BACKGROUND OF THE INVENTION Color television systems encode"luminance" (brightness) and"chrominance" (hue) information signals in a composite television signal. The luminance signal Y is equal to a sum of weighted intensity values of the primary hues (red, blue, green). A pair of chrominance signals Cr, Cb contain color information consisting of the corresponding differences between the intensities of the red and blue and the luminance signal. In the SECAM ("sequential color and memory") standard as defined in CCIR Rep. 624-4, the composite signal containing the luminance and chrominance information is provided in frequency modulated form. The composite SECAM signal comprises the luminance signal Y in one frequency band and sequentially alternating, frequency modulated chrominance signals Dr, Db in an adjacent, partially overlapping frequency band. The luminance spectrum extends from 0 to 6 MHz. The

chrominance information modulates a color subcarrier.

The base frequency of the color subcarrier is foR=4.40625 MHz for the Dr signal and foB=4.25 MHz for the Db signal.

The maximum deviation of the subcarriers is given by <BR> <BR> <BR> <BR> <BR> <BR> Af =+350kHz18kHz [35kHz] and-506kHz25kHz [5OkHz]<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> Af =+506kHz25kHz [50kHz] and-350kHz18kHz [35kHz] OB where, provisionally, the tolerances can be extended up to the values given in brackets. The upper limit of the deviation range for the Dr signal (i. e., 4.75625 MHz) is represented by foRmax and the lower limit for the Db signal (i. e., 3.9 MHz) is represented by foBmin- A color television signal contains periodic vertical blanking intervals that are interposed between successive sequences of active line intervals. The luminance and chrominance information is carried in the active line intervals. In the SECAM standard, the video information is sent in 625 lines per frame. There are two fields per frame. Field 1 contains lines 1 to 312.5 and field 2 contains lines 312.5 to 625. Each line begins with a horizontal synchronization signal and each field begins with a vertical synchronization signal.

In the SECAM standard as implemented some countries including Russia, alternating identification signals Dr', Db' (so-called"bottles") are sent during each vertical blanking interval to assist receivers in deciding which of the alternating chrominance signals

(Dr, Db) is being received and thereby adjust FM demodulation accordingly. In particular, the alternating chrominance identification signals Dr', Db' are sent during nine successive line intervals that follow after a series of vertical synchronization pulses. The Dr'signal consists of initial subcarrier oscillations at the base frequency fOR followed by a linear increase in subcarrier frequency to with oscillations held at foRmax for the duration of the identification line interval. The Db'signal consists of initial subcarrier oscillations at the base frequency fOB followed by a linear decrease in subcarrier frequency to fOBmin, with oscillations held at foBmin for the duration <BR> <BR> of the identification line interval. As shown in FIGs.

1A to 1D, the chrominance identification signals Dr', Db'are sent in the vertical blanking interval alternately in lines 7 to 15 of fields 1 and 3 and lines 320 to 328 of fields 2 and 4.

The SECAM chrominance identification signals are complex waveforms. Known SECAM transmission systems generate these signals using analog circuitry that is also generally complex. The components of such analog circuitry have values that can drift over time and thus require periodic adjustment.

SUMMARY OF THE INVENTION A need exists for a capability for generating SECAM chrominance identification waveforms of the type used in

countries such as Russia that is less complex and does not require periodic adjustments. Such waveforms need to be generated in a consistent manner.

The above and other needs are addressed by the present invention which provides a method and apparatus for generating chrominance identification waveforms from digital waveform samples stored in memory. The digitally generated waveforms are converted to analog form and inserted in the proper identification line intervals of a SECAM encoded signal. The digital waveform generation capability of the present invention provides chrominance identification waveforms that are consistent and do not require adjustment over time.

In accordance with the invention, a waveform generator is provided for use in a system in which an encoder provides a SECAM encoded signal that has periodic vertical blanking intervals interposed between successive sequences of active line intervals. The waveform generator provides chrominance identification waveforms for insertion into the encoded SECAM signal and includes a timing circuit which determines a sequence of identification line intervals within vertical blanking intervals from SECAM vertical and horizontal sync signals provided by the encoder. The waveform generator provides first chrominance identification waveforms during corresponding portions of alternate ones of the sequence of identification line intervals within vertical blanking intervals and second chrominance identification waveforms during corresponding portions of intervening ones of the

sequence of identification line intervals. The waveform generator includes memory for storing digital samples representing the chrominance identification waveforms; a selector for selecting the digital samples for the corresponding chrominance identification waveform from memory; and a converter for converting the selected digital samples to an analog representation of the corresponding chrominance identification waveform.

The waveform generator of the present invention is particularly useful in systems that convert digital source signals, such as MPEG ("Moving Pictures Expert Group") signals, to SECAM standard signals.

Accordingly, a SECAM television signal transmission system includes an MPEG decoder, a SECAM encoder and a waveform generator. The MPEG decoder decodes an MPEG video stream to provide digital luminance and chrominance signals. A SECAM encoder encodes the digital luminance and chrominance signals to provide a SECAM encoded signal having periodic vertical blanking intervals interposed between successive sequences of active line intervals. The waveform generator includes a timing circuit which determines a sequence of identification line intervals within vertical blanking intervals from SECAM vertical and horizontal sync signals provided by the SECAM encoder. The waveform generator provides chrominance identification waveforms during corresponding identification line intervals within the vertical blanking intervals. The SECAM encoded signal and the corresponding chrominance

identification waveforms are supplied to a combiner to provide a composite SECAM color television signal.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views.

The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

FIGs. 1A-lD show SECAM chrominance identification signals in vertical blanking intervals of succeeding video fields.

FIG. 2 is a schematic block diagram of a SECAM transmission system having a waveform generator in accordance with the present invention.

FIG. 3 is a schematic block diagram of the waveform generator of FIG. 2.

FIGs. 4A and 4B show corresponding chrominance identification waveforms Dr', Db'stored in the memory of FIG. 3.

FIG. 5 is a schematic block diagram of the logic circuit of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION An exemplary color television transmission system 10 in accordance with the present invention is now

described with reference to FIG. 2. The system 10 converts MPEG source signals to SECAM standard signals and includes an MPEG decoder 14, a SECAM encoder 16, adder 20 and waveform generator 22. The MPEG decoder 14 receives a stream of MPEG video packets on input line 12 and decodes the stream to a digital luminance signal Y and two digital chrominance signals Cr and Cb. The digital luminance signal Y and the chrominance signals Cr, Cb are supplied to the SECAM encoder 16 which encodes the digital video signals into an analog SECAM signal on line 18. In addition, the SECAM encoder 16 provides vertical ("VSYNC") and horizontal ("HSYNC") synchronization signals to a waveform generator 22 which provides chrominance identification waveforms on line 24 that are combined with the SECAM encoded signal in adder 20.

The MPEG decoder 14 can be implemented using an IBM model number CD20 decoder. The SECAM encoder 16 can be implemented using a Philips Semiconductor model number SAA7182A digital video encoder device.

As shown in FIG. 3, the waveform generator 22 includes a timing circuit 104, a selector circuit 106, a memory 108 and a D/A converter 110. In a preferred embodiment, the timing circuit 104 and the selector circuit 106 together comprise logic circuitry 102 that is described further herein. The timing circuit 104 determines a sequence of identification line intervals within each vertical blanking interval from the SECAM vertical and horizontal sync signals provided by the encoder 16 (FIG. 2) and provides proper timing

information to the selector circuit 106. The sequence of identification line intervals includes portions of lines 7 to 15 in fields 1 and 3 and portions of lines 320 to 328 in fields 2 and 4 as shown in FIG. 1.

The waveform generator 22 stores digital samples representing the chrominance identification waveforms Dr', Db'in a static RAM 108. The digital samples for the corresponding chrominance identification waveform are selected from the SRAM 108 by the selector 106 on line 107 based on the timing information provided by the timing circuit 104 which indicates the proper waveform (Dr', Db') to select. The D/A converter 110 converts the selected digital samples to an analog representation of the corresponding chrominance identification waveform and outputs the analog waveform on line 24.

Referring now to FIGs. 4A and 4B, time-domain waveforms of the standard bottle-shaped Dr', Db'signals are shown. The Dr'signal (FIG. 4A) consists of initial subcarrier oscillations at the base frequency fOR for 4.9 microseconds (to point a) in the so-called bottle neck portion of the signal. The bottle neck is followed by a linear increase in subcarrier frequency to foRmax over 15 microseconds to point b. Oscillations are held at foRmax for the remainder of the line duration (i. e., 37 microseconds to point c). Similarly, the Db'signal (FIG. 4B) consists of initial subcarrier oscillations at the base frequency fOB for 4.9 microseconds (to point a') in the bottle neck, followed by a linear decrease in subcarrier frequency to fOBmin over 18 microseconds to point b'. Oscillations are held at foBmin for the

remainder of the line duration (i. e., 34 microseconds to point c'). In either case, insertion of the Dr', Db' bottle signals into the identification line interval starts 7.1 microseconds after the line interval begins to account for the duration of a leading horizontal sync pulse.

The SECAM standard further provides for switching the phase of the subcarriers fOR and fOB. In particular, the initial subcarrier phase varies from field to field by a predetermined pattern given by 0,180,0,180 degrees. In addition, the subcarrier phase varies from line to line in either of the following two patterns: 0, 0,180,0,0,180 degrees or 0,0,0,180,180,180 degrees. The line-to-line phase rotation combined with the field-to-field phase rotation yields an overall phase rotation pattern that repeats every 12 fields. In the preferred embodiment the former line-to-line phase rotation pattern is implemented; however, it should be apparent that either pattern can readily be implemented in accordance with the principles of the invention.

As noted above, the Dr', Db'bottle waveforms are stored as digital samples in memory 108. The digital samples are obtained by sampling the waveforms shown in FIGs. 4A and 4B at a sampling rate of 27 MHz. Each Dr', Db'bottle waveform includes 1536 samples of 8 bits stored as two lookup tables each, one for 0 degrees phase and the other for 180 degrees phase.

Referring now to FIG. 5, an embodiment of logic circuitry 102 of the waveform generator of FIG. 3 is shown. The logic circuitry 102 includes the timing

circuit 104 and selector circuit 106 (FIG. 3) and can be implemented in a field programmable array (FPGA) device.

The timing circuit 104 includes a sync circuit 104A and a counter circuit 104B. The selector circuit 106 comprises SRAM interface 106A. The sync circuit 104A synchronizes the incoming HSYNC and VSYNC signals to a 27 MHz system clock. The counter circuit 104B includes three counters (frame, line and sample) which are synchronized using timing outputs from the sync circuit 104A.

As noted in the background, video information is sent in 625 lines per frame in SECAM systems. There are two fields per frame. Field 1 contains lines 1 to 312.5 and field 2 contains lines 312.5 to 625. Each line begins with an HSYNC signal and each field begins with a VSYNC signal. The SECAM frame rate is 25 Hz; therefore, the line frequency is 625*25=15625 lines/second. With the 27 MHz system clock, each line of video has 27000000/15625=1728 clock-cycles. Given these system parameters, the counters 104B count the frames/fields, lines and samples/pixels (SAMPLECNT, LINECNT) from the clock-cycles to generate positioning/gating signals.

That is, the counters precisely define the position within a 12 field video sequence with a resolution of a single sample/pixel. The counter circuit 104B outputs control signals (SAMPLE-START, SAMPLE-STOP) to the SRAM interface 106A which starts and stops a sequential counter which steps through SRAM addresses (ADDR, READ) to retrieve individual samples from the SRAM 108 (FIG.

3) to form the proper Dr', Db'signals. To account for

the 7.1 microsecond starting interval, a DELAY signal is provided to SRAM interface 106A.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.