1. Field of the Invention
The present invention relates to video technology using composite video signals.
2. Description of the Related Art
Almost all contemporary video equipment utilize composite video inputs or outputs. Laser discs, cable TV, and broadcast TV are sources of composite video that generate composite video outputs. Video devices such as VCRs, laser disc players, camcorders, and TV monitors receive and convert composite video in recording or playing video data, thereby processing composite video inputs. A composite video signal is an encoded signal that includes a luminance signal, Y, and a chrominance signal, C. When a video signal is separated into both a luminance signal and a chrominance signal, the signals are termed Y/C video. The luminance or brightness signal represents the black and white information of a video signal, and a chrominance signal represents the color information of a video signal. A composite video signal is specifically a summation of the luminance signal and the chrominance signal. A composite video signal permits the chrominance signal and the luminance signal to share bandwidth such that the signals overlap in a portion of the same frequency range, minimizing the bandwidth of the composite video signal. In addition to black and white information provided by a luminance signal, a luminance signal includes a horizontal sync pulse and a vertical sync pulse. The composite video signal includes a color subcarrier burst which is sent with each horizontal sync pulse. The color subcarrier burst serves as the reference point for the chrominance signal.
The luminance and chrominance signals of a composite video signal occupy the same frequency space by the process of frequency interleaving. The energy spectrum of a static composite video signal is concentrated in clusters separated by the horizontal scan rate. The luminance signal does not have a continuous distribution of energy across its 4.2 MHz bandwidth. Instead, the luminance signal exists as clusters of energy, each separated by 15.734 kHz. The chrominance signal is contained in clusters spaced at 15.734 kHz intervals across a bandwidth from about 2.1 MHz to 4.2 MHz. By providing the color subcarrier at an odd harmonic (455) of the horizontal scanning frequency divided by 2, the chrominance signal clusters are centered exactly between the luminance signal clusters. As a result, the luminance and chrominance signals are frequency interleaved. Y/C separation of a composite video signal is achieved using a filter such as a digital comb filter. A comb filter separates the luminance and the chrominance signal by generating a frequency response that selectively passes either the luminance signal or the chrominance signal.
Y/C separation involves certain artifacts. The cross-luminance artifact is composed of alternate light and dark patterns which are present on the vertical edges of transition regions between sharp changes in color on a screen. The cross-chrominance artifact is in the form of high frequency luminance information that generates colored xe2x80x9crainbowxe2x80x9d regions on a screen rather than black and white regions. These artifacts thus detrimentally affect video quality and are a function of any inaccuracies in Y/C separation.
A video decoder, which serves to decode an analog composite video signal into digital form, may include a comb filter and color separation circuitry for generating a burst gate signal to detect a color burst. A color burst takes the form of a number of cycles of the color subcarrier. A burst gate signal is required to lock onto the color burst. Detection of a color burst is necessary for identifying the timing location of the color information in a composite video signal. In high quality video applications, the inaccuracies of color separation circuitry internal to a video decoder have provided unacceptable levels of cross-luminance and cross-chrominance. Because an internal burst gate signal has been required to lock onto a color burst, additional color separation circuitry external to the decoder to improve video quality has not been an option. Because a composite video signal is in analog form, externally generating a burst gate signal from the composite video signal is a prohibitive undertaking in terms of technical complexity and cost.
Briefly, the present invention generates a simulated burst gate signal and a video synchronization key. A video decoder generates a horizontal sync pulse which is programmed to envelop a color burst, thereby simulating a burst gate signal. The offset to the horizontal sync pulse due to simulating a burst gate signal may be compensated at a video memory subsystem receiving the horizontal sync signal in order to maintain an accurate determination of when active pixels are provided by the video decoder. Alternatively, counter circuitry external to the video decoder may be used to generate a simulated burst gate signal by counting the number of pixel clock cycles between the horizontal sync pulse and the color burst. Unlike a burst gate signal generated within a video decoder for use with color separation circuitry in the video decoder, a simulated burst gate signal allows for use of color separation circuitry external to the video decoder. Detecting a color burst using external color separation circuitry is thus also disclosed. Further, since the programmable horizontal sync pulse of a video decoder is capable of being programmed and later compensated, the horizontal sync pulse may also be used as a synchronization key for video devices.