Archivio Storico:- ex Dipartimento di Musica e Spettacolo - Universita' di Bologna
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Note:
Although DAW producers advertise their signal processing being compatible to CD-standards, careful verification of the format along CD-Red Book specification is necessary, which calls for linear PCM audio at a 16-bit word length and 44.1kHz sample rate. Special care has to be taken if sound documents from unknown sources are not accompanied by the appropriate digitisation side information. Chaining of different noise shaping or dithering concepts can produce distortions quite above the hearing threshold, although each of them is inaudible when listening to them alone.
Linear PCM Audio 16-bit is not obsolete! Audio technology has to be seen transitional today. With both DVD-Audio and Super Audio CD on the horizon, libraries and archives will face new challenges before classic digital audio formats have been acquired sufficiently. A psychological turning point will probably come when the CD is no longer seen as the best available sound quality format by the public. Just as the CD in comparison to the LP and the cassette on its way up, the new formats will gain consumer acceptance rapidly. Nevertheless, CD format: linear PCM audio at a 16-bit word length and 44.1kHz sample rate is not obsolete. It will remain the only viable consumer digital delivery format building the mainstream in consumer audio electronics for quite a while. An estimation of millions of CD-recorders to be sold per year represent a lot of hardware for the survival of the format. Recordable DVD is still in discussion on different formats (and size: 12 cm, 8 cm?); the projection that one DVD can possibly not be run on two different laptops is confusing the salespeople as well as the users. DVD as Digital Video Disk will even depend on the manufacturers choice to make it playable in more than one geographical region.
Signal Enhancement and Signal Restoration: Although Quality Control may report signal degradations which could be repaired by means of digital signal enhancement algorithms, a strictly linear archive copy should be produced in any case. This advice is justified because any signal enhancement to be considered can be performed on digital copies of the linear archive document with no loss of generality. As digital copies are 100% exact replica of the original no information loss takes place; whereas signal filtering or any other processing carried out already at the time of digitisation would inevitably introduce nonlinear transfer functions which frequently cannot be inverted later on. As a general rule nonlinear manipulations of the audio signal, such as lossy data reduction, compression algorithms, filtering and signal restoration, should be carried out at the end of the transmission chain only and never on archive copies. Some manufacturers of digital audio workstations and audio software packages provide typical audio signal processing algorithms, working either in the time or frequency domain, such as:
- DeNoiser for the reduction of broadband noise (hiss)
- RepairFilter for elimination of quasi-static noise (mains hum, hum from dimmers, stereo pilot tones)
- DeScratcher for elimination of scratches on vinyl record and shellac recordings
- DeClicker for automatic elimination of clicks
- DeCrackler for automatic elimination of crackles
- De8Clipper for automatic elimination of digital clipping
- DropOuter for automatic drop-out restoration
- VPIs for remastering and sweetening (parametric EQ, 1/3 octave EQ, linear phase EQ)
The list above can be completed by additional sound conditioning functions which belong to the standard equipment of a sound recording studio, usually appreciated by the professional sound engineer; these are among others:
Compressors, Limiter, Loudness Maximizer, De-esser, Free Shaper (redithering / noise shaping), stereobasis manipulator etc. For exact metering (level control) tools such as PhaseScopes (stereoscope) , , 1/3 octave Analysers, , MatrixScopes etc. are applied.
Archivio Storico:- ex Dipartimento di Musica e Spettacolo - Universita' di Bologna
