bill@videovax.UUCP (William K. McFadden) (02/24/86)
Since I have received almost two dozen requests for my article on MTS audio, it would seem that people are very interested in the subject. Hence, I have decided to repost the article to the net in lieu of e-mailing dozens of individual copies. Anyone who has asked me a question can still expect a reply, but I will no longer answer simple requests for my article, since I am posting it here. Thanks to all who responded. Subject: Stereo TV ----------------------------------- From: net.ham-radio: In article <1178@mhuxt.UUCP> rma@mhuxt.UUCP (ATKINS) writes: >Does anyone know where I can find out the scheme used for >stereo sound transmission on broadcast TV signals? Looking at >the signal it seems to be some kind of sub-carrier system, but >not (L+R) and (L-R) as in FM radio. For those people unfamiliar with FM stereo broadcasting, it uses a system of subcarriers to encode the stereo information. The mono (L+R) information is broadcast as the normal baseband signal. This is done to remain compatible with mono FM receivers. The difference (L-R) is transmitted as a 38 KHz subcarrier using double sideband, suppressed carrier AM (DSB-SC). A 19 KHz pilot tone is provided to allow stereo receivers to decode the subcarrier (also provides an indication that an incoming broadcast is in stereo). To decode the left- and right-channel information, the subcarriers are combined as follows: (L+R) + (L-R) = 2L (L+R) - (L-R) = 2R Thus, a simple matrix is all that's needed to decode stereo FM. Mono receivers filter out the subcariers, which leaves the mono (L+R) component. Now, on to stereo television. Being an EE in the Television Products Division at Tektronix, I have become familiar with the American stereo TV system, known as MTS or BTSC. MTS does transmit L+R and L-R, but not the same as FM radio. First, the frequencies involved are different. MTS uses 15.734 KHz for its pilot and 31.468 KHz for the L-R instead of the 19 and 38 KHz of FM radio. 15734 Hz is the horizontal scanning rate in US TV's, so it was chosen as the basis for MTS to reduce interference between the horizontal circuits and the stereo decoder. TV people call this frequency H. Thus, the pilot is at H and the L-R is at 2H. The principal difference between TV and FM is that the L-R subcarrier in BTSC is compressed before transmission. This was done to reduce noisy reception in fringe areas. Although designed by dbx, the companding system used for stereo TV is not the simple linear 2:1 system used in their tape noise reduction systems. In TV stereo, the companding is both amplitude and frequency dependent. This makes the expander circuit fairly complex and I'm afraid no one has managed to squeeze it into a single IC (yet). (Actually, I have heard of one company that it trying to develop a digital TV stereo chip, but that's probably still a couple of years away). The present decoders have lots of discrete components and about 8 trimpots, so you can see there is a lot of room for improvement. For the sake of completeness, I should mention that BTSC provides for two other subcarriers, SAP at 5H and PRO at 6.5H. SAP stands for Second Audio Program and is intended for bilingual broadcasts (e.g., SAP can broadcast a dubbed foreign language version of the audio). SAP is FM modulated and dbx compressed. I believe the frequency response is somewhere around 12 KHz. The PRO (PROfessional) channel is for data or low-grade voice transmission and is not intended for the general public. So far, there are several stations using SAP (to transmit in Spanish). There are about 200 TV stations broadcasting stereo now, with more on the way soon. Most of these stations carry NBC, the only commercial network using stereo. ABC and CBS plan to go stereo soon. For additional information, see the following articles: "TV Multichannel Sound - The BTSC System," C. G. Eilers, _IEEE_Transactions_on_Consumer_Electronics_, vol. CE-30, pp. 236-240, Aug. 1984 "A Companding System for Multichannel TV Sound," L. B. Taylor, M. F. Davis, W. A. Allen, _IEEE_Transactions_on_Consumer_Electronics_, vol. CE-30, pp. 633-640, Nov. 1984 I hope this helped! -- Bill McFadden Tektronix, Inc. P.O. Box 500 MS 58-594 Beaverton, OR 97077 UUCP: ...{ucbvax,ihnp4,uw-beaver,decvax}!tektronix!videovax!bill GTE: (503) 627-6920 "How can I prove I am not crazy to people who are?"