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1989-04-05
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##########################
# #
# COMPACT DISCS #
# ¯¯¯¯¯¯¯¯¯¯¯¯¯ #
# A HISTORY #
# ¯¯¯¯¯¯¯¯¯ #
##########################
I recently read somewhere that Atari were holding off from releasing
their CD ROM drive until a standard could be established. Hasn't anyone
told them there is already a standard available, and being used
vigourously in the PC world?.
This alone should convince them that there is no point in waiting for an
alternative now. IBM has already set it. Remember the battle between 3"
and 3 1/2" discs? What happened to 3" discs/drives when Big Blue choose
the 3 1/2" version?
Anyway, I went trawling the BBSs and found this article on the history of
CD development. Don't know who wrote it but thanks if you are reading
this.
Dave Mooney
~~~~ sof ~~~~
Note of Success comes from CD
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Philips has made more than its fair share of mistakes during the past ten
years. Laservision video disc came too late. Metal audio tape was
announced too early and withdrawn because of technical problems. By the
time it was ready for relaunch, the trade and public had lost interst.
The N1500 video recorder came too early and could only record for an
hour. Its successor, the long play N1700 VCR, came too late to compete
with VHS and Beta.
V2000 video came even later and died a spectacular death. The relaunch of
Laservision as CD Video has failed, too. The early promise of CD
Interactive. a mixed media disc, has never been honoured. That, too may
now arrive too late to succeed.
These disasters tend to obscure the fact that Philips changed the world
in the 1960s with the compact audio cassette and has changed it again in
the 1980s with compact disc audio.
Although Philips granted licences on the compact cassette and policed
them to maintain a world standard, the Dutch company never made a penny
out of royalties. The Japanese only adopted the system in return for
royalty free licences.
But the situation on cd is different. Philips took Sony on board as
codedeveloper, screwed the hardware and software industries to the ground
and now earns a royalty on every disc and player made anywhere in the
world. Instead of sitting back and counting the free money that rolls in,
Philips continues to develop the system.
The latest development, by far the most radical since cd was launched in
1982/83, is called bitstream. This is a grassroots rethink on the way
digital signals read by laser from a cd are decoded. Several Japanese
companies, for example Technics, are working along similar lines. But
Philips appears to be in the lead.
Music is recorded on a cd in digital code. This code is made up from
words, each containing 16bits of information. Each word describes the
original analogue signal which has been measured or 'sampled' 44,100
times a second.
So far cd players have done the logical thing and converted the 16bit
digital words back into an analogue waveform. Even though the digital to
analogue converter is working on 44,100 measurements a second, the result
can never be a smooth waveform: it is stepped like a staircase. An
analogue filter must be used to round off the sharp edges.
Initially it was thought that this filter must have a very step curve, to
slice off spurious noise signals above 20kHz, while leaving the music
below 20kHz untouched. But steep filters tend to distort the phase of the
music signal. So Philips developed a technique called oversampling. Each
digital sample is processed several times, to spread the noise over a
wider frequency range and allow the use of a more lazy filter curve.
The Japanese followed the Dutch lead and oversampling is now a standard
trick in cd players.
But a problem still remains. A conventional 16bit dac has to process some
bits with an accuracy 65,536 times greater than for other bits. However
carefully the dacs are made and adjusted at the factory, and however many
times they oversample the digital signal, they are still working with
16bit binary code words.
In any digital system that relies on binary code words, some bits in each
word will be much more 'significant' than others. Any errors in these
most significant bits (msbs) can make the stepped output signal jump
wildly up or down, introducing nasty random noises into the music.
Each bit has equal significance and together they describe the way the
signal is changing. Up a little, down a little, down a little more, up a
little and so on -rather like instructions to a car driver backing into a
tight corner.
Because all bits have the same 'significance', errors are never
catastrophic - they just steer the signal slightly off course and the
mistake is easily corrected.
Obviously, the faster the bits stream, the more accurately they describe
the signal. BitStream does 256 calculations on each of the 44,100 digital
words available each second. This creates a stream of single bits running
at 11.2MHz, more than 11 million a second.
Taken together, the density of the bits paint a highly accurate picture
of the waveform, because each bit is a snapshot of the signal taken at a
very brief moment in time.
Philips will start using BitStream chips in the most expensive cd players
this winter, and by 1991 all Philips digital audio equipment will rely on
them. By the mid 1990s all the electronics for a cd player will be
integrated into a single BitStream chip.
~~ o0o ~~~
One bit wins the decoding race
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
The 'bit race' is officially over. The four and eight times oversampling
systems used by CD players are now 'out', and so are 18 and 20 bit
decoding. One bit decoding is the 'in thing'. The break with 6 years of
CD tradition has arisen because the Japanese are offering a single bit
decoder that rivals the Bitstream system designed by Philips. The
Japanese system is called multistage noise shaping (mash), and was
developed by Nippon Telephone and Telegraph.
Bitstream and mash do not directly convert the stream of 16bit digital
words read from the disc into an analogue waveform by digital to analogue
converters (DACs) usually made by US company, Burr Brown. Instead they
both convert the words into a more rapid stream of single bit
instructions.
Whereas Bitstream fashions the bits into bunches which vary in density,
mash bits vary in width. While conventional DACs are referred to as pulse
code modulation devices (PCMs), Bitstream uses pulse density modulation
(PDM), and mash pulse width modulation (PWM).
Supplier Technics admits that mash has been ready for more than a year,
but the company has sat on it for fear of creating confusion. Now Philips
has pre-empted Technics with the Bitstream launch. Technics must follow
suit with mash.
All the major Japanese companies are following with single bit
processing. Most will buy mash chips from Technics, although Sony looks
likely to favour Bitstream - not good news for Burr Brown.
According to Tadashi Abe, the man in charge of CD development at
Technics: "Mash is very difficult to explain - especially after so much
talk about oversampling and 20bit decoding. But it is now well understood
in Japan and suddenly popular. We will now try to explain it in Europe.
We have to because mash will eventually take over from 'old type' DAC
decoding.
"We started using mash in some CD players last October but did not tell
anyone. Now we are using it in all but two of our new CD players.
Actually we wanted to introduce mash one and a half years ago, but we
were fighting the 18 and 20bit battles - the idea was opposed. The sales
force did not understand the system. Thanks to Philips, which has started
to promote the idea of one bit processing, we can now join in."
Seiji Kaji, director of Technic's audio division, added: "We now want to
use mash technology for all digital equipment, amplifiers and so on."
Firing the first shot in the single bit battle, Technics' Japanese
engineers have produced a simplified explanation of the system, which is
far clearer than the muddled Bitstream documentation from Philips.
Technics' simple explanation stems from an analogy with water: it
compares conventional pcm 16bit decoding with filling a bucket using
water from 16 different sized cups. In theory each cup's size is an exact
multiple of the others: in practice the sizes are never quite right, so
each decoded sample shows an error.
This means the system is non linear as different steps have different
errors, resulting in audio distortion. Technics prefers to fill the
bucket many times using one cup only. That way all cupfuls are exactly
the same size and any error is linear: the bucket either fills too slowly
or too fast, which is equivalent to an overall gain error in audio.
The clearest demonstration of non linearity can be given with a digital
recording of music, recorded at a very low level so that background noise
becomes audible. With PCM decoding the music distorts as quite passages
fall into the noise floor. With pdm and pwm there is less distortion.
In a mash decoder a digital filter looks at each 16bit word many times to
oversample, or more accurately resample it. Mash currently works at 32
times rate, but will later work at 64 times and eventually 256 times when
chips are available which can work fast enough to do all the calculations
involved.
At 32 times oversampling, each of the 44 100 words per second is examined
32 times. The filter also looks at the words which came before and after
the word under examination and splits the difference between them into 32
slots. In this way the filter can construct a smooth curve between words,
like a graph with many lightly registered points between the main points
on the curve.
This has the beneficial effect of spreading the random noise that
pollutes any digital signal. It is caused by chopping the waveform into
discrete segments over a very wide range. The noise in the audible band
is reduced to one thirty second of its original value.
Next a noise shaper (the part of the system which is actually the mash
circuit) pushes the noise towards the high frequency end of the range
reducing the amount of noise left in the lower frequency audible band.
At the same time all the many samples are circulated in a mathematical
calculator loop, which works like the negative feedback loop of an
analogue amplifier. It calculates the value of each sample in an
interactive process.
A rapid stream of values describing the signal very accurately comes out
of the loop. With more values there is improved accuracy - but the
circuit must work faster, bringing a cost penalty because consumer chips
have a speed ceiling.
Each sample is represented by a pulse of fixed level but the width can
vary between 11 values, which is equivalent to a 3.5bit word. This sounds
a course measurement, because at 32 times the normal 44.1kHz sampling
speed the pulses are streaming so fast they work together to accurately
describe the value of the signal.
Because all the pulses have the same level, and their speed is rigidly
locked by a quartz clock, there is no risk of non linear distortion.
As a final step the pulses are easily converted into an analogue signal
by passing them through a filter which rolls off high frequencies.
~~~ o0o ~~~
'No good reason why we can't commercialise this'
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Industry experts were stunned. Tandy, a company best known for a chain of
cut price electrical stores, was claiming to have developed an audio cd
player that could record and re-record blank discs. It cost just $500.
Competitors were sceptical and noted Tandy said it would be 18 to 24
months before it launched the product. But the company was defiant.
"There is no good reason why we can't commercialise the technology," John
Roach, Tandy's chief executive, told the press.
Four years later, and Tandy is still finding reasons why it cannot
commercialise the technology, at least not within the time it promised.
So the task of bringing cd recorders to the market has been left to other
manufacturers. And although the several products launched in the last few
months are priced for the professional market, cd recorders could one day
become affordable to consumers, if only those in the top end of the hi-fi
market.
Unlike Tandy's dye polymer technology, which promises cds that can be
written and erased hundreds of times, the recently launched cd recorders
use a write-once medium. And while Sony is promoting its magneto-optic
mini disc as a rewritable cd, due to be launched this year, the discs
cannot be played on a conventional cd system.
The current generation of cd recorders began to reach the UK in the first
half of last year, with Yamaha selling a handful of its 12,000 pound
machines to the broadcast industry.
Although that price has dropped to 8000 pounds, it has taken Philips to
bring the cost down to a more realistic level. This year, the company has
begun manufacturing recorders under its own trade names and badged for
other companies.
This led to a flurry of recorders reaching the market including a 3,000
pound model badged for Micromega of Paris, a 3,500 system from Marantz,
the trade name Philips uses for its top end audio equipment and machines
from Mission, Studer and Meridian.
Blank cds cost 29 pounds and the recorders are being marketed for storing
radio jingles, sound effects and other applications that require digital
sound and random access. Matsushita, Kenwood and Ricoh are also
developing recorders or have launched them in the US and Japan.
Write-once cds have, ofcourse, been available to computer users for some
years. But the failure of manufacturers to agree on a single optical
technology and a set of specifications means a disc written on one system
can rarely be read by another. With audio cds so well established,
manufacturers have ensured their recordings are compatible with existing
players.
There is, nevertheless, a move to standardise the recording process and
ensure blank and partly written discs can be interchanged between
recorders.
Last October, Philips and Sony, joint inventors of audio cd, linked with
Meridian Data, Ricoh, Teac, Sun Microsystems, Kodak and other computer
companies to form the Frankfurt Consortium, with the aim of promoting a
standard called cd-recordable (cd-r). Its specifications are enshrined in
the Orange Book, a superset of previous cd standards. This means cd-r can
read and write not only audio cd, but also cd-rom, cd rom-xa and cd-i.
"Our objective is to gain international acceptance for cd-r standard that
ensures complete compatibility for the end user," said Frederick Meyer,
the consortium's spokesman. "Such a standard will enable the end user to
place a disc in a cd-r drive knowing it will work regardless of who
manufactured the drive or who manufactured the disc."
Just as Philips makes most of the cd-r machines, so are the blank discs
made by a single manufacturer, the Japanese giant Taiyo Yuden. It, too,
allows companies such as Philips and Yamaha to resell them under their
own names. Kodak is also putting its name on the gold coloured discs for
the Photo CD system it has launched. TDK has recently developed its own
discs, but these are still being evaluated for cd-r specifications.
Also developing discs are Ciba-Geigy and the Cambridge based form
Plasmon. When Plasmon approached the Swiss chemicals group for improved
coatings for the optical discs it manufactures, it discovered Ciba-Geigy
was already working on a disc that was far more stable than current
technology. So the companies formed a joint venture to produce recordable
cds guaranteed to retain data for up to 50 years.
Bob Longman, Plasmon's technical director, said tests show uv light can
cause some discs to degrade after as little as three years, which can
cause a loss of data. The venture aims to have mass production beginning
at Plasmon's UK factory in December. "It is a major breakthrough," said
Longman.
The companies are saying little about the composition of the surface
chemicals they are developing, but physically the discs are identical to
Taiyo-Yuden's, ensuring they are compatible with the cd-r specification.
Blank cd-r discs have a transparent plastic substrate engraved with a
spiralling grove to guide the laser. This is covered with a special dye
that forms the recording layer. On to this is laid a gold reflective
layer with a plastic cover forming a protective layer.
To write data, the laser focuses through the substrate on to the
recording layer, heating a small spot to about 250 centigrade. This
causes the dye to melt, and the substrate swells to fill the space in the
recording layer. In this way the laser effectively burns a hole in the
dye and builds up a series of pits corresponding to a prerecorded cd.
The way the data is arranged differs from a conventional cd in that
additional information is written in the innermost tracks. This
information consists of a program memory area (pma) and a program
calibration area (pca).
As a recording is made, the track numbers and their start and stop points
are written to the pma. Parts of the recording, such as faults, can be
marked invalid in the pma, which will be skipped when the disc is played.
The pca is used to test the optimum power the laser needs to write, which
for any one disc can be between 4 and 8mw. This value is then stored in
the pca and read before every recording. When the user decides that the
recording is complete, the machine writes a table of contents into an
area corresponding to the lead-in of a conventional cd. A lead-out is
also written at this time to designate the end of the recording. Only
then will the disc work on a conventional cd player, so a music producer
cannot, for example, take a partially recorded disc home for the weekend
to play on his hi-fi.
One of the reasons the 8,000 pound Yamaha player is more than twice as
expensive as its competitors is that it effectively updates the table of
contents as the disc is filled, rather than waiting until the the end to
write it. This ensures the cd can be played on a normal system any time.
Cd-r is a complicated process, which is why the machines cost so much.
And as some of the most sophisticated parts are mechanical rather than
electronic, the price is unlikely to fall as fast as early cd players,
said Chris Barron, technical service manager of HHB, the UK distributor
of the Marantz recorder.
"Technologically, it's not possible to achieve the level of pricing of
consumer units, because of the amount of processing and servo assisted
laser control that are required to achieve the correct recording pit
size", he said.
"Compatibility is very important. If the pit size is not absolutely
correct the disc will not play back on many machines".
Furthermore, Barron does not see the price of blank discs falling below
14 pounds in the next two years, because of the inherently expensive
manufacturing process.
So perhaps the greatest hope of a mass market cd recorder still lies with
Tandy. The latest word from the company is that it will introduce a write
once machine this year "at an affordable price".
As for the $500 erasable system, don't hold your breath. "It will be a
good product for the 1990s," said Ed Junge, marketing director. "We hope
it will be the early 1990s".
~~~~ eof ~~~~