e003046.pdf

GENERAL INTEREST

new audio formats

high resolution and lots of features

Digital audio has become

commonplace. Many things have

changed since digital audio was

introduced into the consumer

market in the 1970s. The technology

has been improved, and media

capacities have increased

considerably, partly due to the

development of new generations of

semiconductor lasers.

Many people find it difficult to imagine that music shops

looked completely different twenty years ago. In a remark-

ably short time, vinyl records have been forced to make way

for a very broad assortment of CDs, and recently also DVDs.

The vinyl phonograph record has now practically disap-

peared from the face of the earth, along with the equipment

used to play it.

A WORLD FULL OF CHANGES

After the first initiatives to digitise audio information for the

consumer market in the 1970s, and the appearance of the first

audio CDs in the market over fifteen years ago, a lot of effort

has gone into developing successors to the CD that can lift

the quality of the audio information to even higher levels.

Although the audio CD represents the ultimate musical expe-

rience for many of us, there are certain technical weaknesses

in its design. New developments are certain to change this

situation.

A BIT OF HISTORY

The development of the audio CD in the ’70s and ’80s of the

last century was in many regards a tremendous technical

breakthrough. Traditional analogue recording techniques,

which up to then consisted of grooves in vinyl disks or mag-

By H. Steeman

One of the first audio CD players from Philips.

Elektor Electronics

3/2000

netic tracks on tapes, were discarded.

The development of A/D and D/A converters was a diffi-

cult task in the early days of digital audio. A multibit PCM

coding scheme with a relatively low sampling rate was chosen

for digitising the audio signals. Based on an audio bandwidth

of 20 kHz, the sampling rate was set to 44.1 kHz, with a reso-

lution of 16 bits. At that time, these were top-level specifica-

tions that allowed very good quality to be achieved.

side of the disk, as well as by using both sides of the disc. Let’s

have a closer look at all four of these formats.

DVD-5 (4.7 GB)

Single-sided, one layer

This format is the most elementary member of the DVD fam-

ily. The disc has a storage capacity of 4.7 GB. Only one of the

two substrates has an information layer. However, the two

substrates are combined (glued together) to produce a single

substrate that is 1.2 mm thick.

C HANGES ON SEVERAL FRONTS

The digital recording of audio information was not the only

innovation. The use of the CD as a storage medium was also

a technical breakthrough. For the first time, a semiconductor

laser was used in a consumer product to read a digital opti-

cal storage medium at a high data rate. Nowadays, it’s impos-

sible to imagine our daily life without audio CDs.

The manufacturers of consumer electronics products find

that the time is ripe to raise the quality of audio reproduction

to a new level. Unfortunately, they have not been able to

agree on a single new standard. Two new audio media have

been introduced into the consumer market, where they will

compete with each other in the coming years: Super Audio

CD and DVD-Audio.

Label

Polycarbonate

Bonding layer

Reflective layer

Polycarbonate

000022 - 11

Figure 1. Construction of a DVD disc with one optical

data layer (DVD-5).

DVD-9 (8.5 GB)

Single-sided, two layers

This DVD variant is single-sided, but it uses two layers on the

one side. This results in a capacity of 8.5 GB, which is a bit less

than twice 4.7 GB. The difference is due to measures used by

the manufacturers to improve the readability of the second

layer. The resulting bit pattern on both layers is thus 10%

larger than that of the DVD-5 and DVD-10 formats.

Each layer is located on a 0.6-mm thick substrate, with one

of the layers being semitransparent (gold coloured). The two

substrates are glued together using a transparent (optically

neutral) adhesive to make up a single assembly. The storage

capacity is more than adequate for recording a complete fea-

ture film with high quality (MPEG2 format), and it also pro-

vides room for the storage of supplementary data, including

eight tracks of audio information.

H IGH - DENSITY MEDIA

Any improvement in the quality of digital audio goes hand

in hand with an increase in the amount of digital data that

must be stored. Given that the dimensions of a CD (4 3 / 4 inch

or 120 mm) has more or less become an industry standard, it

is inconceivable that a physically larger disk (and thus yet

another new format) would be used. Furthermore, compati-

bility with existing devices and/or software is an important

prerequisite for the successful introduction of a new system.

This means that the storage capacity of the optical medium

must be improved. It took a long time for consensus to be

reached within the industry regarding a new generation of

CDs, but this has finally come in the form of the DVD (Digi-

tal Versatile Disc).

DVD: ONE CONCEPT , FOUR FORMATS

Although a DVD appears to be the same as a normal CD to

the naked eye, it is considerably more sophisticated. In the

first place, a different type of laser is used to read a DVD. This

laser uses light with a shorter wavelength (635 to 650 nm

instead of 780 nm). This makes the light spot that is used to

read the DVD much smaller. This in turn means that the

length of the digital data pits can be reduced from 0.83 to 0.4

micron. At the same time, the tracks can be made narrower,

so that adjacent tracks can be placed closer together. The track

pitch of the DVD has thus been reduced to 0.74 micron, com-

pared to 1.6 micron for a normal CD. These measures com-

bine to increase the storage capacity from 640 MB to 4.7 GB.

However, even this is inadequate for some applications. Con-

sequently, there are four formats specified for the DVD:

DVD-5, DVD-9, DVD-10 and DVD-18. In addition to these

formats, there are also agreements regarding (re)recordable

DVDs, but these fall outside the scope

of this article.

The smallest DVD has a storage

capacity of 4.7 GB, while the largest

has a capacity nearly four times as

large (17.1 GB). All DVDs employ two

substrates (transparent plastic layers

carrying the digital information) that

are 0.6 mm thick. With traditional

CDs, only one substrate is used, with

a thickness of 1.2 mm. Since the total

capacity of a single substrate layer is

4.7 GB for all DVDs, it follows that the

maximum capacity can only be

achieved by using two information

layers on top of each other on each

Label

Polycarbonate

Reflective layer

Bonding layer

Semi-reflective layer

Polycarbonate

000022 - 12

Figure 2. Construction of a DVD disc with two optical

data layers on one side (DVD-9).

DVD-10 (9.4 GB)

Two-sided, one layer per side

This DVD variant uses both sides of the disc, with a digital

Table 1. Comparative physical specifications of DVDs and standard CDs.

Specification

DVD

Number of layers

single

double

Substrate thickness

1.2 mm

0.6 mm

Number of substrates per disc

Track separation

1.6 micron

0.74 micron

Minimum pit length

0.83 micron

0.4 micron

0.44 micron

Reading speed (m/s)

1.3

3.49

3.84

Laser wavelength (nm)

780

635/650

Numerical aperture (NA)

0.45

0.6

Modulation

EFM

8-to-16

Elektor Electronics

3/2000

storage layer with a capacity of 4.7 GB on each side. In order

to play back both sides, the CD must be turned over. For the

time being, this must be done manually, although it is in the-

ory possible to make CD players that can read both sides of

the disc without requiring it to be turned over by hand.

disc with two data layers is used. The information layer that

a normal CD player reads lies at the standard depth of

1.2 mm, while a special layer with high storage density is

located at a depth of 0.6 mm. The laser of a standard CD

player, operating at a wavelength of 780 nm, focuses on the

optical layer located at a depth of 1.2 mm and thus looks

through the high-density layer at 0.6 mm. The laser of a

SACD player, with a wavelength of 650 nm, focuses on the

high-density layer and does not see the ‘standard CD’ layer.

Polycarbonate

Reflective layer

Bonding layer

Label

CD layer

(reflective)

Reflective layer

Polycarbonate

000022 - 13

HD layer

(transparent at 780 nm

reflective at 650 nm)

Figure 3. Construction of a DVD disc with one optical

data layer on each side (DVD-10).

000022 - 15

DVD-18 (17.1 GB)

Two-sided, two layers per side

The DVD-18 format is the variant with the largest imaginable

storage capacity. A double data layer is present on each side of

the disc, so that each disc contains a total of four data layers.

With this format, the two data layers on each side must be

placed on a single polycarbonate substrate. A photographic

process is used to produce the data layer that is located in the

middle of the substrate. With regard to the manufacturing

process, this means that a production system that transfers

the bit pattern to the photosensitive polymer must also be

available, in addition to the usual DVD press. The produc-

tion of DVDs according to this specification is very difficult

and presently quite expensive.

Figure 5. Construction of a Super Audio CD. Two differ-

ent optical data layers are present on one side of the

disc. The layer that is read depends on the type of laser

used.

M ORE INFORMATION ON THE DISC

Up to now, we have described how the industry has

increased the amount of data that can be stored on a disc

with a diameter of 120 mm from 640 MB to a maximum of

17.1 GB. In order to make optimum use of this storage capac-

ity, and to also attach a user advantage to it, the recording

and coding techniques have also been improved.

Polycarbonate

Semi-reflective layer

Reflective layer

Bonding layer

Reflective layer

Semi-reflective layer

Polycarbonate

000022 - 14

f m

f s

f s-m

f s

f s+m

000022 - 16

Figure 4. Construction of a DVD disc with two optical

data layers on each side (DVD-18). The fabrication of

this format is presently very difficult and expensive.

Figure 6. When an audio signal is sampled, a mirror

image of the sampled frequency spectrum (f m ) is created

on the other side of the sampling frequency (f s ).

S UPER A UDIO CD:

THE ODD MAN OUT

As already mentioned, the music

industry has introduced the Super

Audio CD in addition to the DVD. As

a data medium, this is a mixture of a

DVD and a normal CD. The develop-

ers of this concept, Philips and Sony,

needed a storage medium with DVD

capacity to accommodate the desired

improvements in the quality of the

audio information, but they

absolutely wanted to maintain com-

patibility with the traditional CD. The

Super Audio CD (SACD) can thus be

used with all existing CD players as

well as with the new SACD players.

Only in the latter case are the

improved quality and extra features

available. Here again, a single-sided

A compact DVD player for reproducing audio CDs, video DVDs and

special video DVDs containing audio tracks.

Elektor Electronics

3/2000

As already noted, the original audio CD is based on

multibit PCM coding with a resolution of 16 bits. Presently,

digital technology has advanced so much that speed itself

is absolutely no problem. Philips and Sony, who together

invented both the standard CD and the Super Audio CD,

have over the last several years developed a new recording

and coding system that is called Direct Stream Digital

(DSD). This works with a resolution of 1 bit and a sampling

rate of 2.8224 MHz (64·f s ). Figure 6 shows how the audio

signal is digitised. When an audio signal (f m ) is sampled at a

sampling frequency f s , then two ‘mirror ’ images of the

audio signal are generated, one on either side of the sam-

pling frequency. In order to prevent these mirror images

from overlapping, the sampling frequency must be chosen

to be as high as possible. With the standard audio CD, the

bandwidth of the audio spectrum is 20 kHz and the sam-

pling rate is 44 kHz. The original signal and the lowest part

of the mirror signal thus lie quite close to each other. Strong

filters that sharply cut off the audio spectrum just above

20 kHz are thus essential. Such filters can also affect the sig-

nals within the audio spectrum. If a high sampling fre-

quency (64·f s ) is used instead, the filtering problem disap-

pears, since the resulting image frequencies lie in a com-

pletely different part of the spectrum from the audio

frequencies. In addition, the resolution of the A/D converter

can be dramatically reduced, in this case to a single bit. For

each sample, the converter checks whether the amplitude

of the new sample is greater than or less than that of the

previous sample. This type of digitisation, which is called

delta-sigma conversion, forms the heart of a DSD converter.

If the conversion error (quantisation error) of the previous

conversion is taken into account in each new conversion

(which is referred to as noise shaping), the quality of the

sampling process can be improved even further. Figure 7

illustrates this process. The result is a digitised audio signal

with a bandwidth of 100 kHz (five times as large as that of a

normal audio CD) and a dynamic range of 120 dB, com-

pared to 96 dB for a standard audio CD. To achieve these

results, four times as much digital data are needed as for a

standard audio CD. A normal CD recording that takes up

600 MB thus requires around 2.4 GB in DSD format.

The audio specifications of DSD are so good that, with the

present state of knowledge, it is not better to make a better

recording. Philips and Sony have advised all recording stu-

dios to make their master recordings in the DSD format

(although the fact that the DSD format first appeared on the

market in mid-1999 means that very few studios have suit-

able equipment available, but that’s a different issue).

In order to guarantee compatibility with the existing PCM

technology, Sony developed what it calls Super Bit Mapping

Direct. Figure 8 shows how all standard PCM formats can

be derived from a DSD signal using this technique. It is pos-

sible to convert the signal to the PCM format with various

resolutions (16, 18, 20 and even 24 bits). If other formats are

thought up in the future, they can also be derived from the

DSD format.

It is generally thought that the audio industry will be able to

use the DSD format for the next twenty years.

DSD is the best possible recording technique, and the data

stored on the high-density layer of a Super Audio CD are in

DSD format. Since a maximum-length recording (74 minutes)

does not use the full storage capacity, there is room left over

for other information. In this regard, there is consideration

being given to multi-channel recording, graphic information

and text. Although Super-Audio CD players are presently

outside the price range of the average consumer, it is

expected that the prices of DSD decoders will drop so much

in the coming years that they will be built into more and

more equipment.

Loop Filter:

Noise Shaping

and Integration

Quantizer

Input

”

DSP Output

Negative Feedback Loop

000022 - 17

Figure 7. A DSD converter employs a 1-bit delta-sigma

converter. If proper feedback is applied, the noise level

drops and the signal quality is improved.

on the DVD has also been worked on for the last few years.

The resulting formats were definitively laid down in a stan-

dard in April of 1999.

This standard is based on two DVD formats, which are

called DVD-Audio and DVD-AudioV. These two formats

stand in addition to DVD-Video, which is the format that has

been developed for the distribution of films. Future DVD

players should be able to play back all DVD formats, and

computers can work with them as well.

DVD-Audio offers new possibilities for the user, including

improved sound quality (due to an increased sampling fre-

quency and higher resolution), surround sound (multi-chan-

nel sound), longer playing time and supplementary func-

tions. The storage capacity of DVD-Audio with two layers is

adequate for at least two hours of very high quality surround

sound or four hours of stereo sound. The specifications of a

disc with only one layer are, overall, the half of these. In order

to increase the storage capacity, a lossless compression tech-

nique called Meridian Lossless Packing has been specially

developed for DVD-Audio. This increases the storage capac-

ity so much that at least 74 minutes of audio can be recorded

on a single DVD side. The hardware in the DVD player can

Direct

Stream

Digital

2.8224 MHz

44.1 kHz

1 bit

The sampling rate of

Direct Stream Digital

lends itself to simple

downconversion to all

the standard PCM

distribution formats

88.2 kHz

14.112 MHz

1 bit

441

32 kHz

294

48 kHz

147

96 kHz

000022 - 18

DVD-A UDIO

In addition to the Super Audio CD, an audio standard based

Figure 8. The Super Bit Mapping Direct technique can be

used to derive any desired PCM format from the DSD for-

mat, with regard to both sampling rate and resolution.

Elektor Electronics

3/2000

Up to now, only Sony supplies SACD players. These are still quite expensive; the illustrated model costs around £4000.

handle this compression without imposing too much of a

load on the processor.

In addition to all these attractive specifications, additional

measures have been taken with both DVD-Audio and the

Super Audio CD to make copying more difficult. The aver-

age user will (for the time being) not be able to make copies

that have the same quality as the originals. Criminal organi-

sations will have difficulty bringing copies on the market that

resemble the originals closely enough that the unwitting con-

sumer will not be able to tell the difference between an orig-

inal and a copy. The special watermarks that can be applied to

the data side cannot easily be reproduced.

In light of the fact that the anti-copy protection of DVD-

Video discs using the Content Scrambling System (CSS) has

recently been cracked, and that the cracking program DeCSS

was freely distributed via the Internet, it is a good question

how long these security measures will remain effective. A

variation of the CSS is also used for DVD-Audio.

S TANDARD DVD-V IDEO AS AN

AUDIO MEDIUM

As a complement to these audio formats, it is interesting to

note that a DVD-Video disc can also have eight audio chan-

nels. In this regard, the disc manufacturer can make a trade-

off between multiple mono channels, for example for

speech, and multi-channel sound recording. At least three

multi-channel audio recordings can be combined with a

video signal.

In the mean time, several music producers (such as Denon

and Chesky) have been making video DVDs containing only

two audio tracks of 96 kHz, 24-bit audio with very good qual-

ity. As long as no hardware and software are available for

DVD-Audio, these special recordings can be regarded as an

excellent alternative.

After this summary of the new audio formats, we must ask

ourselves if the consumer is really interested in all of this.

DVD-Video can be considered to be a successful new format,

but this clearly represents a qualitatively high-value combi-

nation of video and audio. It is doubtful that the true audio

fan will be overly enthusiastic about two new audio media.

(000022-1)

DVD-A UDIO IN PRACTICE

The maximum data rate for DVD-Audio is 9.6 Mbit/s. This

means that the maximum sampling rate for multi-channel

audio is limited to 96 kHz. In order to make the best possi-

ble use of the available bandwidth, it is possible to apply dif-

ferent specifications to the various channels. For example, the

left, right and centre audio channels

could be sampled at 96 kHz with 24-

bit PCM resolution, while the two

rear channels could ‘make do’ with a

sampling rate of 48 kHz and 16-bit

coding. Other coding schemes can be

used in addition to PCM, such as

Dolby Digital (AC3), Digital Theatre

Sound (DTS), MPEG1 stereo or

MPEG2 multi-channel audio

(although the MPEG audio format

appears to have already lost the race).

DVD-AudioV is a supplementary

standard that combines audio with

video, such that the audio portion

can be played back on special DVD-

Audio player and the video portion

on a normal DVD video player.

Primary DVD-Audio and DVD-Video specifications.

DVD-Audio

DVD-Video

Coding

linear or

linear PCM,

compressed PCM

Dolby AC-3

or DTS

Sampling frequency

44.1/48/88.2/96/176.4/192 kHz

48/96 kHz

Resolution

16/20/24 bits

Number of channels

6 (96 kHz maximum)

2 (176.4/192 kHz maximum)

Maximum bit rate

9.6 Mb/s (linear PCM)

6.144 Mb/s

Elektor Electronics

3/2000

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: