Method, coder and decoder for the processing of helper signals within television signals

Abstract: The present invention relates to a method for the transmission of helper signals within the SECAM standard which allows a compatible 16:9 transmission in existing TV channels. In order to retain backwards compatibility with existing 4:3 receivers in PALplus the letterbox format was chosen together with a transmission of a helper signal HS in the black band regions. The same format conversion technique can be foreseen for an extension to SECAM. However, the use of the helper signal processing of PALplus for the same task in SECAM causes strong disturbances in the black band regions of 4:3 receivers due to the missing FM subcarrier. According to the invention adaptively wheighted artificial subcarriers are arranged in quadrature phase to the helper signal modulated subcarrier to ensure the SECAM compatibility. This allows a compatible reproduction for conventional 4:3 receivers, while for receivers for enlarged picture-aspect ratio the helper signal is separated from the artificial subcarriers by means of a product-demodulator DEM and is used for reconstruction of the original picture signal with the enlarged picture-aspect ratio.

Description: The present invention relates to a method, coder and decoder for the processing of helper signals within the SECAM standard which allows a compatible 16:9 transmission in existing TV channels. Background In every new television broadcast standard the image format 16:9 can be found. Concerning PALplus, the extension to the existing PAL standard, the problems in processing wide screen pictures to retain backwards compatibility with existing receivers are solved and first PALplus receivers are already on the market. Invention It is the object of the invention to disclose a method for the transmission of television signals having a 16:9 format compatible to the SECAM standard. This object is reached by the inventive method disclosed in claim 1. It is a further object of the invention to disclose a coder and a decoder which utilize the inventive method. This object is reached by the coder disclosed in claim 5 and the decoder disclosed in claim 9, respectively. In an extension to the existing SECAM standard, in the following called SECAMplus, the change of the picture aspect ratio from 4:3 towards 16:9 has to be carried out as most essential alteration. In order to keep the additional costs for a SECAMplus decoder small, SECAMplus should have as much communality as possible with the PALplus standard. In PALplus the letterbox format was chosen together with a transmission of a helper signal in the black band regions as described in EP-A-0445177 and EP-A-0451205. Since there is no need to transmit chrominance picture information in the black bands the helper signal is amplitude modulated onto the colour subcarrier. The same format conversion technique can be applied for SECAMplus, however, if the helper modulation is directly adopted from PALplus, strong disturbances of the black band regions by coloured large area flashes arise. This flickering is due to a missing FM subcarrier at the compatible SECAM receiver for low helper amplitudes. According to the invention adaptively wheighted artificial subcarriers are arranged in quadrature phase to the helper signal modulated subcarrier to ensure the SECAM compatibility. This allows a compatible reproduction for conventional 4:3 receivers, while for receivers with enlarged picture-aspect ratio the helper signal is separated from the artificial subcarriers by means of a product-demodulator and is used for reconstruction of the original picture signal with the enlarged picture-aspect ratio. In principle the inventive method consists of the processing of helper signals within television signals with enlarged picture-aspect ratio, which are compatible with conventional television standards, wherein the video signals are decomposed into helper signals and letterbox signals, in which an adaptively wheighted artificial subcarrier is added in quadrature phase to a helper signal modulated subcarrier and the composed signal is inserted into the black band region, the picture signal consisting of the letterbox and the black band signal is transmitted and at the receiving end can be decoded both with conventional 4:3 receivers and with receivers for the enlarged picture-aspect ratio, in which the helper signals are separated from the artificial subcarrier and used for reconstruction of the original picture signal with enlarged picture-aspect ratio. Advantageous additional embodiments of the inventive method are resulting from the respective dependent claims. In principle the inventive coder for the processing of helper signals within television signals with enlarged picture-aspect ratio comprises a format conversion unit, in which the video signal is decomposed into a letterbox signal and a helper signal, a coder for processing the letterbox signal, a modulation unit for the helper signal, which contains: a pre-processing unit, a first modulation unit, where the pre-processed helper signal is modulated onto one or more suitable subcarriers, an amplitude detector circuit, to which a certain helper signal modulated subcarrier is applied and which supplies a normalized detection signal, an adder, in which a control signal is evaluated by subtracting the normalized detection signal from 1, a second modulation unit, in which one or more artificial subcarriers are multiplied with the control signal to control their amplitude, an addition unit, where the one or more artificial subcarriers are added to the respective helper subcarriers in quadrature phase, a post-processing unit for half-nyquist filtering of the one or more composed signal and selection of one filtered signal in the case of more than one composed signals, and selecting means, by which the processed helper and letterbox signals are composed. Advantageous additional embodiments of the inventive coder are resulting from the respective dependent claims. In principle the inventive decoder for the processing of helper signals within television signals with enlarged picture-aspect ratio comprises: filter units, in which the black band signal is nyquist filtered, a unit, which applies the composite video signal to following means, if the black porch region is reached, PLL circuits, to recover the FM carriers from the video signal, a 90 degree phase shifter, selecting means, which select the corresponding nyquist filtered black band signals and phase shifted FM carriers a product demodulater, where the nyquist filtered black band signal and the phase shifted FM carrier are multiplied, a lowpass filter for filtering out carrier components at multiple of the carrier frequency, and a post-processing unit for expansion of the helper amplitude. Drawings Preferred embodiments of the invention will now be described with reference to the accompanying drawings, which show in: Fig. 1 a block-diagram of the processing at the transmitter side. Fig. 2 a block-diagram of the helper signal modulation Fig. 3 a block-diagram of the helper signal modulation in more detail Fig. 4 a block-diagram of the helper signal demodulation at the receiver side Preferred embodiments In Fig. 1 the processing at transmitter side by a coder for television signals with enlarged picture-aspect ratio, which are compatible with conventional television standards, is shown. After the format conversion, in particular from 16:9 to 4:3, in unit FC the video signal is decomposed in the case of a television signal with 576 active lines into a letterbox signal LS with 432 active lines per frame and a helper signal HS with twice 72 active lines per frame. The letterbox signal passes through a standard coder ENC which supplies at its output the composite signal while the helper signal is subjected to a processing unit MOD. The processed helper signals and the letterbox signals are then composed by selecting means SELE1. The inventive insertion of an adaptively wheighted artificial subcarrier in the black band region is shown in Fig. 2. First a pre-processing of the helper signal HS is carried out in unit PREE. By passing through a lowpass filter LPE1 the horizontal bandwidth is reduced, a look-up-table LUT performs a companding, and a further lowpass filter LPE2 is used in order to suppress the harmonics caused by the non-linearity. Afterwards the pre-processed helper signal is modulated in a modulator MODH onto a subcarrier having a frequency, which is one fourth of the used sampling frequency. Then the amplitude of the resulting signal is determined by an amplitude detector circuit AD comprising in series a bandpass filter BP, an amplitude measuring circuit AM, and a non-linearity NL. The amplitude detector circuit ensures that in the absence of a strong helper signal an artificial subcarrier is added in quadrature phase in unit QP, thus keeping the resulting amplitude into sufficient limits. For this purpose the amplitude of the artificial subcarrier is controlled in unit MODA by a control signal CS, which is determined by subtracting the normalised detection signal NS from 1 in unit ADD. The succeeding half-nyquist filter NF realizes a half-nyquist slope at the subcarrier frequency, which has to be used, if a vestigial sideband transmission is chosen. The insertion of the artificial subcarrier in quadrature phase allows after transmission at receiver side a simple separation of the helper signal and the artificial subcarrier by means of a product-demodulator. According to the two different frequencies of the colour difference components in the SECAM standard for a conventional SECAM 4:3 receiver by insertion of only one artificial subcarrier the suppression of the very bothering flickering is possible, but a slight colouring of the black band regions remains. This colouring can be suppresed by using two subcarrier frequencies for the two colour difference components, which is shown in more detail in Fig.3. After pre-processing the helper signal HS is modulated by means of modulators MODH1 and MODH2 onto two different subcarriers with frequencies f S /4 and f S /4-Δf , respectively, in which for digital SECAM decoding the used sampling frequency amounts to f S =17.75 MHz. For the D R component the high frequency subcarrier and for the D B component the low frequency subcarrier is selected by selecting means SELE2 and applied to the amplitude detector AD which yields the normalised detection signal NS. Subtracted from 1 this gives the control signal CS, which controls the amplitude of two artificial subcarriers, also with frequencies f S /4 and f S /4-Δf by the modulators MODA1 and MODA2. The respective subcarriers and artificial subcarriers are added in quadrature phase in units QP1 and QP2 and the resulting signals are applied each to a half nyquist filter NYQE1 and NYQE2, respectively. Between the filtered signals is switched over by selecting means SELE3. At the receiver side the helper signals can be demodulated like shown in Fig. 4. The black band signal is first half-nyquist filtered by filter units NYQD1 or NYQD2, which are identical to the used filter units at transmitter side NYQE1 and NYQE2, respectively. Between the two nyquist filtered black band signals is switched over line by line by first selecting means SELD1. Parallel to the black band processing the necessary helper carriers have to be reconstructed. For this purpose a unit GAT applies the composite video signal VS to second selecting means SELD2, if at the beginning of the TV line in the back porch region BP the FM burst, which corresponds to the quadrature phase of the carrier used in the encoder, is transmitted. The selecting means SELD2 switch over line by line between two separated PLL circuits PLL1 and PLL2, in which the two different FM carriers are recovered. After third selecting means SELD3 for line-alternating switching between the two FM carriers a 90 degree phase shifter PS becomes necessary for a synchronous amplitude demodulation of the helper signal. A product demodulator DEM, where the nyquist filtered black band signal and the phase shifted FM carrier are multiplied, yields the demodulated helper signal, which needs to be lowpass filtered in order to get rid of the carrier components at twice the carrier frequency. After filtering in unit LPD a helper post processing has to be carried out in unit POSTD which takes into account the expansion of the helper amplitude. A bandlimitation of the output signal like in the encoder is not necessary. The use of two artificial subcarrier yields a reduction of the colouring in the black band region of conventional SECAM 4:3 receivers to neglectable values. Another advantageous improvement of the invented method is the insertion of a constant or even adaptively amplified subcarrier amplitude where the helper signal is significantly different from zero, if the clamp or black level in the helper signal is slightly increased in order to guarantee a minimal difference between the syncronization impulse and the most negative value of the black band signal. The artificial hum corresponding to this offset can be removed by the clamp circuit at the compatible receiver. The invention can be used e.g. in SECAMplus TV receivers and VCRs.

Claims: Method for the processing of helper signals for a TV system, which is the extension to the existing SECAM standard by processing wide screen pictures having an enlarged picture-aspect ratio such as the 16:9 format while retaining backwards compatibility with existing receivers, wherein the video signals are decomposed into helper signals (HS) and letterbox signals (LS), the helper signals being amplitude modulated onto the colour subcarrier, an artificial subcarrier (ASUB) is added in quadrature phase to a helper signal modulated subcarrier (HSUB), the composed signal (BS) is inserted into the black band region, the picture signal consisting of the letterbox signal and the black band signal is transmitted and at the receiving end can be decoded both with conventional 4:3 receivers and with receivers for the enlarged picture-aspect ratio, in the latter receivers the helper signals (HS) being separated from the artificial subcarrier and used for reconstruction of the original picture having a full vertical resolution with an enlarged picture-aspect ratio. Method according to claim 1, characterized in that the helper signals (HS) are modulated onto two subcarriers (SUB11, SUB12) with different frequencies, in which the first frequency is nearly equal or identical to the subcarrier frequency of one colour difference component and the second frequency is nearly equal or identical to the subcarrier frequency of the other colour difference component, and that an adaptively weighted artificial subcarrier (ASUB1 or ASUB2) is added to each of the helper signal modulated subcarriers (HSUB1 or HSUB2) in quadratur phase, wherein between the two composed signals is switched over line by line of the picture signal. Method according to claim 2, characterized in that a constant or even adaptively amplified subcarrier (ASUB1 or ASUB2) with non-neglectable amplitude is also inserted for helper signal amplitudes significantly different from zero and the black level in the helper signal is increased. Method according to any of claims 1 to 3, characterized in that the transmitted television signals with enlarged picture-aspect ratio are compatible with the SECAM television standard and the enlarged picture-aspect ratio amounts to 16:9. A coder for the processing of helper signals for a TV system, which is the extension to the existing SECAM standard by processing wide screen pictures having an enlarged picture-aspect ratio such as the 16:9 format while retaining backwards compatibility with existing receivers, related to a method according to any of claims 1 to 4, comprising a format conversion unit (FC), in which the video signal is decomposed into a letterbox signal (LS) and a helper signal (HS), a coder (ENC) for processing the letterbox signal, a modulation unit (MOD) for the helper signal, which contains: a pre-processing unit (PREE), a first modulation unit (MODH), where the pre-processed helper signal is modulated onto one or more suitable subcarriers, an amplitude detector circuit (AD), to which the output signal of said first modulation unit is applied and which supplies a normalized detection signal (NS), first combining means (ADD), in which a control signal (CS) is formed by subtracting said normalized detection signal (NS) from 1, a second modulation unit (MODA), in which one or more artificial subcarriers are multiplied with the control signal (CS) to control their amplitude, second combining means (QP), where the output signals of said second modulation unit are combined to respective helper subcarriers in quadrature phase, a post-processing unit (POSTE) for half-nyquist filtering the output signal of said second combining means and for the selection of one filtered signal in the case of more than one composed signals, and selecting means (SELE1), by which the processed helper and letterbox signals are composed to form said compatible television signals. A coder according to claim 5, where: the first modulation unit (MODH) comprises two modulation units (MODH1, MODH2) for modulation of the pre-processed helper signal onto two subcarriers corresponding to the two colour difference components in SECAM and first selecting means (SELE2) for line-alternating switching between the two modulated subcarriers, the second modulation unit (MODA) comprises two modulation units (MODA1, MODA2), in which two artificial subcarriers corresponding to the two helper subcarriers are multiplied with said control signal (CS), the second combining means (QP) comprise two addition units (QP1, QP2), where the respective helper and artificial subcarriers are added in quadrature phase, the post-processing unit (POSTE) comprises for each output signal of said addition units a filter unit (NYQE1, NYQE2) for nyquist-filtering and second selecting means (SELE3), which select the output signals of said filters in a line-alternating way, for inserting the correct processed and modulated signal into the black bands. A coder according to claim 5 or 6, where the pre-processing unit (PREE) for the helper signal includes in series a lowpass filter (LPE1), a look-up-table (LUT) and a further lowpass filter (LPE2). A coder according to any of claims 5 to 7, where the amplitude detector (AD) comprises a bandpass filter (BP), an amplitude measuring circuit (AM) and a non-linearity (NL). A decoder for the processing of helper signals for a TV system, which is the extension to the existing SECAM standard by processing wide screen pictures having an enlarged picture-aspect ratio such as the 16:9 format while retaining backwards compatibility with existing receivers, related to a method according to any of claims 1 to 4, comprising: filter units (NYQD1, NYQD2), in which the black band signal (BS) is nyquist filtered, first selecting means (SELD1) for line-alternating switching between the two nyquist filtered black band signals, a unit (GAT), which applies the composite video signal (VS) to second selecting means (SELD2), if the black porch region (BP) is reached, PLL circuits (PLL1, PLL2), to recover the FM carriers from the video signal, second selecting means (SELD2), which connect said unit (GAT) with one of the PLL circuits (PLL1, PLL2), between which is switched over line by line, third selecting means (SELD3) for line-alternating switching between the two FM carriers, a 90 degree phase shifter (PS) downstream of (SELD3), a product demodulator (DEM), where the nyquist filtered black band signal and the phase shifted FM carrier are multiplied, a lowpass filter (LPD) connected to said product demodulator for filtering out carrier components at multiple of the carrier frequency, and a succeeding post-processing unit (POSTD) for expansion of the helper amplitude.

Inventors: BOIE WERNER, BOIE, WERNER
Applicants: THOMSON MULTIMEDIA SA, THOMSON MULTIMEDIA
Application year: 1996
Main classification: H04N7
IPC codes: H04N7, H04N11