ben@catnip.UUCP (Bennett Broder) (02/09/86)
The current (March 1986) issue of Radio-Electronics magazine contains
plans for a stereo tv decoder. I was pleased as punch to see this, since
I have been looking for a cheap and convenient way of adding stereo decoding
to my video system. It looks like it could be built for about $40.
Here is the rub. The circuit does not contain any noise reduction.
Apparently, the compression technique used is proprietary to dbx, and
the chips necessary to do it are available only to licenced OEMs.
So, my question:
- Is it worth building one of these?
- How noisy will it be without the dbx chips?
- Will there be any other unwanted side effects (breathing noises
or frequency response abberations) without the dbx?
If I get enough responses and/or actually build one of these things,
I will post a summary.
Ben Broder
{ihnp4, decvax} !hjuxa!catnip!benbill@videovax.UUCP (William K. McFadden) (02/11/86)
In article <218@catnip.UUCP> ben@catnip.UUCP (Bennett Broder) writes: >The current (March 1986) issue of Radio-Electronics magazine contains >plans for a stereo tv decoder. I was pleased as punch to see this, since >I have been looking for a cheap and convenient way of adding stereo decoding >to my video system. It looks like it could be built for about $40. > >Here is the rub. The circuit does not contain any noise reduction. >Apparently, the compression technique used is proprietary to dbx, and >the chips necessary to do it are available only to licenced OEMs. >So, my question: > > - Is it worth building one of these? NO! NO! NO! > - How noisy will it be without the dbx chips? About as noisy as FM Stereo, when compared against FM mono (e.g., S/N is degraded by 23dB on weak signals). > - Will there be any other unwanted side effects (breathing noises > or frequency response abberations) without the dbx? YES! Only the L-R channel is compressed. If you try to rematrix it with the L+R without first expanding it, the separation, at best, could be described as terrible! This is the primary problem with the R-E circuit. > >If I get enough responses and/or actually build one of these things, >I will post a summary. > >Ben Broder >{ihnp4, decvax} !hjuxa!catnip!ben I too, was surprised when I saw "Build This Stereo TV Decoder" on the front cover of _Radio-Electronics_. My first thought was, "Gee, they finally came out with a chip to do the dbx decoding?" I quickly turned to the article and, looking at the schematic, the first words out of my mouth were, "This won't work!" I am tempted to write a letter to the editor of R-E, telling them they shouldn't have jumped the gun. In my opinion, you would be better off buying a real MTS decoder and not wasting your time building this circuit until the dbx chips are here. Otherwise you will be dissappointed. On the positive side, the article is educational. It explains how the MTS system works. A few months ago I wrote an article on the workings of stereo television. If anybody is interested, e-mail me and I will forward it. Also, feel free to ask me any questions you might have about MTS. If I don't know the answer, there are lots of other people around here who do. Bill McFadden TV Measurement Systems Tektronix, Inc. -- 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?"
jimb@tekcbi.UUCP (Jim Boland) (02/12/86)
> The current (March 1986) issue of Radio-Electronics magazine contains > plans for a stereo tv decoder. > ...... a cheap and convenient way of adding stereo decoding ...for about $40. > > Here is the rub. The circuit does not contain any noise reduction. > Apparently, the compression technique used is proprietary to dbx > So, my question: > > - Is it worth building one of these? I read that article, also. Actually, it can be built for about $10 depending on what you have lying around. About a dozen caps and resistors, an led, a 2N2222 (or similar npn), and an LM1310 (or similar stereo decoder chip). It's worth it as an experiment in the interest of science. Actually, the method used for mts stereo is similar to the one used in FM broadcasting. The only difference is that the FM stereo uses a 19Khz pilot with a 38KHz suppressed carrier and stereo TV uses a 15+ KHz (horizontal freq.) pilot and a 31+ KHz suppressed carrier for the difference signal. Therefore, all the circuit in RE is doing is changing the oscillator frequency on that chip. Nothing magic there. > - How noisy will it be without the dbx chips? The article does mention that the circuit doesn't have the dbx noise reduction. How noisy it will be, I don't know. The article does say that you can add some noise reduction to improve the system. But what they don't tell you (directly), and here is the rub, is that in stereo TV, the dbx reduction is only on the difference channel, not the main channel. (It does show it in the diagram). Obviously, they cannot modify the main channel without affecting all existing tv's. Therefore, you would really need to add the noise reduction on the difference signal prior to summing it with the main channel (L+R). You cannot get to that point on that chip. Therefore, you would not be able to do the noise reduction where it should be done. > - Will there be any other unwanted side effects (breathing noises > or frequency response abberations) without the dbx? > I don't know enough about dbx to comment. However, I do have all the parts and a few National LM 1894 DNR (Dynamic Noise Reduction) chips so I think I'll build this thing, try it out, and let you know.
seifert@hammer.UUCP (Snoopy) (02/12/86)
Looking at the skiz in RE, there appears to be no place to put in the proper dbx expansion. Also, the 75us deemphases is applied to the L and R outputs, not the L+R signal. It appears that one might be able to leave off the 75us RCs, regenerate the L+R and L-R signal, apply the 75us deemphases to the R+L, apply the dbx expansion to the L-R, and then get the L and R signals back again. But this method would involve a lot of extra and unnecessary processing. Questions: Is there a reasonable way to get at the L+R and L-R signals? (Presumably using a different chip) What exactly does the dbx expansion involve? Is it a straight linear expansion? What ratio? Any deemphases required? Also, audiophiles will want to upgrade the parts used. Gee, if a software hacker like me can find all this wrong with their design, I hate to think what a good analog person could find! Snoopy tektronix!tekecs!doghouse.TEK!snoopy
hansen@mips.UUCP (Craig Hansen) (02/12/86)
> > The current (March 1986) issue of Radio-Electronics magazine contains > > plans for a stereo tv decoder. > But what they don't tell you > (directly), and here is the rub, is that in stereo TV, the dbx reduction is > only on the difference channel, not the main channel. (It does show it in the > diagram). Obviously, they cannot modify the main channel without affecting all > existing tv's. Therefore, you would really need to add the noise reduction > on the difference signal prior to summing it with the main channel (L+R). > You cannot get to that point on that chip. Therefore, you would not be able to > do the noise reduction where it should be done. Actually, it is very easy to get the original L+R and L-R channels back from a monolithic multiplex stereo decoder chip. Take the L and R outputs and add them to get the L+R channel, and subtract them to get the L-R channel. A couple of Op-Amps can do it easily. (This technique is widely used for cable & subscription decoders, where the pay sound is in the L-R channel.) You can then put the L-R through dbx compensation, and add and subtract them again to get the real L and R channels. Craig Hansen MIPS Computer Systems ...decvax!decwrl!glacier!mips!hansen
dsi@unccvax.UUCP (02/13/86)
> The current (March 1986) issue of Radio-Electronics magazine contains > plans for a stereo tv decoder. I was pleased as punch to see this, since > > Here is the rub. The circuit does not contain any noise reduction. > Apparently, the compression technique used is proprietary to dbx, and > the chips necessary to do it are available only to licenced OEMs. > So, my question: > > - Is it worth building one of these? > - How noisy will it be without the dbx chips? Don't bother. According to the National Association of Broadcasters Engineering Handbook, Seventh Edition (just hot off the NAB press) the signal to noise ratio of the uncompanded BTSC subcarrier is roughly 20 dB at 5000 Hz. Both wideband decibel linear companding AND sliding band equalisation is used; the result is not the same as DBX "B" system used for consumer audio. Get a real BTSC decoder if you want satisfactory stereo programme service... dya
jimb@tekcbi.UUCP (Jim Boland) (02/14/86)
> In article <218@catnip.UUCP> ben@catnip.UUCP (Bennett Broder) writes: > >The current (March 1986) issue of Radio-Electronics magazine contains > >plans for a stereo tv decoder. > > > > - Is it worth building one of these? > > NO! NO! NO! > > > - How noisy will it be without the dbx chips? > > About as noisy as FM Stereo, when compared against FM mono (e.g., S/N is > degraded by 23dB on weak signals). > > > - Will there be any other unwanted side effects (breathing noises > > or frequency response abberations) without the dbx? > > YES! Only the L-R channel is compressed. If you try to rematrix it with > the L+R without first expanding it, the separation, at best, could be > described as terrible! This is the primary problem with the R-E circuit. > OK Folks. I had all the parts and a few hours so, I built this thing. Note: the schematic does not show you that you must apply VCC to pin 1. Also, I put decoupling caps on the outputs. I'm not to fond of sending 12V through shielded cable to my amp. I used the MPX output from my Sony 2700 and 2710. Both had the same result. Yes, the LED recognizes whether there is a stereo signal present there or not. That is about the only positive thing I can say about this circuit. When on a station that is only mono, the sound is fairly normal. The same is true when on a stereo station and detuning the osc. frequency on the decoder. However, when the decoder osc. is adjusted correctly, the output sound is noticeably in stereo (through headphones) but EXTREMELY bad. How extreme you say??? Of course, it is all subjective, but how about this: a. Very noisy b. very "hissy" c. hum and noise levels varying as the picture varies in level, etc. d. signal to noise ratio as being close to 0 db. ie, 1:1 or worse yet, 1:2+ In a nutshell, those are my experiences. someone else may have others. > > In my opinion, you would be better off buying a real MTS decoder and not > wasting your time building this circuit until the dbx chips are here. > Otherwise you will be dissappointed. I agree 100%. The article did warn you that it would not be ideal but that you need to start building circuits sometime. I think that "sometime" is still in the future. Look at the simple schematic in RE. Then go to your local radio shack and look at the schematic for their tuner. Ignore the tuner and amp part and just check out the decoding part. Big difference and a lotta lunch in that box. I think we're still far from getting it as simple as in the RE article. > On the positive side, the article is educational. It explains how the MTS > system works. Yes. and of course, these opinions are only mine. tektronix!tekcbi!jimb
fish@ihu1g.UUCP (Bob Fishell) (02/15/86)
> >Here is the rub. The circuit does not contain any noise reduction. > >Apparently, the compression technique used is proprietary to dbx, and > >the chips necessary to do it are available only to licenced OEMs. > >So, my question: > > > > - Is it worth building one of these? > > NO! NO! NO! > > > > - Will there be any other unwanted side effects (breathing noises > > or frequency response abberations) without the dbx? > > YES! Only the L-R channel is compressed. If you try to rematrix it with > the L+R without first expanding it, the separation, at best, could be > described as terrible! This is the primary problem with the R-E circuit. > I haven't seen the article descibed, but I think the problem could be solved (at least theoretically) by re-extracting, via a differential amplifier, the L-R information, expanding it (I think NS has a single-chip expander), and then rematrixing it with the L+R signal. I don't know what compression dbx uses on the MTS system; it's 2:1 on their type II noise reduction systems, so that might be a place to start. In any case, you can vary the expansion with a pot until it sounds right. A problem with [cheap] compansion systems, though, is that, even if you lick the breathing problem, hum gets picked up by the expander and modulated with the audio. Very annoying. Since I already have an MTS decoder, I probably won't try this myself, but I thought I'd advance the idea anyway. -- __ / \ \__/ Bob Fishell ihnp4!ihu1g!fish
bill@videovax.UUCP (William K. McFadden) (02/18/86)
In article <772@ihu1g.UUCP> fish@ihu1g.UUCP (Bob Fishell) writes: >> >Here is the rub. The circuit does not contain any noise reduction. >> >Apparently, the compression technique used is proprietary to dbx, and >> >the chips necessary to do it are available only to licenced OEMs. >> >So, my question: >> >> > - Will there be any other unwanted side effects (breathing noises >> > or frequency response abberations) without the dbx? >> >> YES! Only the L-R channel is compressed. If you try to rematrix it with >> the L+R without first expanding it, the separation, at best, could be >> described as terrible! This is the primary problem with the R-E circuit. >> >I haven't seen the article descibed, but I think the problem could be solved >(at least theoretically) by re-extracting, via a differential amplifier, >the L-R information, expanding it (I think NS has a single-chip expander), >and then rematrixing it with the L+R signal. > >I don't know what compression dbx uses on the MTS system; it's 2:1 on their >type II noise reduction systems, so that might be a place to start. In any >case, you can vary the expansion with a pot until it sounds right. A problem You're on the right track, but there is a problem. The dbx compression used for MTS is not the same as their tape noise reduction system. The compression on the tape NR system is linear 2:1, as you said, so it is only amplitude dependent. The MTS compression, on the other hand, is amplitude *and* frequency dependent. I've seen the curves, and they look pretty strange. Chances are, using the dbx type II NR system would eliminate a lot of the hiss, but would not improve the separation much. And since separation is important in a stereo decoder, you are probably still better off with mono until you can get a *real* MTS expander chip. For those who are interested, I have found the following articles informative: "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 -- 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?"