Abstract:
It is disclosed a method for changing channel in a television appliance. Upon reception of a user command to tune on a desired channel ( 301 ), the television appliance is tuned on the desired channel ( 302 ) and audio and video packets are received. Video and audio packets are buffered in relative buffers, so that audio and video output can be generated by processing the buffered packets. Video output frame rate is increased from a first, slower, frame rate to a predetermined final frame rate. Independently from the frame rate increasing law, video output frame rate is raised to the final frame rate as soon as an audio output can be generated from the buffered video packets which is synchronized to the video output. A television appliance implementing the method is also disclosed.

Description:
TECHNICAL FIELD 
     The present invention relates to the field of television appliances, like set-top-boxes or TV sets, which are adapted to receive broadcasted digital television signals. In particular, the invention relates to methods for controlling channel change in a television appliance. 
     BACKGROUND ART 
     As it is known, a television signal comprises both audio and video that shall be synchronized when played. 
     DVB (Digital Video Broadcasting) standards provides for broadcasting transport streams each comprising a plurality of channels (services). Each transport stream comprises a plurality of elementary packets transporting audio, video and data. 
     Each frame of the television signal can require a different bandwidth for its transmission, e.g. I-frames are less compressed then P and B-frames. Each frame can therefore be divided in a different number of elementary packets and transmitted in one or more transport streams. 
     As a result, audio and video are not always transmitted in the same transport stream; actually audio can arrive up to 3 s later then the respective video. 
     When a user changes TV channel, audio and video are displayed only when the television appliance has been able to receive both of them. The result is that when a user changes channel, a black screen is displayed for few seconds or milliseconds depending on the transmission delay between audio and video. 
     If time delay is long, often user can change channel thinking that no signal is available; this often happens during zapping, wherein the user changes channel very rapidly to have an overview of what&#39;s on TV. 
     WO2009/047273 faces the problem of channel zapping time and discloses a method for an early start of audio-video rendering. 
     WO2009/047273 discloses several methods wherein after video stream reception, video rendering is started with a speed which is slower than the normal rendering speed. Rendering speed is then accelerated until a video buffer is filled and video is rendered at normal speed. 
     According to the different embodiments disclosed by WO2009/047273, audio can be rendered synchronized or desynchronized to the video during video speed acceleration. In case audio is rendered desynchronized to video, this can be annoying for the user. In case audio is rendered synchronized to video, an audio pitch algorithm is necessary in order to compensate for audio distortion introduced by the reduced video speed; such an algorithm can be expensive and not that efficient in case of audio rendering at very slow speed. If audio pitch algorithm is not available, according to WO2009/047273 audio is not rendered until video can be rendered at normal speed; this solution has the drawback that audio that could be rendered, is not rendered and the user therefore loses information carried by the audio. 
     OBJECTS AND SUMMERY OF INVENTION 
     It is therefore an object of the present invention to present a method for solving the drawback of channel change in prior art television appliances. 
     In particular it is an object of the present invention to present a method for channel change that reduces probability that a user changes channel thinking that no signal is available. 
     It is also an object of the present invention to present a television appliance, and in particular a set-top-box, implementing the method for channel change. 
     These and further objects of the present invention are achieved by means of a method and a television appliance comprising the features of the annexed claims, which are integral part of the present description. 
     Inventors have thought of a method for changing channel in a television appliance wherein, upon reception of a command to tune on a desired channel, the television appliance is tuned on the desired channel and audio and video packets are received. Video and audio packets are buffered in relative buffers, so that audio and video output can be generated by processing the buffered packets. 
     Video output frame rate (i.e. frequency at which consecutives frames are produced) is increased from a first, slower, frame rate to a predetermined final frame rate. Independently from the frame rate increasing law, video output frame rate is raised to the final frame rate as soon as an audio output can be generated from the buffered video packets which is synchronized to the video output. 
     This solution has the advantage that in case of channel change, video is rendered in short times, before audio is received. At the same time, video and audio are rendered synchronized as soon as this is possible, without waiting for a video buffer to be filled in. 
     In one preferred embodiment, video output frame rate is increased according to a monotone law, preferably an exponential one, before said synchronized audio and video output is generated. In this way user does not feel that the television appliance is not working, but has the impression that it is actually starting to present the TV program. 
     Preferably video output frame rate is increased up to a second predetermined value, preferably lower than the final frame rate, and then video output frame rate is kept constant at said second predetermined value until audio packets are received and synchronized audio and video is output. This solution allows synchronization of audio and video also when audio arrives with a huge delay. 
     In another embodiment, video output is generated after a predetermined time interval from reception of the first video packets. 
     In one aspect, the invention is directed to a computer program comprising program code means for performing all the steps of the method according to the teachings of the following description and claims, when said program is run on a computer. 
     In another aspect, the invention is directed to a computer readable medium storing computer-executable instructions performing all the steps of the computer-implemented method according to the teachings of the following description and claims, when executed on a computer. 
     In another aspect, the invention is directed to a television appliance comprising an input block for receiving a transport stream comprising audio and video packets of a TV channel. A memory block allows buffering audio and video packets, and a video output block and an audio output block allowing output of audio and video. The television appliance further comprises a user interface block for receiving a user command to change TV channel. A control unit is operatively connected to the input block, to the user interface block, to the memory block, to the audio output block and to the video output block. The control unit is suitable for tuning input block on a TV channel selected by a user and for generating synchronized audio and video outputs respectively on the audio block and on the video block. The control unit is further adapted to generate a video output ( 307 ) with a first frame rate and to increase video output frame rate from said first frame rate to a predetermined final frame rate; frame rate is raised to the final frame rate as soon as said memory block contains audio and video packets which can be processed by the control unit to generate synchronized audio and video output. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present invention will become apparent in the detailed description of preferred non-exclusive embodiments of a coding method and of a decoding method, and of relative systems according to the invention, which are described as non-limiting examples with the help of the annexed drawings, wherein: 
         FIG. 1 , schematically represents a television appliance according to the present invention; 
         FIG. 2  shows audio and video output according to an embodiment of the present invention, 
         FIG. 3  is a flow chart of a method according to an embodiment of the invention, 
         FIGS. 4 and 5  shows audio and video output in different cases based on time delay between audio and video. 
     
    
    
     These drawings illustrate different aspects and embodiments of the present invention and, where appropriate, like structures, components, materials and/or elements in different figures are indicated by the same reference numbers. 
     DETAILED DESCRIPTION OF THE INVENTION 
     While the invention is susceptible of various modifications and alternative constructions, certain illustrated embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims. 
     In the following description and in the figures, like elements are identified with like reference numerals. The use of “e.g.,” “etc.,” and “or” indicates non-exclusive alternatives without limitation unless otherwise noted. The use of “including” means “including, but not limited to,” unless otherwise noted. 
       FIG. 1  schematically illustrates a television appliance  100 . For sake of clarity only most important functional blocks of the television appliance  100  are represented in  FIG. 1 , while it is intended that further features can be provided on television appliance  100 . In the following examples, television appliance  100  is a set top box, nevertheless, in other embodiments television appliance  100  can be any device suitable to output a video signal or to directly display a video signal on a screen; as an example, the television appliance can be a TV set, a DVR (Digital Video Recorder), a mobile phone or a palm computer. 
     Television appliance  100  comprises a front end  101  for receiving a digital television signal, in particular a transport stream comprising MPEG compressed video packets. Front end  101  comprises a tuner for tuning on a user selected video channel and for providing an IF (intermediate frequency) signal to a processor  102 . 
     Processor  102  comprises a CPU (central processing unit,  1020 ), an audio processor  1021 , a video processors  1022 , a system interface block  1023 , a connectivity block  1024 . Communication between these blocks is achieved by means of a bus  1025 , e.g. an IC2 bus. 
     An user interface block, in particular an infra-red (IR) receiver,  107  is used for receiving user commands from a remote control (not shown in  FIG. 1 ). 
     When a user selects a TV channel, processor controls front end  101  (in particular a tuner) to tune on the requested TV channel. Television signals from front end  101  are then received by processor  102 , wherein they are demodulated before a parser, executed by CPU  1020 , separate video and audio packets of the received transport stream. While video packets are processed by video processor  1022 , audio packets are processed by audio processor  1021 . 
     System interface block  1023  allows communication with a memory block  103  comprising different type of memories: non-volatile memories  1030  (e.g. FLASH, NAND, NOR), volatile memories  1031  (RAM, DRAM) and storage devices  1032  (e.g. Hard Disk drivers HDD or solid state driver SDD). 
     Non-volatile memories stores drivers and applications necessary for the correct boot up and operation of the television appliance  100 , while storage device are preferably used for storing recorded video streams. 
     Connectivity block  1024  is used managing connection with external components  104 , like USB ports  1040 , Network Interface Cards  1041  communicating via Ethernet protocol, and so on. 
     Processor  102  is therefore adapted to process MPEG A/V signals stored in storage device  1032  or received via connectivity block  1024 , e.g. IPTV (Internet Protocol Television) signal. 
     Television appliance  100  further comprises audio I/O block  105  and video I/O block  106 . Blocks  105  and  106  are used to receive audio and video from different sources, like a DVD reader, a Blu-Ray disk reader, an analog amplifier, a Video Cassette Recorder and so on. Blocks  105  and  106  further represent audio and video outputs, e.g. HDMI output to be provided to a TV set. 
     When processor  102  receives a command to change TV channel, it starts the method hereby described with reference to  FIGS. 2 and 3 . The command to change channel can be issued by a user and received via a user interface ( 107 ) or can be issued by a processor, e.g. in case a timer is set for recording and/or displaying a video. 
     At time t0 processor  102  receives a channel change command (step  301 ) via the IR receiver  107 . In response to the user command, processor  102  controls (step  302 ) tuner of the front end  101  and therefore it starts receiving video packets relative to a TV program broadcasted on the requested TV channel. 
     Elementary video packets are buffered in volatile memory  1031  (steps  303 ). 
     As video packets are received, the method therefore provides for starting a timer to check if audio packets are received before a predetermined time (t1). This timer is represented in  FIG. 3  by blocks  304  and  305 . 
     During time interval t0 to t1, therefore, processor  102  checks if audio packets are received (step  304 ). If yes, synchronized audio and video is output (step  306 ). 
     If audio and video is not received before t1, then the method continues with step  305  to check if time interval t1 has not elapsed. If t1 is passed since first video packets were received, then at time t1 processor starts outputting (step  307 ) the first video frames, herein after also called intermediate video frames. Intermediate video frames are generated from the received video packets and are therefore output for displaying before audio packets are received. In the preferred embodiment, start time t1 for intermediate video output is preferably selected as the minimum time between 500 ms and time difference between PTS and PCR: t1=min(500 ms, PTS-PCR), wherein PTS (“presentation time-stamp”) and PCR (“Program Clock Reference”) are values of MPEG coding defined by ISO/IEC 13818-1: 1994(E). 
     As it can be seen from  FIG. 2 , video frames are output at a reduced frame rate. Most common frame rate in television field is usually 24 FPS (frames per seconds) or 25 FPS or 30 FPS, nevertheless the present invention applies to any frame rate, therefore in the following example the normal frame rate of the video signal is normalized to 1. Considering to be 1 the normal frame rate, in the embodiment of  FIG. 2  video frames are initially output at frame rate equal to 0.2, i.e. one fifth of the final frame rate. 
     Frame rate is then increased up (step  308 ) to reach a frame rate of 1 at time t3, once audio data synchronized with the currently displayed video are output. In other words, frame rate in increased up to reach the final frame rate. From a logical point of view, a loop is created wherein after increasing of the frame rate, a check is made (step  309 ) to verify if audio packets have been received which are ready to be synchronized with video frames. If they are, then synchronized audio and video is output (step  310 ), if not the loop is repeated and frame rate increased. 
     Preferably frame rate is increased according to a monotone law. 
     Preferably increasing of frame rate is exponential. 
     In the embodiment of  FIG. 2  first audio packets are received at time t2, nevertheless, they will be related to frames already displayed and therefore they are not output. Since at time t2 video frame is still displayed at a reduced frame rate, at time t3 audio packets are synchronized with video frames and are ready to be output. 
     Since delay between audio and video (t0−t2) is not known, increasing in frame rate follows a predetermined law which is based on experimental analysis and which is based on expectation that audio will arrive after 2 s after video, in other words t2=2 s. 
     In one preferred embodiment, video frames are output after a delay (t1) of 0.5 s, while frame rate is increased from 50% to 75% from a to t1+0.5 s, and from 75% to 90% from t1+0.5 s to t1+1 s and from 90% to 95% from t1+1 s to t1+1.5 s. 
     In one preferred embodiment, frame rate increment follows a law according to which it is increased from a first predetermined value (preferably comprised between 20% and 50%) to a second predetermined value which is less than 100%, in a predetermined time interval (e.g. 1.5 s). After this predetermined time interval, frame rate is kept at the second predetermined value, so as to allow easier synchronization with audio as shown in  FIG. 4 , wherein audio is shown arriving at a time t2 equal to 2.2 s. This is represented in  FIG. 3  by a verification step  311 , wherein after step  310  a check is made on whether frequency rate (F) has reached this second predetermined value, i.e. its maximum value before synchronized audio and video is output. 
     In the embodiment of  FIG. 4 , video is output at time t1 after 0.5 s with a frame rate equal to 50% of the final frame rate. Frame rate is then increased up to 90% which value is reached at time t4=2 s. Frame rate is then kept at 90% until processor  102  is able to output synchronized audio and video (time t3). As it can be seen from  FIG. 5 , once audio and video can be output synchronized, video output frame rate is immediately raised to the final frame rate; increasing of frame rate to the final value therefore depends only on availability of audio that can be synchronized with video, it is therefore independent of the frame rate increasing law between t0 and t3. 
       FIG. 5  shows the case wherein audio arrives before expected and therefore frame rate has completed its incremental law. In this example, frame rate is increased according to the same law of  FIG. 4 , yet audio arrives at a time t2 equal to 1.5 s and therefore processor  102  is ready to output synchronized audio and video after 1.6 s. 
     While the invention presented herein has been depicted, described, and has been defined with reference to particular preferred embodiments, such references and examples of implementation in the foregoing specification do not imply any limitation on the invention. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader scope of the technical concept. 
     The presented preferred embodiments are exemplary only, and are not exhaustive of the scope of the technical concept presented herein. Accordingly, the scope of protection is not limited to the preferred embodiments described in the specification, but is only limited by the claims that follow. 
     It can be easily recognized, by one skilled in the art, that the aforementioned appliance and method for channel change may be performed and/or controlled by one or more computer programs. Such computer programs are typically executed by utilizing the computing resources in a computing device such as personal computers, personal digital assistants, cellular telephones, receivers and decoders of digital television or the like. Applications are stored in non-volatile memory, for example a flash memory or volatile memory, for example RAM and are executed by a processor. These memories are exemplary recording media for storing computer programs comprising computer-executable instructions performing all the steps of the computer-implemented method according the technical concept presented herein.