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.
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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.
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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.
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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.
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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.
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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.
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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.
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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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