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Howdob Dbx-Tywork! How Dob Spfrai Companding Prat The situation is worse for the SAP channel. Because of its stereo subcarrier would have had only about 43 dB of dy- subcarrier's higher frequency, even more noise is intro- namic range (measured from the peak signal to unweighted- duced. The SAP channel also uses a frequency modulated rms noise level). The dynamic range of an "unprotected" subcarrier, which is subject to "buzz beat," an intermodula- SAP channel would have been worse, around 26 dB! tion of the picture with the audio, causing a particularly ob- noxious type of non-harmonically-related distortion. The most practical engineering solution to these problems was to apply a sophisticated companding noise reduction system HOW DOB DBX-TY WORK! only to the stereo-difference and SAP channels. TO COPE WITH SUCH NOISY CHANNELS, AN ENTIRELY NEW NOISE reduction system was designed. In order to consistently mask background noise during low-level, predominantly low-fre- WHAT IS TN PURPOSF Of TN DBX-TV quency passages, substantial amounts of pre-emphasis (tre- ble boost during transmission) would be required. But if ex- STSHM! treme pre-emphasis were permitted to remain in effect during high-level, high-frequency passages, the broadcast I In- comcaNDING sysTEra DEvEt.ocEar B Y D R X, L I K E T H E O T H E R S channel would be overloaded and gross distortion would re- submitted for examination, was designed to: sult. The solution combines the traditional approaches of • provide significant noise reduction even in poor reception wideband companding and pre-emphasis with an innovative areas, system (called spectral companding) that varies the pre-em- • preserve input-signal dynamic range without loss of head- phasis to suit the characteristics of the audio signal. room or incursion of other audio anomalies (distortion, noise pumping, breathing, alteration of frequency response, and so forth), • prevent the stereo subcarrier from interfering with overall TV-transmitter power levels, HOW DOB SPFRAI COMPANDING • ensure reliable and effective performance even in the face PRAT of severe man-made noise and transmission/reception sys- tem impairments (such as multipath and low signal strength), A SUBSECTION OF A DBX-TV CO MPANDER -THE SPECTRAL CO M- and pressor—examines the input signal to determine the ratio • perform well at reasonable circuit cost and simplicity. between its high- and low-frequency information and varies The simultaneous achievement of these five goals was not the pre-emphasis according to these rules: For predominant- easy, and it was necessary to use unusual companding tech- ly low-frequency signals, large amounts of pre-emphasis are niques to overcome some difficult engineering challenges. applied to boost the high frequencies far above the noise. For For example, the dynamic range of the channels before noise predominantly high-frequency signals, where the signal it- reduction was very low. Sixty miles from a transmitter, where self hides the noise, the boost is reduced to prevent overload. picture quality is only slightly snowy, an uncompanded ( C O N T I N U E D O N P A G E 5 2 ) L7 R 50 kHz - R BTSC PILOT 25 kHz - TONE SAP 15 kHz - CHANNEL PROFESSIONAL 5 kHz - CHANNEL 3 kHz - , 1 50 Hz 15 kHz 15.734 kHz 31 468 kHz 78 67 kHz 102 3 kHz FREQUENCY FIGURE 2 THE BASEBAND SPECTRUM OF A BTSC SIGNAL NOT SHOWN TO SCALE THE L RAND I - R MODULATIONS , TINTED AREAS, CARRY THE STEREO INFORMATION 51 .
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