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Clarification of DVD resolution

Some recent letters to the editor have suggested that DVD pictures are considerably better than VCR pictures because of the poor resolution of the latter. In particular, they have spoken of the little more than 200 lines of resolution provided by a VCR, with one letter contrasting this with DVD's 576 lines.

This requires clarification. DVDs and VCRs produce the same vertical resolution. Nominally 625 lines, some of these lines are used during the vertical blanking intervals (during which the electron beam returns from the bottom of the screen to the top). For PAL, the number of displayed lines was traditionally 585. For DVD, 576 lines was settled upon. The difference is insignificant (most displays lose several lines through overscan at both the top and the bottom).

This 580-ish horizontal lines spread over the vertical dimension is the same whether the picture is from VCR, broadcast or DVD (assuming a full screen DVD picture).

The resolution sometimes specified for VCRs is often stated as '200 lines' or maybe a little more. This is not the vertical resolution but the horizontal resolution. Rather than the pixels that have been talked about most commonly with the advent of computers, the resolution of TV pictures was traditionally specified in terms of the number of vertical lines that could be discerned across the width of the display. This is where the '200' comes from.

By contrast, DVD players are typically capable of between 500 and 540. Note that the picture on a PAL DVD is actually encoded as a 720 pixel wide by 576 pixel high image, so hardware DVD players lose around 17% of the horizontal resolution during the conversion from a digital picture to an analog PAL signal.

Stephen Dawson,

Gilmore, ACT.

Right of reply on DVD widescreen

I'm glad to see my letter in the September 2001 issue generated so much interest and comment.

What disappointed me, was I feel, most of you missed my point. My gripe was never against DVD and I never once said that DVD doesn't give a better picture than VHS. It's the film industry that manipulates the film and has the final say on how they will do it. I have already mentioned "What Women Want" and "Titanic". Now I include "True Lies" and "Airforce One". How many other movies are being done this way?

"Airforce One" was a blatant example. I acquired a Zone 1 NTSC copy of the DVD and the beauty of this one was it is double-sided, one side Widescreen 2.35:1 while the other is Full Screen 4:3. The widescreen, as you guessed, was the 4:3 picture black-barred top and bottom; ie, 44% of the original picture removed. What really annoyed me, the blurb on the cover of this, as it does on most VHS movies, said "Film reformatted to suit 4:3". What a joke this is, when in these cases, it actually happened the other way around.

If the film makers are filming in 4:3, why are they bothering with the pseudo widescreen? As I stated in September 2001, I thought widescreen was more picture on the sides, not less top & bottom. So why would anyone in their right mind spend upwards of $4000 on a 76cm widescreen TV when you get the same result with a $2000 to $3000 80cm 4:3 set?

My point is I guess, how are movies today initially filmed and then eventually presented. Next time you go to the cinema, take a note of the trailers to up and coming movies. Most are presented in 16:9, however when they become the feature they're suddenly 2.35:1.

All you "couch potato experts" have put your two bob's worth in but I wish the real experts from the film industry would comment. Also maybe the excellent team at SILICON CHIP could explain the workings of the compression technique used on DVD and what trickery is done to get 500 lines resolution out of a 4:3 and supposedly the same from 2.35:1.

Simon Kareh,

via email.

More on transformer ratings

Congratulations on securing the copyright of EA/ETI as noted in the Publisher's Letter in the January 2002 issue.

On a different note, in reference to Ben Lowe's letter (Mailbag, January) relating to transformer ratings, I am afraid that I must side with Mr Lowe. Perhaps I can elaborate a bit on what he wrote.

The power rating of a transformer depends on how hot the transformer is allowed to get during use. That in turn depends on two main sources of heat: iron (eddy current) losses in the core and copper (I²R) losses in the windings. Here, "I" represents the RMS current through any winding of the transformer and "R" is its resistance. At 50Hz we can safely assume that eddy current losses are small compared to I²R so it is the loss due to the resistances of the primary and secondary windings that is the more important.

In any practicable transformer design there is only a limited space available in which to put the windings. If you want more power you need thicker wire (to reduce I²R losses). This means that you need more space and hence a bigger transformer.

In general, the optimal space available for the secondary winding(s) is practically the same whether you have one secondary winding or two. If you design for a centre-tap configuration you would need twice as many turns on the secondary winding(s) as you would for a bridge configuration. This means that thinner wire must be used and the effective winding resistance would be doubled.

Imagine a transformer with two identical secondary windings. You can connect the windings in parallel for a bridge configuration or in series for a centre-tapped configuration. The parallel connection has half the resistance of the centre-tapped configuration for the same no-load output voltage. Therefore the I²R loss under load is halved, which translates into 1.4 times as much total power for the same I²R loss.

Although for reduced duty cycles the power loss is increased compared with the output power, the duty cycle makes no difference to the advantage offered by the bridge configuration over the centre-tapped configuration. This is because for any given duty cycle the RMS current is the same. Hence the I²R loss for the centre-tapped configuration is always double that for the bridge configuration.

Incidentally, if you include the I²R loss in the primary winding, which is independent of the output configuration, the advantage of the bridge configuration over the centre-tapped one is somewhat less than otherwise but the principles remain the same.

Herman Nacinovich,

Gulgong, NSW.

Officious letters not necessary

I have been away from home for some time and started to catch up on some reading, SILICON CHIP included! I was amazed at some of the correspondence and indeed the tone of some of it. I wish particularly to take issue with Mr Ian Stanley-Eyles' letter published in your November issue.

I gather by the tone of the letter that Mr Stanley-Eyles is either a civil servant or a solicitor, in whichever case there was no need for the very thinly veiled threat embodied in the correspondence, essentially demanding a legal disclaimer and suggesting that a magazine such as yours was in any way acting outside the law deliberately!

I do not believe a letter of that tone is needed in the columns of a magazine like this!

I wish also to make comment regarding the ongoing debate regarding electrical installations and who may carry out electrical work. It seems to me, as it has to others, that the arguments coming from the electrical safety authorities are nothing other than the oft-repeated views of a bunch of electricians turned civil servants trying to keep the "old boys club" going by excluding not only competent laymen, but also highly qualified technicians and engineers, from doing what they are more than capable of doing and doing, in many cases, better than a so-called qualified electrician.

The civil servants charged with the duty of public safety in matters electrical should move aside and let people do their own wiring.

Steve Newson,

via email.

Don't change electronics content

Firstly, it's great to see that SILICON CHIP has not strayed from its original philosophy with regards to content. I have been collecting (much to the anguish of my wife) all the Australian local electronics magazines since 1980 when I was studying electrical engineering at the UNSW and have been a subscriber to SILICON CHIP for a number of years now.

I was horrified when the "other" locally produced electronics magazine made a severe change of content direction. Its recent withdrawal from publication probably reflected the same opinion from other readers. I guess what I'm trying to say is that you people at SILICON CHIP are doing a tremendous job and should keep doing what you do best.

Con Andrews,

via email.

Virtues of negative feedback

I need to comment a little on your response to the reader "G. W." who asked questions about the Ultra-LD Amplifier in the February 2002 issue, page 91.

I notice that you advertise audio books by Self and Hood in the same issue, pp 86, 87. G. W. and the friends who advise him, would do well to read both of these books. Self and Hood don't always agree but their books are clear, informative, useful and fun to read.

G. W. mentions "a few people I know ... anyone can achieve low THD with lots of feedback". Many people who know a little of electronics find feedback difficult and seem to want to regard it with suspicion and as an unnatural "black art". I am willing to admit that in my student days, I built quite a few amplifiers that oscillated - and oscillators that didn't. But difficulty doesn't justify suspicion.

Let me try to dispel the biggest part of the feedback myth first. Feedback is not at all unnatural; nature uses it by the truckload. Feedback is what enables us as humans to walk about on two legs. Anything tall and skinny with bendy bits in all sorts of peculiar places is unstable and this becomes conspicuous when we are drunk, asleep or dead - we fall over.

In our legs, we don't just have muscles and bones, we have umpteen nerves dedicated to monitoring what bit is pointing where. Behind our ears, we have special balance canals filled with special fluid and zillions of tiny hairs, so we can know which way is up; we also use our eyes.

When doctors ask us to stand on one leg and shut our eyes, they are checking for a certain social disease. It damages nerves and people with that disease don't get enough feedback and fall over.

Moving away from nature a little, feedback is what enables us to drive a car. Most drivers pay attention to where the car is on the road. If too far to the left, they steer to the right; if too far to the right, they steer to the left. Back to bodies: feedback is what keeps us alive. Many, probably most, of the chemicals sploshing about in our bodies are regulated by feedback systems. Too little oxygen, our heart beats faster; too much carbon dioxide, we breathe faster. Many illnesses happen when some feedback mechanism breaks down. No feedback: no us!

Let me move on to the "less feedback is better" myth. It isn't. The authoritative explanation is: P. J. Baxandall, "Wireless World", December 1978, pp 53-56. In layman's terms, a little bit of feedback can do only the easy bit: suppressing the low-order distortion components. However, it can't do the difficult bit: suppressing the high order distortion components.

Indeed, the way it suppresses the low-order ones is by turning them into high-order ones. A little bit of feedback makes things worse, not better. It is really dumb to do gross, brutal things like using Class A to reduce feedback.

The implication that "anyone can use lots of feedback" is a myth too. That is the big problem with feedback. It is necessary but difficult to use lots of it. That is why I made amplifiers that oscillated while I was a student. Feedback is much more like porridge than like sex. Like porridge, feedback needs to be "just right"; bad feedback is bad and definitely not, "better than nothing".

But getting it "just right" isn't easy. It is necessary to understand it well, to do a few moderately difficult calculations, and to do even more testing to double and triple check that it is "just right". But once it is right, then it is RIGHT, and it deserves no more suspicion than a person who can stand on one leg with their eyes shut.

Keith Anderson,

Kingston Tas.

Australian 3-pin plug and socket

I am able to provide some background to the questions posed by Dick Smith (Mailbag, Jan 2002) regarding the origin of the Australian 3-pin mains plug. The design was adopted, around 1930, as a result of a "Gentlemen's agreement" (there was no Standards Australia then) between Fred Cook of Ring-Grip, Geoffrey Gerard of Gerard Industries, and Brian Harper Miller of the SECV.

They based the design very closely on an existing American plug, and although that American design has apparently now faded into obscurity, I believe that same American design is the reason that Argentina and Chile and probably also China, also use essentially the same design. New Zealand copied the Australian design as Australian electrical appliances and equipment were exported across the Tasman.

One of the reasons behind the adoption of that particular design by the three Australian gentlemen was that it was cheap to make; the flat pins could be easily stamped out of sheet brass, in contrast to round pins or thicker rectangular ones used in other countries. This was also a consideration when the Chinese authorities officially adopted the design in relatively recent times, despite the considerable inroads the British plug had made due to its use in Hong Kong.

The Chinese made one change, however; the earth pin is at the top. This is considered to offer some protection should a conductive object fall between the plug and the socket. An interesting piece of trivia is that this is referred to in Hong Kong as the "emu foot" or "emu track" plug, as the socket when viewed with the earth slot uppermost does resemble the track of an emu.

I thank Dennis Perry and Norm Zerner (presently and formerly of Gerard Industries) and Max Steen, a former colleague of mine in the Approvals Branch at ETSA, for their help with this information.

David Inkster,

Meadows, SA.

TV for collector/restorer

I have just been given a Healing B/W valve TV set, not working due to blown input fuse. The set is in great condition and may be suitable for a collector. It is a Healing 17, with the circuit diagram in the rear indicating a Chassis 5M150-5M075-4M150-958 RUN 3.

I would hate to throw it away and would be happy to pass it on to anyone who could do something with it.

Ashley Hosking,

PO Box 73, Basket Range, SA, 5138.

Phone (08) 8390 3442.

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