conditions of the FIFO buffer 1 1 are set to leave enough room for the step inputs to go to the positive or negative limits. The maximum allowable step change in input is given by the 34M line being switched from a -20ppm line to a +20ppm line. This gives a step input of 40ppm swing. This will always be the maximum amplitude and a first approximation of the FIFO depth requirements are to equal this with a centre at the mid point. Using a 40ppm step the max. fill is given as This is solved to give a step amplitude of 12699 bits, or 1588 bytes. Consideration should also be given to the steady state error introduced by the variation in centre frequency of the VCXO. Taking this to be typically +/-25ppm, the swing from this is greater than the input line frequency. The effect of this is to skew the FIFO fill about a point not equal to the mid point. For example, if the VCXO is supplied with an open loop centre frequency of +25ppm, this would mean that the output frequency would be at +25ppm offset when the input voltage was at 2.5V. Thus to drive a nominal Oppm line offset, a steady state drive voltage of 0.5V is required. This 0.5 V will correspond to a buffer "fill" of 7936bits. In a practical implementation, the FIFO will be given a fixed centre, such that the fill, during any of the input conditions will never cause the read and write address printers to coincide or crash. Therefore in the simple case this will be equal to half the maximum swing amplitude plus a margin for frame jitter and pointer bursts. Nominally this will be 794 bytes, from the solution to equation 1 1. However, if a steady state error exists, this will form an additional offset to the FIFO centre. A preferred way of overcoming this is to use the fact that the centre frequency is a steady state value which may be tuned out at manufacture. This may be effected by providing an additional steady state error register which is used at manufacture to set the open loop centre frequency. This value is stored in non volatile memory and written to the mapper at power up. The magnitude of the steady state error is important to balance against the dynamic range of the VCXO. The +/-25ppm variation corresponds to a maximum of 1V of range required on a 50ppm/V device. The available dynamic range for line tracking is therefore reduced to 3V as +/-1.5V. At 50ppm this allows for 150ppm tracking which can adequately cover the line variations in a second order control system. However, this is just inside the limit for the more complex control situation following. (A 30ppm VCXO would be unsuitable, as the 50ppm variation would require 1.8V to tune out. This would leave a range of 2.2V corresponding to 66ppm. This is just over the required 40ppm, but would not allow the following cases to function). The primary limiting factor in the second order loop is the static error. This should not be confused with the previously described steady state error. The static error is that generated by the frequency deviation of the 34M line. This deviation causes a residual fill in the buffer which in turn drives the VCXO in the closed loop control system. This error is constant, after settling, and directly proportional to delay (with a constant of proportionality of 2.91 x10" 8 seconds per bit at 34.368Mbit/s line rate). With the system simulated, a FIFO size of 1588 bytes is required centred about 794bytes. The delay through this system is be determined by the fill state of the buffer, which in turn is determined by the input frequency deviation. A deviation of -20ppm will have a nominally zero delay and a deviation of +20ppm will have nominally 370 microseconds. The 100 microsecond maximum delay specification is therefore compromised. It is preferred to remove the delay dependence on input frequency deviation by eliminating the static error. In order to drive the static error to zero it is necessary to provide effectively an "imaginary" static error. This takes the form of providing a drive to the VCXO when the system has settled to zero error in the buffer. An integrator is therefore necessary to provide an output when its input is zero. This turns the loop into a third order system by summing the integral of the error scaled to the filtered scaled error term driving the VCXO. The characteristic equation is now given as s{(K + Gτ)s + Kτ) , .r + τ.r + {K + Gτ)s + Kτ Note that when the scalar K is set to zero, the characteristic resumes to the second order original of equation 1. The effect of the integral term may be kept to a minimum if the constant scalar K is around an order of magnitude less than G. Both of these constants are compound, and both share the terms, G and Gf, so the effect can only be achieved by setting Ki an order of magnitude or so less than G i The simplest means of ensuring a better than 0 76ppm/s step response, is to add the third order gain term at tenth gain in parallel with the second order term and adjust on resulting simulations of 16 bit step responses The time constant required reduction and the gam of the integrating term were reduced to approximately a thirtieth The method used here was to adjust the gam down until the roots of the denominator of the third order polynomial in s become real This gives a damping factor of 1 and the third order loop is accordingly critically damped

Claims: CLAIMS:- 1 . An arrangement for desynchronising video signals broadcast over a synchronous network, the arrangement including a feedback loop incorporating a tunable oscillator adapted to provide a video line clock from demapped video information, there being means tor controlling the oscillator frequency such that the rate of information flow around the loop is substantially constant 2. An arrangement for desynchronising video signals broadcast over a synchronous network, the arrangement including a feedback loop incorporating a buffer store for demapped video iπfoi mation atunable oscillator adapted to provide a video line clock from the demapped video information there being means for controlling the oscillator frequency and for reading the video information from the store at a rate determined by the oscillator frequency such that the rate of information flow around the loop is substantially constant 3 An a rrangement as cla imed in claim 2 wherein the stoe comprises a first-in-first-out (FIFO) store into which ed -ideo bytes are written and from which said bytes are read 4 An arrangement as claimed in claim 3 wherein the output of the oscillator is coupled to the FIFO store via an integrator 5 An arrangement as claimed in claim 3 or 4, wnerem the output of the FIFO store is coupled to the oscillator via an amplifier and a transfer function adapted to provide the loop with a critically Odinped response 6. An aπ angement as claimed in any one of the preceding claims wherein said oscillator is a voltage controlled crystal oscillator 7 A telecommunications subscriber terminal provided with a video desynchronising arrangement as claimed in any one of tne preceding claims 8. A method of desynchronising video signals broadcast over a synchronous network, the method including demapping a video information stream from the broadcast signal, passing the demapped information around a feedback loop incorporating a buffer store for the information and tunable oscillator providing a video line clock controlling the frequency of the oscillator, and reading information from the store at a rate determined by the oscillator frequency such that the rate of information flow around the loop is maintained substantially constant

Inventors: SHIELDS JAMES ALEXANDER, SHIELDS, JAMES, ALEXANDER
Applicants: NORTHERN TELECOM LTD, SHIELDS JAMES ALEXANDER, NORTHERN TELECOM LIMITED, SHIELDS, JAMES, ALEXANDER
Application year: 1996
Main classification: H04J3
IPC codes: H04J3