rma@mhuxt.UUCP (ATKINS) (10/03/85)
This may not quite fit into the ham radio catagory, but
I found no info in net.audio or video and knowing the vast
pool of knowledge in ham-radio circles(!) I thought I`d try here.
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.
Thanks, Bob Atkins (KA1GT)bill@videovax.UUCP (William K. McFadden) (10/07/85)
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. I tried to reply to this when you asked about it in net.audio. Unfortunately, the system somehow ate my reply before I could get it properly posted. In my frustration I gave up. Since no one else has replied I thought I would try again. 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 indtended for bilingual broadcasts (e.g., SAP can broadcast a dubbed foreign language version of the audio). SAP is FM modulated and dbx compressed. I beleive the frequency response is somewhere around 10 KHz. The PRO (PROfessional) channel is for data or low-grade voice transmission and is not intended for the general public. So far, the is one station using SAP (to transmit in Spanish). The TV industry predicts there will be 200 stereo TV stations on the air by the end of the year. 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?"