Solution to interference with STBs

I am writing about Robert Parnell’s letter concerning electrical interference to his digital set-top box, in the July 2005 issue. I solved my electrical interference problems as follows.

The glitching on my reception was quite intolerable (every few seconds), so rather than go back to analog I sought a solution – which maybe I have. I have heard a number of stories of people in my area who are not using their STB because of electrical interference. This issue of electrical interference is rapidly becoming a major factor impeding digital uptake in Australia.

I put an earth stake in the ground and connected it to the antenna system signal ground, just before the distribution amplifier. It made a huge improvement and I do not have quad-shield coax!

One possible explanation for the improvement is that cleaning out the LF garbage in the coax braid has removed spikes that were sufficient in amplitude to drive the tuner front-end into non-linear overload.

If there is any part of the VHF spectrum that you can filter, then do this. I fitted a -50dB VHF filter (Kingray DPO-UV diplexer) and this made a difference but I am lucky because all my stations are UHF-only.

The best general alternative is a triplexer (Kingray FL3BPMH) that can be used to filter out all non-TV frequencies and any unused TV bands.

Electrical trade wholesalers such as Lawrence and Hanson, Middendorps, Active Electronics, etc sell 1.8m copper-plated earth stakes for about $20. I located mine in the wettest part of the garden, minding any underground gas pipes, etc. This was a shade-clothed fernery with a built-in mist-spray watering system and it also happened to be close to where the TV distribution amplifier is located. I also purchased 6m of some very heavy mains (green-yellow) wire – the copper was about 5mm diameter.

I soldered the wire and clamp that comes with the stake and soldered the other end to the shield/braid at a splitter just before the distribution amplifier. Leave out the soldering if that is too hard but it does help reduce the earth path impedance.

This arrangement almost completely eliminated audio/video drop-outs, pixellation and "no signal" indications on my digital tuner.

I would not recommend using the mains earth for the antenna as this is very "dirty" and makes matters worse.

Those people with STBs that run extremely hot might also benefit from my clean earth idea as the problem is eliminated if you don’t have isolated TV outlets. Before I earthed my antenna it "measured" 2000V! This is fairly common, not usually dangerous and is caused by leaky TV sets but it does cause tuner modules to run very hot.

Interference not the only problem with STBs

I read with interest the email about the digital set top box in the July 2005 edition, from Robert Parnell. Having investigated digital reception myself, and speaking to many people on the subject, I agree that the analog system is unlikely to be shut down any time soon, if ever. I believe, however, that one of the major reasons for people not switching to digital is being over-looked, that being network programming.

Many people I know (including myself) are watching a fraction of the television that they used to. In fact, most of the time now the television set is off, unless I’m watching a DVD. The vast majority of what the networks try to pass off as entertainment is absolute garbage and if you have no interest in football or cricket (a significant number of people don’t, despite what the networks think), there is not much reason to turn it on.

With these points in mind, why would any one bother spending money on a digital set-up? Nobody needs to see garbage in high-definition, along with channel logos and banners. Television stations have become, like most companies, only interested in pleasing shareholders.

The government will have a hard time getting enough people over to digital, until the networks are made to lift their game. I would not hold my breath though.

Photocopying is a cost issue in schools

Leo Simpson is totally correct when he registers his concern (June 2005) about the rampant photocopying that takes in a whole variety of industries. In schools, one core reason for this practice is the fact that good quality texts for the teaching of electronics to teenagers are difficult to find and some of them are ridiculously expensive, or aimed at the juvenile level (one UK book I am considering for my classes is priced at $A149!).

There are heaps of electronics training manuals and more senior texts available but they don’t "talk" to teenagers. Teachers are thus forced to compile piecemeal workbook and copy masters for classroom use, adding their own teacher-speak to explain the engineering concepts in novice terms. Everybody involved in the electronics teaching industry is absolutely flat-out just teaching and can’t find the quality time required to write and publish a good affordable text for teenagers.

There are some tactics SILICON CHIP could try, apart from justifiably hammering the copyright authorities. For example, I want 25 copies of the excellent current series on PICs in schools. I really can’t afford to buy 25 sets of the whole series to do this. It would cost hundreds of dollars to do so and I would just be covering PICs. Perhaps SILICON CHIP could collate them and sell them as a series package and I’d gladly buy a class set.

Another tactic would be to include photocopying authorisations for individual schools so that teachers who felt professionally accountable could pay a yearly fee for photocopying rights. I’d certainly be quite happy to pay a fee of $30 or so for this service.

Whatever happens, SILICON CHIP needs the support of all educators in this regard if it is to remain the world class magazine that it is. Don’t photocopy SILICON CHIP! Simple!

Welcome to a SILICON CHIP convert

I must have been in another world to have missed SILICON CHIP until now. I dragged wartime copies (WWII) of "Radio and Hobbies" out of the cellar at the Scout Hall when I was "cub". It was still "Radio and Hobbies" when I used to borrow it from the boy next door and when I started to buy my own, it was "Radio TV and Hobbies". I absorbed the change to "Electronics Australia" and later on, on the Council of IREE, I worked with such luminaries as Neville Williams and Neville Thiele.

I had never been a very regular subscriber to "EA" or its predecessors, almost always buying it from the newsagent on a regular basis. Then I began to miss it but try as I might, I couldn’t find a replacement. Recently I have been conserving a 1930s radio chassis and my net search for bits led me to an email address for a previous "EA" staffer. Next step: SILICON CHIP!

Now I am a subscriber and I have just got my first issue. I feel like I am back home! It’s a wonderful feeling.

It is interesting to note that I had picked up SILICON CHIP editions twice at the newsagent but thought "Naah!" It was the list of topics on the website that showed me that SILICON CHIP is what I have been looking for and now I am back in familiar territory. Thanks – it’s a great magazine.

Ross Tester’s July article on "WiFi", particularly his comments on page 12 about possible illegality of sharing communications with the "NDN", brought back some interesting memories. I couldn’t help but think how legislators, once having forbidden something, are reluctant to relinquish control. Ross probably does not remember the "Wireless License" every house had to have.

My mother used to freak out if the "wireless licence" was a day overdue. It was a hurried walk to the Post Office to buy a new one. Inspectors could knock on your door at any time, and they did, to check that you were not listening to an unlicensed radio. Can you imagine it? So an extension speaker across the fence to let the "poor old dear next door" keep a check on the fate of her son away "at the war" was a breach of your licence and a loss of revenue.

All that has gone, and good riddance. But I suspect that only the licence requirement was revoked, with the "sharing with a neighbour" part of the Act left untouched. Ahh! – the seduction of legislative power.

Using a Train Detector with Command Control

Could I suggest that R.S. of Bundaberg (Ask SILICON CHIP, July 2005) have a look at the Level Crossing Detector from the March 1994 issue of SILICON CHIP?

Assuming that he only wants to know where a train is, this circuit could be used by placing the Hall Effect sensors so that the exit from one section is the start of the next. Then use suitable LEDs on the track circuit board which will light and go out as the train proceeds around the track.

The only problem I have with this setup is the magnet attached to the underside of the wagons (see page 43, March 1994 issue). If you use magnetic decoupling with magnets between the rails, trains come to a sudden halt at times and wagons can derail when the two magnets come together.

I use miniature glass magnetic reed switches (Jaycar SM-1002) set between tracks at just below rail level and ferrite magnets (Jaycar LM-1616) cut in halves and suitably fixed on edge under the wagon.

I am sure the Hall Effect pickups could be easily changed to reed switches.

N. J. Guy,
Cohuna, Vic.

Coolmaster circuit not energy-efficient

I am the author of the "ultra-efficient fridge" article in Jan/March "Renew" magazine, issue 90.

I got your SILICON CHIP magazine and read about your "Coolmaster" freezer thermostat kit. Perhaps you and your readers may be interested in its shortcomings.

(1). The "Triac" part (that switches 240V to the freezer compressor) consumes at least 0.85W for 24 hours; ie, about 20 watt-hours per day when the fridge becomes connected to its socket.

(2). The "plugpack" that you suggest using to power the 9V electronics, if carefully chosen, will consume about 1.7W for 24 hours; ie, about 40 watt-hours a day.

Hence, the power consumption of your thermostat alone (with fridge connected but not working) is about 60Wh a day. Now, you may remember that the entire fully working fridge with my thermostat consumes only about 100Wh a day. Can you see how mediocre your thermostat really is?

It seems that you have never been exposed to living with limited energy resources. You haven’t even noticed the challenge of zero-standby power consumption.

The thermostat that I currently use for my fridge has zero standby 240V power consumption, essential for power-sensing inverters and people who can use every bit of saved energy that they generate.

It consumes about 70mA from a NiMh battery to monitor the fridge temperature and has a built-in fast battery charger that tops up the battery when the compressor is turned on (about two minutes per hour), so that the entire system can work for many years on one battery.

Please let me know if you are interested to publish, test or improve my version of the thermostat.

Comment: it is true that the Triac does dissipate about a watt but only when it is turned on and since it is in series with the fridge, it does not add anything to the overall power drain.

Odd Panasonic S-video behaviour

We recently sold a popular infrared remote-controlled AV switcher to a customer at Glen Innes for the purpose of switching audio and S-video from his DVD player, digital STB and satellite STB, to his Panasonic TX-68P100Z which is a large flat-screen model about four or five years old.

All these sources worked just fine with the Panasonic on an individual basis but with the switcher in circuit the customer reported that the set would automatically switch to 16:9 format and stubbornly refused to stay put at 4:3.

I tested a sample switcher before offering them for sale and am aware of a DC offset on all the video outputs (composite, luma and chroma) of about 0.5V across a 75Ω load. It seemed to me that this DC offset might be the culprit but why? I made a little box with two S-video connectors and isolating capacitors (470μF non-polarised) in series with the luma and chroma and sent it off.

It didn’t work as expected though. The format switching stayed at 4:3 but the picture was now unwatchable, with gross distortion and over-saturated colour. How bizarre!

I contacted Panasonic at Belrose and a few emails flashed back and forth to an engineer there. Apparently, the set has an automatic format switching feature which is controlled by a DC offset on the chroma signal. I was unable to extract any specific information about this feature which Panasonic refer to as "S2 switching" and I was also unable to find out if there are any other features controlled in a similar manner (are there S1/S3/Sx?), the voltage levels, pulse widths, etc.

Perhaps the Panasonic service manual has some details (who knows?) but I was not inclined to spend the money to find out. I think that Panasonic could make such information readily available. We technical types need to know about "features" like this. The engineer did reveal that they requested the feature, apparently popular in Europe, be disabled in Australian sets but the cost for modified versions was too great.

Anyway, I retrieved the box from Glen Innes, connected the Y (luma) signal straight through, put a 100nF MKT capacitor in series with the C (chroma) signal and sent it back to our customer who reported a success! This was because, as you would know, the chroma is a symmetrical AC signal at 4.43MHz (or 3.58MHz for NTSC), so the average DC level is 0V but what about the luma?

What about AC-coupled outputs? Why did the Panasonic set totally lose control of AC-coupled video? Surely they clamp the luma. Do they use the luma for any tricky controlling? I dunno!

The problem is this: I see DC offsets on the outputs of a lot of recent equipment from Asia. I guess they are unaware of this S2 switching feature too. The engineer at Panasonic did tell me that other manufacturers have the same features in their equipment, citing Sony, Philips, NEC, and Toshiba as examples. He likened it to the fast blanking used in SCARTs but that is a different sort of feature as far as I know, used for keying in RGB graphic overlays such as external teletext or picture-in-picture, etc.

Maybe there is that sort of functionality but I can’t seem to find anything on the web or in my reference books. There are implications for manufacturers of active switchboxes and distribution amplifiers, etc. Maybe the TV servicing fraternity may be aware of the details. There might even be scope for a project that exploits these features. I know I would be fascinated to find out about it.

Right and wrong way to wire a 3-pin plug I was studying the photographs on page 42 of the Coolmaster article in the June 2005 issue and I noticed that the socket has been wired at the wrong end of the cable. It appears to me that the Neutral/Earth conductors are crossed, even though the socket is no doubt wired correctly. This is the "wrong" way to wire a 3-pin mains plug & socket. Here, the plug and socket have been wired to the wrong ends of the cable, so that the wires have to cross each other to reach their correct terminals. It is the lay of the cable that is my real concern. To demonstrate, I have made up two short extension leads which have correct and incorrect ("wrong" end of cable) cable lay. Both are wired in accordance with the details provided by HPM. Please note that in the "correct lay" extension cable, none of the conductors "crosses" any of the others to reach their correct positions on the plug top or socket. Also, note the distance (2-3mm) from the opaque plastic of the plug/top socket to the other insulation of the 3-core cable. And this is the correct way - by reversing the direction of the mains cable, the Active, Neutral & Earth wires can be run to their correct terminals without crossing each other. I had no difficulty in making the connections and assembling the plug top and socket. Ray Smith, Hoppers Crossing, Vic. Comment: you make an interesting point and one that we have not considered before. While it makes no difference to plug safety it possibly does make the job of wiring plugs and sockets a little easier – and it looks neater!