United States Patent [191 [11] Patent Number: 4,691,234 Albean [45] Date of Patent: Sep. 1, 1987 [54] NOISE REDUCI‘ION CIRCUIT FOR G. Eilers, IEEE Transactions on Consumer Electron TELEVISION MULTI-CHANNEL SOUND ics, 8/84, vol. CE-30, No. 3, pp. 236-241. [75] Inventor: David L. Albean, Indianapolis, Ind. “TV Multichannel Sound—Encoding and Transmis sion” by Lee et al., IEEE Transactions on Consumer [73] Assignee: RCA Corporation, Princeton, NJ. Electronics, 8/84, vol. CE-30, No. 3, pp. 242-246. [2]] Appl. No.: 761,216 “TV Multichannel Sound-Reception and Decoding” by Mycynek et al., IEEE Transactions on Consumer [22] Filed: Jul. 31, 1985 I Electronics, 8/84, vol. CE-30, No. 3, pp. 247-250. [51] Int. Cl.4 ............................................. .. H04N 7/04 [52] US. Cl. ............................ .. 358/144; 358/198 Primary Examiner—Tommy P. Chin [58] Field of Search .............. .. 358/144, 198; 381/1-4, Assistant Examiner-David E. Harvey 381/7 Attorney, Agent, or Firm-P. J. Rasmussen; P. M. Emanuel; R. H. Kurdyla [56] References Cited PUBLICATIONS [57] ABSTRACT “The DBX Television Multichannel Sound Com A television receiver includes an audio signal process pander: Characterization” by L. B. Tyler, published by ing arrangement for expanding a second audio program Electronic Industries Association, l2/83, pp. 22-56. (SAP) signal compressed in accordance with the “dbx” “TV Multichannel Sound—The BTSC System” by C. system in a relatively simple manner. The signal pro G. Eilers, IEEE Transactions on Consumer Electron cessing arrangement includes a ?rst ?lter for providing ics, 2/85, vol. CE-31, No. 1, pp. l-7. a ?rst amount of deemphasis. A controllable gain ampli “A Companding System for Multichannel TV Sound” ?er has its input coupled to the ?lter and its gain con by Tyler et al., IEEE Transactions on Consumer Elec trolled by a ?ltered signal representative of its input tronics, 11/84, vol. CE-30, No. 4, pp. 633-640. signal. A second ?lter at the ampli?er output provides a “Handbook of Industrial Electronic Circuits”, Markus et al., published by McGraw-Hill Book Company, Inc., second amount of deemphasis. New York, 1948, pp. 5-9. “TV Multichannel Sound-The BTSC System” by C. 5 Claims, 1 Drawing Figure ,{ SIGNAL SOURCE I ll] (11. TUNER. IF. VIDEO SIGNAL 12/ srcnnn 15mm PROCESSING 1k souun IF LIMITER MUN" MAIN AUUIU EHANNEL )6 FM nE‘iEcmR Aumu SIGNAL PRUEESSINE 2“ \ PROCESSINGSAP SIGNAL ml 3“ SAP SIBNAL wnu mums \35 L5 DBXPREEMPHASIS ‘LB SAP AUIJIU PROCESSING 4,691,234 1 2 The inverse of these operations, i.e. “expanding”, NOISE REDUCTION CIRCUIT FOR TELEVISION takes place in the receiver so as to restore the signal to MULTI-CHANNEL SOUND its original condition. The present practice for both the stereo difference signal and the SAP signal, as explained The present invention relates to the ?eld of multi in the afore-mentioned articles, is to provide another channel sound reception, such as may be employed in bandpass ?lter for deriving a control signal representa providing stereophonic (stereo) sound and a second tive of the high-frequency content of the received signal audio program (SAP) in a television (TV) receiver. and to pass the signal through a variable deemphasis The broadcasting of multichannel or stereo sound arrangement responsive to the control signal in an in programs for TV in the United States is in accordance verse manner to that in the preemphasis arrangement in with the system adopted by the Broadcast Television the transmitting end. Systems Committee of the Electronics Industries Asso Stereo signals are derived by matrixing the main ciation. This television multichannel sound system pro (L——R) audio channel and the stereo (L—R) subchan vides for the transmission of the sum of left and right nel. Good left and right channel separation requires stereo audio information (L+R) in a main audio chan close amplitude and phase matching between the main nel in the spectrum space of the television signal pres audio channel after deemphasis and the stereo subchan ently occupied by the monophonic (mono) audio signal. nel, since even a fraction of a dB and a few degrees of A difference of left and right stereo audio information phase error can signi?cantly reduce stereo separation. (L——R) is modulated onto a subcarrier and transmitted The expander in the stereo subchannel path in the re 20 in a subchannel. - ceiver is therefore required to meet stringent require The television multichannel system also provides for ments in matching that in the transmitting end so as not the option of simultaneously transmitting a second to degrade stereo separation to any substantial extent. audio program (SAP) channel which may typically Expanders designed to meet such requirements are carry a'second language version of the program being somewhat complex and therefore relatively expensive. provided in the main (L+R) audio channel. The audio An example of a companding system of the type re frequency response of the SAP channel is in the range ferred to is described in a copending Patent Application of 50 Hz to 10 kHz, which is relatively restricted as Ser. No. 06/593,967, entitled “Apparatus for reducing the effect of noise interference in audio companding compared with the 50 Hz to 15 kHz for the main chan system”, ?led in the name of Max Ward Muterspaugh nel and the (L—R) subchannel. This bandwidth has on Mar. 27, 1984, and assigned to RCA Corporation, been deemed to be suf?cient for the nature of the bulk the Assignee of the present Application. of program material expected to be carried on the SAP Stereo reproduction and SAP are generally not re channel. - quired simultaneously. Therefore, typically only one The stereo subchannel and the SAP channel both use expander is provided, with a switching arrangement to subcarriers at frequencies above the audio range and, allow it to be selectively included either in the (L—R) due to the nature of the frequency modulation (FM) stereo subchannel or in the SAP channel signal path. transmission system used, these channels exhibit rela The present inventor has recognized that, while there tively high noise levels compared with the main audio is ordinarily no objection to sharing the expander in this channel. Companding is therefore utilized for the stereo manner since it is required anyway in a stereo receiver, subchannel and the SAP channel to provide acceptable 40 there is no need for an elaborate expander when stereo noise performance. The companding system presently separation is not a concern, as in a monophonic (mono) utilized is referred to as the “dbx Companding System” receiver with an SAP channel. and is described in'detail, for example, in an article by An aspect of the invention concerns an audio signal Leslie B. Tyler et al, in the IEEE Transactions on Con processing arrangement included in a television _re sumer Electronics, November 1984, Volume CE-30, ceiver for receiving and demodulating a monophonic No. 4, pages 633-640 and in an article by C. G. Eilers, audio signal and an SAP signal, the SAP signal having in the IEEE Transactions on Consumer Electronics, been subjected to variable preemphasis for modifying February 1985, Volume CE-3l, No. 1, pages l—7. the magnitude of its relatively high frequency portions The “dbx Companding System” sets out to improve without substantially affecting the magnitude of its rela the attainable signal to noise ratio. Fixed and variable 50 tively low frequency portions such as in the aforemen preemphasis are applied to the audio signal before trans tioned “dbx” system. The preemphasis is such that the mission. This is intended to avoid two problems which high frequency portions are subject to relatively large may occur when ?xed preemphasis is used alone. First, magnitude compression when the magnitude of the high audio signals which contain predominantly high fre frequency portions is relatively large and relatively quencies would be boosted excessively causing over 55 small magnitude compression when the magnitude of modulation. Second, low level audio signals lacking the high frequency portions is relatively small. The suf?cient high frequency content would not be suf? audio signal processing arrangement for the SAP signal ciently boosted to mask noise. comprises a ?rst ?lter for providing an intermediate In the “dbx” system, a bandpass ?lter and detector audio signal representative of the SAP signal and exhib arrangement at the transmitting end provide a control iting a ?rst predetermined amount of deemphasis. A signal representative of the high frequency content of wideband detector, rather than a detector with a band the audio signal. The audio signal is passed through a pass ?lter at its input as in the “dbx” system, is coupled variable preemphasis arrangement responsive to the to the ?lter for providing a control signal representative control signal. When the signal high-frequency content of the level of the intermediate audio signal. A control is relatively high, the preemphasis arrangement reduces 65 lable gain ampli?er has a gain control input coupled to high frequency gain and conversely, when the high-fre the output of the detector to receive the control signal quency content is relatively low, the high-frequency and a signal input coupled to the output of the ?lter. A gain is increased. second ?lter imparts a second predetermined amount of 4,691,234 3 4 deemphasis to the output signal of the ampli?er. This satisfactory performance. The control signal provided arrangement is far simpler than that of the decoding or by detector 30 is represenatative of the amplitude of the expansion portion of the “dbx” system and has been intermediate audio signal provided by deemphasis cir found to provide very acceptable performance for SAP cuit 24 substantially through the entirety of its fre reproduction without the need for the much more com quency range, unlike the relatively narrow band control ples “dbx” expander as will be explained in greater signal derived in the "dbx” system.