audio digitale, digital audio, cd, dvd, dcc, dvd audio, super audio cd, spdif, aes-ebu
audio digitale, digital audio, cd, dvd, dcc, dvd audio,
super audio cd, spdif, aes-ebu
Audio Digitale
Digital audio is the most commonly used method to represent sound inside a computer, many audio processing device and modern audio storage devices (like CD, MD, DVD).
Digital audio technology is a method of representing audio signal using binary numbers. An analog audio signal is converted to digital by the use of an analog-to-digital (A/D) converter chip by taking samples of the signal at a fixed time interval (sampling frequency). Binary number are assigned to these samples. This process is called sampling. This means that the audio data is stored as a sequence of samples taken from the audio signal using constant time intervals. A sample represents volume of the signal at the moment when it was measured.
This digital stream of data is then recorded onto storage media (magnetic tape, optical disk, hard disk or computer memory) or transmission path (telecommunication network, Internet, digital satellite, digital TV transmission). Upon playback, a digital-to-analog (D/A) converter chip reads the binary data and reconstructs the original analog signal. This process virtually eliminates generation loss as every digital-to-digital copy is theoretically an exact duplicate of the original.
In uncompressed digital audio each sample require one or more bytes of storage. Number of bytes required depends on number of channels (mono, stereo) and sample format (8 or 16 bits, mu-Law, etc.). The length of this interval determines the sampling rate. Normally used sampling rates are between 8 kHz (telephone quality) and 48 kHz (DAT tapes).
The physical device that converts analogue audio to digital audio is called ADC (Analog to Digital Converter) and the device which converts digital audio to analogue audio is DAC (Digital to Analog Converter).
Sampling parameters affect quality of sound which can be reproduced from the recorded signal. The most fundamental parameter is sampling rate which limits the highest frequency than can be stored. It is well known (Nyquist's Sampling Theorem) that the highest frequency that can be stored in sampled signal is at most 1/2 of the sampling frequency. Sample encoding limits dynamic range of recorded signal (difference between the faintest and the loudest signal that can be recorded). In theory the maximum dynamic range of signal is number_of_bits * 6 dB . This means that 8 bits sampling resolution gives dynamic range of 48 dB and 16 bit resolution gives 96 dB.
Compact Disc is a stereo digital audio system based upon a 12cm single sided disc made from a polycarbonate material with an internal reflective layer of aluminium or occasionally gold. The information is read by sensing the presence or absense of reflected light from a tightly focused Laser beam pointing at the pits and bumps in the reflective surface within the disc. The digital audio signal is sampled to 16 bits per channel at a rate of 44.1KHz. The major developers were Philips electronics of Holland and Sony of Japan. In other words the audio format in audio CD is two-channel, signed 16 bit two's complement integer samples, 44.1 kSamples/second, linear PCM. This is defined in an industry-standard publication known as the "Red Book", the first in a series of "rainbow" books that define other formats (DVD, karoke, CD-R, etc. etc.).
According to Red Book specifications, a standard CD is 120 mm (4.75 inches) in diameter and 1.2 mm (0.05 inches) thick and is composed of a polycarbonate plastic substrate (underlayer - this is the main body of the disc), one or more thin reflective metal (usually aluminum) layers, and a lacquer coating. CDs are divided into a lead-in area, which contains the table of contents (TOC), a program area, which contains the audio data, and a lead-out area, which contains no data. An audio CD can hold up to 74 minutes of recorded sound, and up to 99 separate tracks. Data on a CD-DA (audio CD format) is organized into sectors (the smallest possible separately addressable block) of information. The audio information is stored in frames of 1/75 second length. 44,100 16-bit samples per second are stored, and there are two channels (left and right). This gives a sector size of 2,352 bytes per frame, which is the total size of a physical block on a CD. CD data is organized into frames (consisting of 24 bytes of user data, plus synchronization, error correction, and control and display bits) which are intricately interleaved so that damage to the disc will not destroy any single frame, but only small parts of many frames. In most cases error correction (reed-salomon) or error-concealment techniques can still solve those errors.
The legal and illegal copying of CDs has increased during the last few years due to the wide availability of low cost CD-recorders and media. This has resulted in a fall in the demand for legitimate pressed CD albums in some countries, which has made music companies to react to this situation. The new discs now making their way into record stores in the United States and Europe contain countermeasures that prevent playback on computers and, in some unintended cases, normal CD players as well. The controversial new anti-copying technology introduces minute errors to the CDs or changes the location of data on the discs to prevent them from being played back on computers. In theory, most consumer CD players can correct the errors and decipher the structure, unlike the more finicky computer CD drives. All of the big major record labels are experimenting with ways to block consumers from "ripping" or transforming their CD songs into MP3 files and distributing them widely online though on-line file sharing services.
This "CD copy protection" has been a well discussed topic because some record companies have started to use it on some music CDs. Copyright protected cd's do not allow you to replicate them in a cd burner nor do they allow you to rip the audio tracks "digitally" (although can still be done through analog). Copyright protections systems currently in use have downsides: not all CD players (especially computer CD-ROMs and some car plyers) will play those CDs.
The current copy protection techniques generally either corrupt the CD table of contents or actual audio data. The protection techniques that modify table of contents, modif it in in such a way that readers that know about data CDROMs get a tummy ache and see only some trivial data files, and not the audio tracks. This means that the protected discs can't be played on CD players that use CDROM drives for transports, of which there are quite a few. One way to make a CD act like is is to add a CD-Extra file track to the end of the CD, but making that CD-Extra track somehow broken (computer tried to read this track, but fails with error which causes the reading of whole CD to fail in many systems). Another method does corrupt the data. The plan is to put some deliberately-corrupted blocks, corrupted enough that the ECC will fail, into the audio data. It's transparent to analog output because the corrupted samples are in places where the interpolation done by the player when it can't do the digital ECC will end up with the same answer (or very close to it) as was in the original samples prior to corruption. Most PC CD-ROMs can detect the errors caused by data corrupptio, but most CD-ROM drive controllers lack the ability to pass this information of where error back up to the PC or do the error-concealment algorithm in firmware, prior to passing the data to the PC. A lot of the better CD-ROM drives do the error concealment on "ripped" data, so they aren't bothered at all by these "protected" discs. Cheaper CD-ROM drives may not do the error concealment.
Anyway this copy protection might make the copying of audio CD harder, but it does not stop you from doing that if you really want to do this. If you can play back the audio CD through your HIFI system, then there is always a way to copy it some way (through digital or analogue means). When you play back a CD on your normal CD player, you wll always get the correct audio signal from the CD player analogue output and CD player digital audio output (no matter if that CD is protected or not). No CD copy protection can stop that (or it will stop normal playback with normal CD players as well).
There are industry views that this kind copy protection techniques are actually a violation of the "red book" standards, to which everyone who presses music CDs has contractually agreed to comply. According some megazine article Philips has been putting these companies on notice that they may not advertise or label such products as "Compact Discs", and cannot use the CD logo or claim CD compatibility, as these discs deliberately violate the Red Book standard and are thus a technical violation of the license agreements for the use of the logos, etc. At the very least, consumers have the right to insist that any such products be accurately identified (warning stickers, etc.) so that we can make an informed decision about buying them (or not!).
I can't escape the idea that these music companies are trying to latch the door on the barn after the horse escaped. What happens with many of the copy-protected discs is that many users will have problems in playing them properly (lots of angry customers and returned CDs to shops) and that someone breaks the copy protection and distributes the tracks through a file-sharing service. Once a track enters the domain of a file-sharing service, there's no stopping its distribution. People copy those music files at higher priority, simply because they were copy-protected (also those people who would have bought a CD but it did not work for them). This means that applying copy protection which causes problems to use punishes the users which are willing to buy the original CD. If the original CDs do not work reliably anymore, those users might be tempted to go to the route of copying the music instead (in this way they get their own CD copy which works for sure on many different players including their home CD, DVD and PC).
HDCD® (High Definition Compatible Digital®) is a patented encode/decodeprocess for delivering on Compact Discs and DVD-audio better than normal CD audio quality. HDCD encoded CDs are encodedso that they fit 20 bits of information onto the CD while remaining completely compatible with the existing CD format.
DVD audio is a multichannel(PCM) audio format which stores many hours of high quality (better than CD) audio to a DVD disk which can be played on "DVD-Audio" capable DVD players. This audio format makes it possible to store more than 74 minutes of sound in two channels using the highest sampling frequency 192 kHz with 24-bit quantization, or six channels at a 92-kHz sampling frequency with 24-bit quantization. That's better than twice the performance of the CD audio specifications. Thus, dynamic range is 144 dB and frequency response is up to 96,000 Hz. The maximum transfer rate is 9.6 Mbits/second. The DVD-Audio format was originally intended to be finalized in spring 1996, but company infighting stretched that time line.
Even though though version 1.0 of DVD-Audio specifications (the PCM format descibed above) was published on the end of 1998, there is still some fuel on the format fight. Sony and Philips are also behind a competing system, Super Audio CD(SACD), using a one bit sampling 2- and multichannel format called Direct Stream Digital(DSD).
A DVD-Audio discs can also be played on almost any DVD-Video player if it in the Video-section includes a track that follows the DVD-Video audio specs (some DVD-Audio disks have this as an option for compatibility). An optional CD layer can give DVD-Audio software compatibility with some players for CD and DVD-Video.
The record industry is only interested in SACD/DVD-A because of the built-in copy-protection features. Philips and Sony are interested in SACD because of the royalties they'll receive for many years.
Digital Compact Cassette (DCC) is dying digital tape format from Philips. This system was able to storage quite high quality (comparable to MD) MPEG compressed digital audio to a compact cassette format cassette (basically same mechanics, just better tape in it). DCC players were also capable of playing back normal analogue compact cassette (C cassette) tapes. Nowadays you can't buy new DCC devices. This technology lost the battle against MD and CD-R/CD-RW technolologies.
MiniDisc (MD) is a small audio disc format. The disc used in the system is 7 cm (2.75") x 6.75 cm (2 21/32") x 0.5 cm (3/16"), the disc inside is 64 mm in diameter. One MiniDisc give around 160MB storage for 74 minutes in audio mode and 140MB in Data Mode. MiniDiscs are available as ready made records to buy and as re-recordable discs for making your own recording. The audio data in mini disc is compressed using ATRAC algorithm.
MiniDisc is suitable format for small portable audio listening devices and also for making quite high quality audio recordings.
At the consumer level there are basically one digital S/PDIF format which is transported using two different interfaces. One commonly used interface option is electrical coaxial interface, using an RCA-type jack and plug (or sometimes a BNC bayonet connector) and a shielded, 75 ohm impedance cable. Other option or consumer S/PDIF interface is Toslink optical, which transmits the digital data in the form of light down a plastic fiber-optic cable (1 mm diameter fiber center). Professional audio equipments use AES/EBU format defined by standard IEC-958, which uses a 3 pin, balanced XLR-type connector and 3 conductor, 110 ohm impedance cable or multimode fiber optics (AT&T or ST-Glass).
Sometimes there is questions on hifi forums raised should I use coaxial or (Toslink) optical interconnection ? They're both digital, so as long as those conversion processes are reliable, nothing will be lost. When properly implemented their performance is equal. Sometimes people recommend coaxial connection over optical because of less processing on the way. But the processing between optical and electrical signal presentations does not change the data contents. This means that there can't be any difference in frequency response, amplitude distortion or noise. The properties of the signal are encoded in the digital data. That data is unchaged by the conversion process. If errors happen (baddly operating devices), the system will notice those very quicly (S/PDIF has parity bits to check data integrity) and mute the audio output.
One might also see arguments about optical connections introducing jitter. There could be a difference in jitter, but comprehensive measurements fail to show any significant differences. Jitter is way overplayed as an audio issue. As long as you are in the digital domain, jitter is utterly irrelevant. It is only at the moment of conversion to/from the analog domain that jitter is relevant. If you have a situation where the jitter is demonstrably worse with an optical connection, the fault lies not with the optical connection; it's in the D/A converter itself, and is the result of poor clock design in the D/A converter. In some cases, there are some bad implementations of optical. Combine that with a BAD implementation of a D/A converter and you have a system that does not work well. But, take solice in the knowledge that as bad as some people can put together a bad implementation of an optical output, there are others who can put together an equally incompetent implementations of coaxial output as well.
The only benefit I can think of from an optical (Toslink) connection over a coax is the galvanic isolation between the two equipments, which can avoid ground loops. This could be relevant in some audio systems where ground loops are a problem. The downside is that Toslink optical connection has shorter distance limits than coax and optical cable usually costs more than coaxial cable. The general advice is use whatever you have easily available and be happy.
In coaxail interconnection sometimes should I use a special "digital cable" or will a normal "RCA audio cable" work ? The right thing to use in any length is 75 ohm coax with appropriate connectors, such is used for RF and composite video. In short lengths like 1 meter, just about anything that is conductive and look like a cable will work about as well as anything else. A standard 1 meter audio interconnect can do the job just well.
MADI (Multichannel Audio Digital Interface) is a professional multichannel version of AES/EBUstandard for transmitting up to 56 channels of digital audio data over a single coaxial cable terminated with BNC connectors. MADI uses a second cable for word clock, with a fixed data rate of 100Mbps used on large, open-reel digital multitracks. Optical MADI implementations are available.
MADI (Multichannel Audio Digital Interface), also known as AES-10 standard, allows interconnection of two devices to transmit up to 56 channels of 24-bit digital audio with a single coaxial cable or via optical link. It is a standard interface to digital multitrack machines and mixing consoles like the Studer D941 On-Air Mixing console, Studer 950 Digital mixing system, Neve Capricorn or Sony PCM3348HR Digital Tape Recorder. Some manufacturer specific MADI extensions allow up to 64 channels in each direction and can operate at 96 kHz (offering up to 32 channels) or even up to 192 kHz (with 16 channels on one single connection each way).
Tecording industry is worried about copying of their products (CDs). The recording industry has tried putting down the first critical pieces for a system it hopes will keep songs on the Net from being pirated.
The general term 'piracy" refers to the illegal duplication and distribution of sound recordings and takes three specific forms: counterfeit, pirate and bootleg. Counterfeit recordings are the unauthorized recording of the prerecorded sounds, as well as the unauthorized duplication of original artwork, label, trademark and packaging of prerecorded music. Pirate recordings are the unauthorized duplication of only the sounds of one or more legitimate recordings. Bootleg recordings are the unauthorized recording of a musical broadcast on radio or television or of a live concert.
The most talked about technology has been Secure Digital Music Initiative (SDMI). The first phase of the SDMI system requires that portable digital music player manufacturers implement several security components, foremost among them a digital rights management system (DRM). This will allow record labels to securely distribute and track files as they are transmitted over the Net and on to portable players.
Ever since Napster came to prominence, the music CD publishers have been looking for a way to stop people sharing MP3s extracted from their CDs, and now they think they've found it -- by copy-protecting the CD releases. They hope that by making CDs unplayable on computers that this will reduce the number of MP3s getting onto the internet.
CD copy protection is mainly done to make it harder to make copied of CD or converting them to MP3 files on PC. There have been meny technologies tied, but none of them is completely safe or problem free (and there will not be any such for CD). Generally CD copy protection systems more or less try put some type of errors to CD disk content (tale or contensts or audio itself) so that it does not either play on PC or sound gets corrupted on "CD ripping" process. No CD protection which still allows playing the music is bulletproof, even though protection makers say "We can stop all kinds of copying, even on domestic CD recorders." Sensible industry experts are skeptical on CD protection systems promises of them being any long term solution, ebecause hackers have defeated earlier security (promised to be secure) technologies very quickly. The fact for all CD protections is that if you can play the CD with CD-player, analogue copies can be made to any analogue devices (recorded to tape) or current digital devices with analogue inputs (for example PC soundcard) with some loss of quality.
Cactus Data Shield protection for CD is designed to prevent MP3-ripping by corrupting the table of contents so it won't play on a PC. Cactus inserts modifications to original CDs in a way that confuses CD-ROM devices during the copying process. If the CD is copied, however, the copier machine (a PC or disc-to-disc copier) sees some fake control data as music. So when the copied disc is played, there are bursts of distortion as the player tries in vain to decode the garbage.
SafeAudio is an Audio CD protection developed by Macrovision. SAFEAUDIO add samples that sound like bursts of static, and scramble the ECC data around to make it look like an uncorrectable error. Audio CD players will interpolate the samples during playback, but CD-ROM drives doing digital audio extraction generally won't. The result is a disc that plays back correctly on a CD player, but won't "rip" or copy correctly on a CD-ROM drive. SAFEAUDIO v3 changes the music data at the bit level, flipping a fraction of a disc's billions of 1s and to 0s ("very subtle" degree of data corruption). y MusicGuard introduces selective alternative alterations to the EFM data stream (can cause problems for CD-to-CD copying programs).
CDs with the current version of Sony DADC Key2Audio cannot be recognised by standard CD/DVD-ROM, CD-R and CD-RW drives, thus they do not play on PC.
Suncomm MediaClòQ v1.0 protected CDs are designed such that they can not be read by any CD-ROM player or CD burner. The CD has a heavy visible band at about the place where the audio ends. Those CDs generally say "This CD is designed to play in standard Audio CD players only and is not intended for use in DVD players."