Video formats: why bother?

Jim Rowe is a braver man than I, in attempting to explain the reasons behind all those different video input configurations (August 2004), to even reasonably technically "fluent" electronics enthusiasts. I’ve given up trying to explain how and why you use the "16 x 9 scan" facility on TV sets with standard 4:3 tubes, in conjunction with the "16 x 9 output" facility on DVD players and digital set-top boxes!

And nobody seems to have any idea what I’m talking about when I try to demonstrate the differences between Composite, Component and Y-C video. Even I have trouble telling the difference sometimes!

There are a couple of small errors in his explanation of the use of luminance filtering in TV receivers. He states: "The problem here (in early TV sets) was that the low-pass filter ... had to have a cutoff frequency no higher than about 3.2MHz..." In fact, I have never encountered a luminance low-pass filter used in any colour TV receiver, nor is one necessary. With a proper "sync-coherent" colour subcarrier (ie, for PAL, one that is locked to precisely 283.75 times the line frequency plus half the field frequency), the visibility of the colour subcarrier on the screen is quite small.

In most cases, a simple 4.43MHz tuned notch filter is all that is required, simply removing some of the dot pattern from large areas of colour. Although this leaves the chroma sidebands untouched, you really only notice these on colour bar signals and similar electronically generated patterns.

The reality is, a well-designed standard PAL TV receiver will quite happily display luminance frequencies right out to 5MHz. The notch filter produces a small dropout at the colour subcarrier frequency but this is normally quite unnoticeable. This applies equally to the latest AV monsters, as to sets nearly 30 years old. (And yes, a surprising number of those old warriors are still going strong!) It is true that in recent years some el-cheapo TV sets have had some truly atrociously designed video output amplifiers, giving some pretty ordinary pictures, but this trend has now reversed with the proliferation of single-chip CRT drivers in even the cheapest models.

Low-pass filters have to be used with VCRs on the other hand, because the colour recording and playback process involves some extremely rigorous processing to enable the recovered chroma signal to be displayed on an unmodified TV set. If any of the original chroma signals managed to get recorded along with the luminance, you would wind up with a mixture of processed and unprocessed chroma, which would produce unsightly patterning on the colour image.

When VCRs were first introduced, it’s true there was simply no practical means of doing this, other than using simple low-pass filtering. Even low-end broadcast formats such as High Band U-Matic suffered from this limitation. In virtually all cases, the luminance bandwidth is flat to about 2.5MHz and then drops to zero by 3MHz. Using the "80 lines per megahertz" rule of thumb, 2.5MHz equates to about 200 lines.

Although VCR playback signals have lost the subcarrier/sync coherency, the savagely reduced replay chroma bandwidth (typically ±300kHz) means that the TV set’s notch filter can effectively filter this out, so it all works happily enough.

S-VHS ("Super VHS") and the concept of Y-C signals actually dates back to a 1979 proposal by JVC for a "turnkey" portable video theatrette system that would allow current-release movies to be shown in remote districts. This proposal sank without a trace and S-VHS was then reinvented as a "Vi-Fi" consumer format that never really went anywhere and was finally massaged into a reasonably successful professional and low-end broadcast format!

Because there is never any time when the luminance and chroma signals are mixed together during the playback process, it’s perfectly OK for the luminance signal to have residues of the original colour subcarrier in it, which means that you can record and play back a virtually full 5MHz luminance bandwidth from off-air signals. (Of course you get even better results if the luminance is recorded directly inside the camera without ever "seeing" the chroma signal, which was the basis of S-VHS camcorders).

When DVD players came along, some manufacturers started to take advantage of the "S-Video" (Y-C) inputs available on some high-end TV receivers. Technically this was a mere detail: the analog luminance and chroma signals are generated separately in the MPEG decoder chips; all they had to do was divert some of these signals to the Y-C connector as well as combining them to produce a composite video output.

Y-C inputs are capable of quite good results; the problem is that they were only really designed to handle the 300kHz chrominance bandwidth of VHS signals. Although a DVD player could easily provide chroma signals with a wider bandwidth, certain peculiarities of the NTSC system limit any such improvement to only 500kHz (at least without major changes to the decoder circuitry).

Component video inputs on the other hand will deliver the full theoretical 1.5MHz chrominance bandwidth of NTSC signals, and in the present climate of international set design, this is the real reason TV set (and chip) manufacturers have gone for this system, even though S-Video is considerably simpler and cheaper to implement, on both ends. Otherwise S-Video would have been all there ever was!

Also, there seems to be a bit of confusion about the "Zone Plate" test signals shown in the article on the Video Enhancer and Y-C Separator. In theory, there should be no difference between the Y-C and Component input images. The reason we see a difference has more to do with the economics of DVD player design than the signal systems themselves!

This is understandable if you appreciate what actually goes on in the MPEG decoder/PAL encoder chips. In an old-fashioned (20th century) analog chroma encoder, the colour difference signals are fed to a pair of analog balanced modulators to generate the two suppressed-carrier AM signals. These are then combined with the luminance signal to give composite video.

In a modern DVD player, the entire modulated colour subcarrier is synthesised directly, by some very fancy software. This may not sound all that awesome, until you realize that the PAL 4.43361875MHz (and NTSC 3.579545MHz) colour subcarrier frequencies have no direct numerical relationship to the 27MHz master clock frequency most commonly used!

How do they do it? Well, consider an ordinary 4.43MHz unmodulated sinewave being fed to an 8-bit analog-to-digital converter, clocked at 27MHz. What you would get is a 27MHz stream of bytes which if then decoded by a DAC would reproduce the original carrier. There would be no immediately recognisable pattern to the numeric values of the string of bytes though, because the 27MHz clock would be continually sampling at different points on the waveform. However, there is a definite mathematical relationship involved and a fast enough computer can reproduce this pattern with a suitable mathematical algorithm.

From there it is a relatively simple matter to digitally multiply the samples with the values of R-Y and B-Y coming from the MPEG decoder, add the colour burst and voila: a complete modulated PAL subcarrier, direct from the DAC!

While this works very well, they have to cut a few corners, and so very few (if any) DVD players produce a true broadcast-quality sync-coherent colour subcarrier. Unfortunately this means that their wide luminance bandwidth is more prone to generating "cross-colour" artefacts. However, they can cheat somewhat; it is possible to identify and filter out just those luminance components which are most likely to cause cross-colour and that is what most manufacturers do.

I can assure you, if that zone plate was generated by pointing a TV camera at an actual printed chart, the colour artefacts would be much worse!

The reason why the Y-C and Component input displays look different is that in virtually all cases the luminance fed to the "Y-C" output is merely the "doctored" (pre-filtered) "Composite" luminance sans chroma! Apparently, generating a separate unfiltered "Y" signal for the "Y-C" output is simply too hard. I suspect the reason they don’t simply use the Y from the YUV output is that most DVD players also allow you the option of RGB output from the same sockets and the potential for user stuff-ups is simply too large!

Keith Walters,
via email.

Caution on CFL driver power to TV With regard to the CFL Driver in the September 2004 issue, if it is to be used to power a TV, first understand that the power switch on the TV is designed for use on AC where any tendency to contact arcing is quenched during the zero crossings. These switches arc notoriously when switching more than about 100V DC, even on a 2-pole switch. Some TVs use a small power transformer to power the remote control receiver and CPU. This type of receiver MUST NOT be used on DC. It would seem to be a sensible idea to increase the value of the 6.8kΩ resistor to about 7.3kOmega; by putting a 470Ω resistor in series with the original component. This should reduce the maximum voltage from the converter to around 335V while the unit is supplying standby power only or if the 12V line rises to say 14.5V as it could do while the battery is being charged in a vehicle. The reason for doing this is because most switch-mode power supplies work at about this value when operating from a nominal 240VAC mains supply; higher voltages can cause premature power supply failure. The degausser can be converted to operate from DC by charging a 150nF 500V capacitor from the 335V supply through a 330Ω resistor and discharging it through the degaussing coil manually with a robust pushbutton switch. The resistor can be left connected permanently. When the coil is shunted across the capacitor, the circuit will oscillate in a decaying fashion for a short period and in so doing the flux reversals will demagnetise the CRT. The values are those I have used in the past with great success on professional portable monitors converted for battery operation. I have no idea if the values will work for the set mentioned in your article, however I see no reason why one could not experiment. Note if a new TV is modified or even if the back is removed, one is liable to void the warranty. Also fitting a manual degausser with a pushbutton switch and then using the set on the 240VAC mains would render the safety certification for operation null and void so BEWARE. Victor G Barker, VK2BTV, via email.

Slander on CFLs unjustified

Your editorial that CFLs aren’t economic, in the August 2004 issue, deserves a response. Until early this year I was on the body corporate of a large multi-story block of home units with 11 stair-wells that relied on artificial lighting.

The cleaner, who was responsible for changing light globes, came to me asking whether it would be a good idea to fit CFLs instead of incandescent globes. We tried it. He had previously been having to replace all the bulbs at an average of every six weeks. None of the CFLs had failed at the point at which equal cost was reached; ie, purchase price plus cost of power used for both was equal.

Only one CFL had failed within 48 weeks of being fitted, indicating that they met and exceeded the 8000-hour life their manufacturers claimed. In the end, it worked out that by using CFLs the body corporate was saving $600 per year just on the basis of globe costs and power, plus the cleaner’s time savings.

I have used CFLs virtually since they came on the market and have had only one obvious premature failure. You really ought to be asking why you had the problems you did rather then slandering a good product.

Secondly, I must comment on your answer in "Ask Silicon Chip" to D. K., of Innisfail, who asked whether the Smart Mixture Display would work with a diesel engine.

If you were trying to be funny you should be more obvious, otherwise readers might conclude you give answers when you have no idea what you are talking about. Yes, the device will work with a diesel engine if it has an oxygen sensor but of course they don’t, for reasons that are obvious to anyone who knows the basic principles that diesel engines run on. Detecting a lean mixture on a diesel would be pointless. They run lean by design. That’s how they work.

Gordon Drennan,
Burton, SA.

Comment: sorry Gordon, but the answer concerning the diesel motor was fair dinkum and oxygen sensors can be fitted. Properly adjusted, diesels are supposed to run lean. Many don’t.

Voltage warnings supported I fully support your September 2004 editorial regarding the voltage warnings added to the projects in SILICON CHIP. Having some 45 plus years experience in electronics, I have seen (and been guilty of myself) too many accidents ending in smoke and tears. It is not only the beginner but the experienced technician and engineer that have the accidents that can put life and limb at risk. I spent the early part of this year working in an electrical safety testing lab which, excuse the pun, brought me down to earth with a thud! Me, with all the years of experience, had the basics brought home to me in no uncertain terms. Most of my time at the lab was spent testing mains voltage CFLs where the voltages could be in excess of 2500kV peak. Some voltage waveform measurements were carried out with respect to mains earth using an isolation transformer on the CRO and when measurements between output terminals were required, two isolation transformers were used, one on the CRO and one for the CFL. All this with a high-voltage probe to boot! As you correctly stated in your Publisher’s Letter, the main concern is the accidental transposition of neutral for active, which could render the earthy side of the CRO BNC connector live, with disastrous results. So big and bold warnings please. If I skipped them before, after my six months in the testing lab, I will read them now. Michael Abrams, Capalaba, Qld.

Novices will make mistakes

The letter by Otto S. Hoolhorst concerning unnecessary voltage warnings, in the September 2004 issue, has me a bit steamed up. To me, he appears to have no regard for safety, possibly his own but obviously not for novices.

The old saying that "there are two things in life that are guaranteed, death and taxes" should have a third one added: "Novices will make mistakes". Lots of times. That’s how we all learn. Most of us are lucky and no real harm is done.

I have 30 odd years in the electronics game, both military and civilian and even now I screw up sometimes. I screw up through complacency, a novice because of lack of experience.

We are only talking about a cautionary note but they are critical. A novice must be made doubly aware that incorrect actions can lead to disaster, especially with higher voltages. It’s no big deal if you let the smoke out of some component, it’s easily replaceable. But if you fry yourself in the process, maybe Otto is invulnerable but I’m sure not!

Yes, that project may not really be one suitable for a novice but no doubt they will attempt to build it, if it fits a need. Therefore it’s best to err on the side of caution and safety.

The thing Leo may have been too polite to mention in his "Publishers Letter
is the legal aspect. Would Otto like to be in Leo’s shoes if he has to explain to a prosecuting attorney why there was no warning that the project was potentially dangerous? Would Otto like to pay the fine/serve the time if someone died because they were not warned of potential hazards. I think not! Yet these days, a Publisher can be held accountable if an article they approved for printing is found to be a contributing factor in someone’s injury/death.

Sorry Otto, those cautionary notes are there for a good reason. If they bother you that much, then too bad.

Ralph Teichel,
via email.

Separating SMPS transformer core halves I read Keith Farmer’s letter on the subject of separating SMPS transformer core halves with much interest (Mailbag, July 2004). I have rebuilt several computer power supplies and as Mr Farmer found, not all transformers come apart after soaking in paint stripper. I found that after the paint stripper treatment, dunking the transformer in boiling water released the glue holding the core halves together. I would be reluctant to put an SMPS transformer core in a microwave oven in case the microwave energy caused high enough flux densities in the ferrite to produce an irreversible change in the magnetic characteristics. However, if Mr Farmer’s treatment worked and the power supply subsequently operated without a problem, well and good. Keith Gooley, Edinburgh, SA.

Off switch is the best energy saver

Your recent articles on energy conservation and compact fluorescent lighting are timely and welcome. With regard to the former, I found during an energy audit that standby loads were using 1.5kWh per day. As such, I have been promoting the "off switch" as the cheapest energy saver available for some years now. Additional benefits include lower fire risk in the home and longer appliance life.

I have always been a little suspicious of the actual saving achieved by using compact fluorescent lights. Apart from the high cost and short lifespan issues, I believe they may not be as efficient as the label indicates. At a demonstration I attended at the Central Queensland University in Rockhampton, it was shown that the power consumption of a 15W CFL was actually 35-40VA in some cases.

I concur with your view on using fluorescent tube lights where possible. I find that low wattage incandescent globes, 25W & 40W, used in suitable locations for short periods are far more economical in the long term. Your advice to return CFL lights that fail prematurely is the only way we can expect product improvement.

Perhaps LED lighting will be a suitable replacement in the future.

Brian Bartlett,
Rockhampton, Qld.

CFL build quality is the key I have to agree that many CFLs die early. I have even had them fry the instant they were turned on for the first time but as one of the letters in your latest issue also mentioned, certain brands are much better than others. I have not had a Philips unit fail before around 6000 hours and have generally been very happy with them. The ones I have owned have certainly saved me a lot of money over the years. Another good brand is Osram, although they seem to be making their lamps in two places now; the cheap version comes from China while the better version still comes from Europe. You can buy the cheap ones in a pack of two for $5.99 in Coles supermarkets at the moment. And they are a nice warm white. I’m not sure how long they will run – will know that in a year or two I expect! Back when the Osram lamps were called Wotan (I believe you can still get them under that brand), they were made in Germany and were just about unkillable. I know of two people here who have Wotan lamps dating back over 10 years that are still running (one home has several lamps that age and they are all working fine). How many hours these lamps have on them is anyone’s guess, but it would be well over 10,000. Anyway, I pulled apart one of my Wotan lamps after I got tired of waiting it to die after three years. It had much better rated components than any Asian made unit I have seen. For instance, 550V-rated transistors compared to 400V in the cheap lamps, and a 450V filter cap compared to 350V to 400V (there is not much margin on a 350V capacitor on a 250VAC supply!). At the time I pulled it apart it had already paid for itself, despite costing $30 at the time, but had I not killed it during the disassembly then it would have saved me a lot more. So the problem is one of specification more than anything but build quality also comes into it. I had one lamp that was dead when bought, so I opened it up and found two of the tube leads touching each other. Separating them fixed it, and it is still going strong two years later. So my advice is to buy the better quality units, because like most things, you get what you pay for. Lance Turner, Templestowe Lower, Vic.