This invention relates to decoding audio signals and more particularly relates to adjustment of scaling to improve signal quality, such as enhancing stereo separation.
During the 1980s, the FCC adopted the BTSC format as a standard for multichannel television sound (MTS). Typically, the BTSC format is used with a composite TV signal that includes a video signal, as well as the BTSC format for sound reproduction.
The BTSC format is similar to FM stereo, but has the ability to carry two additional audio channels. Left plus right channel mono information is transmitted in a way similar to stereo FM in order to ensure compatibility with monaural television receivers. A 15.734 KHz pilot signal is used, instead of the FM stereo 19 KHz pilot signal, which allows the pilot signal to be phase-locked to the horizontal line frequency. A double sideband-suppressed carrier at twice the frequency of the pilot transmits the left minus right stereo information. The stereo information is dbx encoded to aid in noise reduction. An SAP channel is located at 5 times the pilot frequency. The SAP channel may be used for second language or independent source program material. A professional audio channel may be added at 6.5 times the plot frequency in order to accommodate additional voice or data.
Stereo tuners and demodulator units capable of decoding the BTSC format have been on the market for some time. The front end of the units typically includes analog components or integrated circuit chips that cause variation in the amplitude of the composite signal, including the BTSC portion of the signal. This variation in amplitude reduces signal quality. For example the variation reduces stereo separation of the right and left channel information carried in the composite signal. This invention addresses the problem and provides a solution.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.
One method embodiment of the invention is useful in a demodulator suitable for converting television signals to video baseband signals and audio baseband signals including a pilot signal having an amplitude. In such an environment, the quality of the demodulated audio baseband signals may be improved by a method comprising converting at least the audio baseband signals to corresponding digital audio baseband signals, including a digital pilot signal. At least a portion of the digital audio baseband signals, including the digital pilot signal, is scaled according to a coefficient having an adjustable coefficient value. The amplitude of the digital pilot signal is detected, and the coefficient value is adjusted in the event that the amplitude is outside a predetermined range of amplitudes.
One apparatus embodiment of the invention includes a computer readable medium encoded with a computer program which when executed by a computer improves the quality of audio baseband signals including a plot signal having an amplitude, the audio baseband signals having been converted to corresponding digital audio baseband signals, including a digital pilot signal. The computer program causes the computer to perform a method comprising:
scaling at least a portion of the digital audio baseband signals, including the digital pilot signal, according to a coefficient having an adjustable coefficient value;
detecting the amplitude of the digital pilot signal; and
adjusting the coefficient value in the event that the amplitude is outside a predetermined range of amplitudes.
By using the foregoing techniques, the quality of audio signals may be improved. For example, stereo separation of stereo signals within the audio signals may be improved.
These and other advantages and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings.