|
PROBLEM SOLVER
1. A PROBLEM SHARED --
GETTING A BUZZ?
Back in the May issue we
published a letter from P. Whittled, who complained about a buzzing noise heard
on the soundtracks of some pre-recorded rental movies replayed on his Sony VCR.
Since then several other readers have written in to tell us that they have
experienced similar problems, we have also had a number of suggestions as to
the cause and a possible cure. We’ve decided to investigate and find out what’s
going on; it involves some technical guff, so please feel free to skip the next
couple of paragraphs and get to the bit where we tell you how to make it go
away.
The buzzing sound is
confined to VHS stereo hi-fi video recorders, which use a system known as depth
frequency multiplexing or DFM recording. These VCRs actually have three
soundtracks, a standard ‘lo-fi’ mono linear track, common to all VHS video
recorders, (it’s recorded along the top edge of the tape), and the right and
left stereo tracks which are laid down by an extra pair of heads on the
spinning head drum. In case you were wondering the audio heads are mounted 180
degrees apart but offset at an angle of 138 degrees to the video heads. These
extra heads record the audio tracks as a frequency modulated signal, on a low
frequency carrier, which penetrates several microns into the tape’s magnetic
layer. A fraction of a second later the video heads pass over the same section
of tape (a ‘stripe’ slanted diagonally across the width of the tape), and record the vision signal on a high
frequency carrier. This is directly on top of the audio tracks, and the signal
penetrates to a depth of less than one micron. The gaps in the audio heads are
much narrower than the ones on the video heads, and the track is only around half
as wide, (26 micrometers, compared with 49 micrometres for the video
tracks), moreover the head gaps have
different azimuth angles, to prevent ‘crosstalk’ between adjacent tracks. So far
so good?
When a recording is
replayed on the machine it was made on the same tape heads are used so the
sound and vision signals are extracted and separated without too much bother,
though some signal losses occur as the audio information has to pass through
the video signal. However, if the
recording was made on another machine there will often be microscopic
differences in the shape and position of the heads, recording drum and tape
guide path rollers, and other factors such as tape tension, and mechanical wear
can cause tracking errors to occur. Tracking errors happen where the heads do
not precisely follow the recorded tracks. It’s not normally a problem, most
hi-fi VCRs have automatic tracking systems, which monitor the amplitude of the
video signal, and make minute adjustments to the speed of the head drum, or capstan
motor, to physically shift the tape, in relation to the heads. Using the video
signal as a reference can make the audio heads mis-track slightly, and pick up
what’s known as FM switching noise. This occurs at the end of each track, when
one head is leaving the tape, and the other one is just starting its sweep, if
the changeover point happens a fraction of a second too early or too late a
buzzing sound may be heard.
Here endeth the
techno-speak. The cure is relatively simple, and normally all that needs to be
done is override the VCRs auto tracking system and adjust the tracking manually
to a compromise position. This will usually eliminate the buzzing sound, and in
most cases won’t affect the picture. However, if the difference in tolerances
between the recording and replay machines are too great then the picture will
deteriorate when the buzzing disappears. Alternatively, if the tracking is
optimised to favour the picture the sound may suffer and the VCR may even
default to mono sound, or worse still, chop between the two soundtracks, which
is even more annoying.
Manual tracking solves the
problem in ninety percent of cases but on a small number of machines it may not
go away. There’s two possible causes for this. The replay VCR may require
adjustment, or there could be excessive wear on the deck mechanism; in both
cases it will require expert attention. The other, less likely possibility is
that the recording contains major tracking errors that the VCR cannot hope to
resolve. These days, however, most duplication houses use high-speed thermal
printing equipment, rather than copy tapes on banks of ‘slave’ VCRs, and errors
are normally spotted and corrected before the tapes reach the market.
2. HOME
ENTERTAINMENT UNTANGLED
THE SCART CONNECTION
How’s your French? Does
Syndicat
des Constructeurs d'Appareils
Radio Recepteurs et Televiseurs
mean anything to you? Probably not. How about Peritel, or Euroconnector? No,
then you’ll be relieved to known that all three refer to those large multi-pin
sockets fitted to the back of most
VCRs, TVs and satellite receivers these days.
The SCART connector, to
give it its more familiar name, is a peculiarly European invention, dreamt up
by committee to be all things to all people but dismally failing to please
hardly anyone. SCARTs first started appearing in the early 1980’s primarily as
a means of simplifying the interconnection between a TV and VCR. Philips were
pioneers of AV integration and did much to popularised the concept of
connecting video and audio equipment together, they even came up with a trendy
new name for SCART, calling it a Euroconnector, however, it was the French who
managed to make it almost mandatory on European consumer electronics products.
In 1983 the French sought
to stem the flow of imported TVs and VCRs into the country, which they saw as a
threat to their own electronics industry. They came up with the Poitiers plan,
which insisted that all imported TVs and video recorders had to be individually
examined by Customs agents, in the town of Poitiers. This was to make sure they
complied with European specifications, and it included the fitting of a SCART
connectors. There were only nine customs agents in town, so it quickly had the
desired effect and Poitiers became a bottleneck. Potiers was an apt, deliberate
and some say witty choice, it was the town where the French armies stopped a
Saracen invasion in 732...
However, the plan
backfired, the Germans got shirty because their goods had to pass through
Potiers as well, and eventually the regulations were changed to cover only
products made outside the EEC, but it was too late. The Japanese were setting
up manufacturing plants across Europe and they were careful to ensure their
products used enough locally sourced components to get around import tariffs
and regulations, but by then SCART sockets had become a permanent fixture.
So how exactly does a SCART
connector work? There are 20 pins in all, plus a metal shield which is
connected to electrical ground and the screen around the cables. The pin
allocation appears somewhat arbitrary, as the accompanying table shows, but
there’s basically only three types of signal, configured for video and audio
information, and control.
TABLE 1
PIN ALLOCATION
1. line audio out, right channel
2. line audio in, right channel
3. line audio out, left channel
4. audio earth
5. blue video earth
6. line audio in, left channel
7. blue video in
8. composite video status switching
9. green video earth
10. D2B data line (inverted)
11. green video in
12. D2B data line
13. red video earth
14. video blanking/D2B earth
15. red video in/S-Video chroma
16. RGB video status
17. composite video earth
18. RGB status earth
19. composite video out/S-Video luminance
20. composite video in
21. casing earth
Most of the connections are
reasonably straightforward, but a few might need a little explanation. It’s
also worth noting that some SCART leads can be ‘directional’ and need to be the
correct type, or connected the right way around, unless all of the pins are
connected, many are not. The red, green and blue or RBG ‘component’ video
signals are used by some home computers and video games instead of normal
‘composite’ video to produce crisper, sharper graphics. S-Video or Y/C signals
are used instead of composite video by some VCRs and camcorders, this helps
improve image quality, and reduces patterning in the picture, caused by parts
of a composite video signal interacting with each other. Pins 10 and 12 were
originally set aside for data and control signals for a system known as the
Domestic Data Bus or ‘D2B’. The idea was to centralise and integrate control
systems for a wide range of household appliances, from toasters to VCRs and
home security. This was another bright idea from Philips who eventually dropped
the idea of using a SCART connector in favour of a specially designed 3-pin
plug and socket system. The D2B disappeared without trace a couple of years
ago, largely due to lack of interest from the rest of the consumer industry.
Basically it’s a mess.
Fully wired SCART leads are expensive, thick and heavy and put an undue strain
on the sockets, leading to a lot of failures and hard to isolate faults.
Partially wired leads can be troublesome and more often than not you end up
with the wrong type. SCART does have a few good points, though; a single cable
is quite a tidy arrangement and devices can be daisy-chained together. In the
early days Philips reckoned up to eight components could be strung together in
this way, though in practice most people are unlikely to want to connect more
than three AV products together. Conflicts in the switching and control signals
can also occur, some TVs use them to automatically select external AV inputs.
Unfortunately it looks as though we’re going to be stuck with SCART for some
time to come, until someone comes up with a universally acceptable
alternative.
3. INSTANT KNOW-HOW
WHAT MAKES A GOOD TV PICTURE?
Why do TVs have a colour
control? That might seem like a daft question but the fact is there is a
correct level for colour saturation that could be factory set, and
automatically adjusted to compensate for ambient lighting and even the ageing
effects of the tube. It’s there because people want it, and probably wouldn’t
buy a TV without one. On the majority of TVs the saturation is set way too
high, making colours appear over bright and unnatural, after all, if you’re
paying £83 for a colour TV licence, you want your money’s worth...
Go into any high-street
multiple electrical store and look at the rows of TVs on the shelves, chances
are the picture on each one will look slightly different, but which one is
right, and what guarantee do you have that the TV you buy will look the same as
the one in the shop? There’s no easy answer to that, picture quality is hard to
define, and as the colour control facility demonstrates, mostly about personal
preferences. Nevertheless there are a number of things to look out for when
buying a new TV.
The most obvious one is the
name on the front panel, though even that’s not as important as it used to be.
The number of companies manufacturing picture tubes and the microcircuits that
generate the picture has fallen dramatically, so there’s a good deal of
commonality between TVs from the various brands, wherever they’re built;
however, it’s still fair to say that the big name brands are generally more
advanced designs, have better reliability, and will normally have faster and
more efficient service back-up.
Picture geometry and
linearity come next. Look at the edges of the picture, particularly the top and
bottom of the screen. Does the picture fill the screen? If not you may notice
flashing dots or lines, that’s teletext data and control signals which should
remain out of sight on unused picture lines. If you can see them it indicates
that the factory set-up was either incorrect, or has drifted, both bad signs.
Conversely the picture may be over-scanning, in which case everything will look
elongated. The tops of heads will be cut off
and captions and subtitles at the bottom of the screen may be partially
obscured by the screen surround; again this shows the set has not been aligned
properly. The sides of the picture should be parallel with the edges of the
screen, look at vertical picture elements -- door frames or street lights -- do
they bend in the middle? This is known as pin-cushion distortion and is another
sign of shoddy quality control at the factory. Finally check to see the picture
is square, with the sides of the screen surround.
Colour purity is very
important and should be consistent across the whole screen area. Any patches of
colour indicate that the metal shadowmask or aperture grille inside the picture
tube may be distorted or magnetised. The latter is not a serious problem, and
all TVs automatically ‘degauss’ themselves when they’re switched on, but
persistent colour staining on sets from
the same manufacturer may indicate the’re using low grade tubes.
The best test for colour
accuracy is flesh tones, they’re the hardest colours for a TV screen to render,
and if faces look wrong, all the other colours will be wrong as well.
Definition, or a TVs ability to resolve fine detail has a big impact on picture
quality but it’s really only possible to check it thoroughly with a static test
pattern, and with more or less round the clock TV, test cards have become quite
rare. The best time to see one is during the early morning, though ironically
most of the other tests are best carried out in the late afternoon, when the
TVs in the shop will have been on for a few hours. Look at the bands of thin
lines that get progressively closer together, if possible compare pictures, to
see which sets display the finest lines.
Test cards can reveal a
number of other aberrations, including subtle colour decoding and processing
faults. Check for colour displacement on white vertical and horizontal lines,
and smearing along the edges of black and white blocks in the picture. Finally
view the picture under strong ambient lighting, and look for potentially
annoying reflections.
Recent advances in display
technology are worth keeping an eye on. Sets with 100Hz pictures reduce the
amount of flicker that can sometimes been seen out of the corner of the eye.
Digital processing is also playing a growing role, and can improve reliability
and consistency, though this technology can interact with some types of TV
picture, giving a slightly jerky effect when there’s fast movement in the
picture, or when the image has been taken using a camera with an electronic
shutter. In the end, though the best measure of picture quality is your own
eyes, so use them, and look at the screen first, then worry about the price
tag.
4. GREAT QUESTIONS OF OUR TIME
A QUESTION OD STANDARDS...
Television has evolved into
a global communications medium and pictures from anywhere on earth can be
flashed around the world in a matter of seconds. That’s the glossy hyped-up
image. The truth is somewhat less fanciful, world-wide television technology is
insular and divided, and a TV or video recorder brought in any other EU
country, say, will not work in the UK. Television broadcasting technology is
stuck in a 1950’s timewarp, conceived at the height of Cold War paranoia, long
before the advent mass communication, freedom of travel and satellite TV, it
has stagnated. The result is a score of
incompatible television broadcasting standards used around the world,
spread out on vaguely geopolitical lines. A TV map of the world clearly
illustrates empires, allegiances and conquests from a bygone age.
There is hope, though, and
the coming of digital TV promises a one-world standard but that’s for the
future, right now we have to live in a world of incompatible standards and the
good news is there are now ways and means to get around almost every standards
conflict, but first it’s worth look at what the differences actually mean.
The most fundamental
problems concern the number of lines in the TV picture, the frame repetition
rate (i.e. the number of times a picture is shown each second) , and the way in
which the colour information is processed. There are basically three colour TV
systems: NTSC (National Television Standards Committee), PAL (Phase Alternate
Line) and SECAM (Sequential Colour A’ Memoire). Historically NTSC was the
first, developed in the USA in the 1940’s. The picture is made up of 525 lines,
with a frame rate of 60Hz; incidentally, the last figure is normally based on
the local mains frequency, early TVs used the mains to provide an accurate
timing signal. NTSC is used throughout North America, Japan and the Pacific.
PAL was developed in Germany during the 1950’s, the picture is made up of 625
lines, and the frame rate is 50Hz. PAL is used in the UK, much of Europe, the
Middle East, parts of Africa and Australasia.
SECAM was developed by the French and is technically very similar to
PAL, with a 625-line/50Hz picture; legend has it that it’s just different
enough to avoid infringing the PAL patents. SECAM is used in France and its
former colonies and protectorates. It’s also used throughout the former USSR
and eastern bloc, who were naturally averse to adopting either American or
German technology when they were casting around for a colour TV system in the
late 1950’s.
Video recorder and
laserdisc technology is inextricably tied to television, so these products are
designed to work exclusively on one of the three colour TV standards, with
tuners and mains supplies tailored to suit local conditions. In fact the
relatively small differences between PAL and SECAM has meant manufacturers use
the same recording and replay systems, so the information that ends up on the
tape or disc is the same for both systems.
The biggest problem,
though, is the incompatibility between PAL and NTSC and tapes or discs brought
in the US will not normally play on PAL equipment. However, in the last few
years the picture has changed and now a significant number of VCRs and some laserdisc players can replay
NTSC material. It’s not true standards conversion as such, rather a clever
technological conjuring trick which relies on the fact that TVs made for NTSC
and PAL markets in the last five years or so, use a number of common
components, including the colour processing microchips. The on-screen results
are usually quite good, though the partial processing carried out by the VCR or
disc player is not sufficient for the signal to be re-recorded on another VCR.
Sadly it only works one way, and there are no equivalent products in NTSC
markets, that can replay PAL recordings, apart from expensive
standards-converting VCRs.
Digital-based systems, like
Video CD, for example, are transparent to TV standards because the information
stored on the disc or tape is in the form of numbers, and it’s up to the player
to convert the information to whatever local TV standard is in use, there’s a
few ifs and buts still to be ironed out, but it looks as though the days of
software incompatibility are indeed numbered...
5. GREAT QUESTIONS OF OUR TIME
WHY DO SCRAMBLED SATELLITE CHANNELS LOOK SO BAD?
Are we being short-changed
by satellite TV, is the new age of broadcasting simply a way of selling us
sub-standard material? That’s the view of some satellite TV subscribers who
have commented on the poor picture quality of SKY’s movie subscription
channels, compared with unscrambled broadcasts.
It’s fairly obvious the
encryption process must degrade the picture to some extent simply because the
signal has to go through several additional stages of processing, but how
significant is it, and are there any other factors at work? It’s worth taking a
closer look at how the Videocrypt system used by BSKYB actually works, and
what, if any steps can be taken to minimise quality losses.
Videocrypt is a hybrid of
technologies, brought together by a company called News Datacom, owned by
Rupert Murdoch’s News Corporation, who also have the controlling interest in
BSKYB, so it’s pretty much a family affair. Videocrypt uses a system known as
line cut and rotate, which on the face of it appears quite simple, but it
requires some very heavy duty technology and computing power to get it to work.
At the transmission end of the chain line cut and rotate works by reading each
TV picture line into a digital memory where it is cut into two parts, they’re
turned back to front and joined back together again, before being sent on its
way.
The beauty of the system is
that it’s virtually uncrackable, without some very sophisticated and expensive
technology, and the structure of the TV signal is unchanged. As far as the
uplink transmitter, broadcasting satellite, satellite receiver and your TV are
concerned, they’re handling a normal 625-line PAL TV signal. That’s important
because it means that a Videocrypt TV channel is no more susceptible to
interference or processing errors than any other satellite TV channel
throughout the greater part of the transmission chain. Therefore any quality
losses must be occurring before or during the encryption and decryption stages.
For the set-top receiver to
unscramble the picture it has to reassemble each picture line individually, but
in order to do that it must know precisely where each line was cut, and that’s
where the Viewing card each subscriber must have (slotted into their receiver)
comes in. The card contains a microprocessor which amongst other things, has a
computer program called an algorithm which contains the necessary information,
(used in conjunction with data sent over the satellite link), to work out how each picture line was cut
and rotated.
This process happen in
real-time, in other words the decoder is unscrambling 625 picture lines, fifty
times a second, which is one helluva lot of computing . Moreover, the signal,
which started out in analogue form, has been converted to a digital format,
hacked about, then converted back to analogue again, before it starts its
journey; it then goes through the whole process again in the receiver. Quite
frankly the question should be how come scrambled satellite TV broadcasts look
so good?
Now we’re getting close to
the truth. The actual quality losses are surprisingly small, one of the best
ways to illustrate this is to look at the picture quality of encrypted channels
that broadcast material sourced from live feeds or video tape. SKY Sports is a
good example, picture quality is normally very good, comparable with
terrestrial TV much of the time, although the TV signal has been through the
Videocrypt mill, and made a 75,000km round trip to your dish.
From that is clear that it
is the source material, rather than the encryption process that has the biggest
impact on picture quality. Movies shown on the subscription channels are
generally dubbed from film on to one-inch video, a process which inevitably
leads to some degradation; then there’s the question of the care taken during
the transcription process, and the quality of the original prints, which can
vary enormously. The official BSKYB line is that there is no significant
difference in picture quality between their encrypted channels, and unscrambled
channels, we agree, but when it comes to the quality of the original material,
now that’s another story.
---end---
1994 2505
|
