Audio Recording

What is the Red Book?

Red Book is the set of specifications created by Philips and Sony to define the essential parameters for Compact Disc-Digital Audio (CD-DA). First released in 1980, Red Book has been adopted an international standard (IEC 60908:1999, Audio Recording — compact disc digital audio system) and forms the foundation for all other compact disc standards.

What types of audio CDs can CD-R and CD-R/RW recorders write?

CDs were originally designed for audio so it’s only natural that CD-R and CD-R/RW recorders write discs in the official Compact Disc-Digital Audio (CD-DA) Red Book format for use in any CD audio compatible player. Just like their mass produced prerecorded (pressed) cousins, CD-R and CD-RW discs can hold up to 80 minutes of CD quality audio (44.1 Khz, 16 bit) using as many as 99 separate tracks. In addition to Red Book discs, recorders also write compressed digital audio CDs which, instead of holding conventional tracks, contain MP3, WMA or other compressed audio files. Depending upon the scheme used, one compressed CD-R or CD-RW disc holds as much as ten to twenty ordinary audio CDs and can be played back in devices enhanced for compressed digital audio listening such as compatible computers, personal, home and car CD players as well as many DVD-Video players.

What types of material can be used as sources for audio CD recording?

Depending upon the capabilities of the recorders and software used, CD-R and CD-RW audio discs can be written from either digital or analog sources. Digital material such as existing MP3 files or CDs are conveniently read directly from the hard drive, recorder or from a separate CD or DVD-ROM drive. To record analog sources such as LP records, cassette tapes, microphone or radio tuner inputs connected to a home stereo, signals are first digitized through the computer’s sound card.

Digital

Digital audio material comes in many forms including compressed and uncompressed computer files such as MP3s, WMAs and WAVs, Compact Discs (CD), MiniDiscs (MD), Digital Audio Tapes (DAT), Digital Compact Cassettes (DCC) and Alesis ADAT. How they are handled by the computer for writing to CD varies depending upon the capabilities the individual recording system.

Digital Files

A popular way to create audio discs is to use uncompressed (WAV, PCM, etc.) and compressed (MP3, WMA, etc.) computer files as the recording sources. When producing a compressed digital audio CD these files are written to disc just as they come and, depending upon the recording software used, may be accompanied by MultiAudio or other navigational information. In the case of a Red Book audio CD, compressed files must first be uncompressed and translated into the correct format before recording. Historically, this had to be accomplished manually but most recording software now performs the conversion process automatically during the writing process. As with any audio recording it’s important to remember that the sound quality of a written disc will be no better than the source material used. Higher resolution digital audio files obviously will produce better results.

CD

Recording CD to CD is much simpler than recording from analog sources since most CD and DVD-ROM drives are capable of transferring audio directly (Digital Audio Extraction) without the necessity of converting from analog to digital. As a result, CDs can often be recorded disc to disc using a CD or DVD-ROM drive as the audio source. Where a suitable drive is not available the recorder itself can be used as the audio source. In this case, the audio is read using the recorder and stored temporarily on the computer’s hard drive until written out again.

DAT, MD, DCC, ADAT

Although the contents of DATs, MDs, DCCs and ADATs are already in digital form, most computers lack the proper connections for directly importing digital material. As a result the analog outputs from the source must be connected to the computer’s sound card and the audio re-digitized. Since there is always a loss of quality in the conversion process, some manufacturers offer special sound cards that connect to the digital outputs of the source deck to transfer the audio to the computer while keeping it in quality digital form.

Analog

Analog audio equipment such as LP record players, cassette tape recorders, microphones and radio tuners use continuous electrical signals of varying voltages to record and play sound. Computers, on the other hand operate, in the digital world where everything is represented in binary form. Thus, before the computer can manipulate or record analog audio sources to a hard drive or a CD-R/RW disc, the sounds must be converted into digital form through the computer’s sound card. Higher quality sound cards will produce better results but most cards are capable of recording at the 44.1 kHz frequency, 16 bit resolution used by audio CDs. Some recording software automates the conversion and writing processes into a few simple steps by performing time saving tasks such as detecting the silences between songs to automatically split the music into separate tracks.

What is MultiAudio?

The MultiAudio specification was created by OSTA in 2001 to provide a standardized structure for compressed digital audio files (MP3, WMA, etc.) written onto removable optical media, such as CD-R and CD-RW discs. This uniformity allows these discs to be played back in the same way on any compatible device including MultiAudio-compliant computer software media players and consumer electronics devices such as CD and DVD audio and video players. To accomplish this goal MultiAudio requires that, in addition to compressed digital audio files, an appropriately formatted disc also must contain a defined table of contents which the playing device will use for file navigation. In addition, the specification allows playlists to be created to organize material so it can be accessed by categories such as genre, album, artist or even in custom groupings created by the user. MultiAudio formatted discs are created by standard recording software packages and CD-R/RW recording-enabled audio jukebox applications which support the MultiAudio specification. Written discs can then be played back in MultiAudio compliant devices and even in compressed digital audio units not supporting the specification, albeit in a more limited fashion. Since MultiAudio is simply an organizing system it’s important to remember that the types of discs and specific compressed digital audio file formats supported depend upon the individual capabilities of the particular devices or software employed.

Can consumer compact disc audio recorders write to any CD-R or CD-RW media or are special discs required?

Even though general purpose CD-R and CD-RW discs and their consumer audio versions appear for all practical purposes identical, only blank media bearing the “Compact Disc Digital Audio Recordable” (CD-DA Recordable) and “Compact Disc Digital Audio Rewritable” (CD-DA Rewritable) logos can be written in consumer audio recorders. The reason for this restriction is to comply with international copyright agreements. A special Disc Application Code present in the ATIP information of a CD-DA Recordable/Rewritable disc’s pregroove wobble identifies it specifically for audio use. Consumer audio recorders are programmed to reject discs not containing the correct code. By adopting this safeguard various countries and other authorizing jurisdictions may selectively apply copyright levies to the price of blank discs intended for consumer audio use while exempting those destined for computer or professional applications.

Does using lower CD-R recording speeds and lower capacity media produce better sounding discs?

High speed CD-R writing often creates discs with low I3 and I11 signal amplitudes (optical signals generated from the smallest and largest marks) and 80 minute discs achieve their capacity by packing marks and lands more tightly together. These result in reduced recording and playing margins and sometimes lead to perceptible sound degradation, especially in older CD audio players which may not employ equalization (signal boosting). Consequently, many high speed recorder manufacturers recommend creating audio discs at reduced writing speeds while some recorders even limit their maximum speed to 24x when writing audio discs. In addition to slower recording speeds, some manufacturers also suggest using 74 minute instead of 80 minute discs. Several of the latest recorders even offer special writing modes which record audio discs with longer marks and lands than would normally be the case, albeit at the expense of some capacity. For example, an 80 minute disc written with longer marks and lands might only hold 74 minutes of audio and a 74 minute disc just 68 minutes of material.