Record restoration
From Wikipedia, the free encyclopedia
Record restoration, a particular kind of audio restoration, is the process of converting the analog signal stored on gramophone records (either 78 rpm shellac, or 45 and 33⅓ rpm vinyl) into digital audio files that can then be edited with computer software and eventually stored on a hard-drive, recorded to digital tape, or burned to a CD or DVD. The process may be divided into four separate steps: 1) recording the record with a computer; 2) processing the resulting sound file with software in order to reduce noise and remove transients resulting from damage (like clicks, pops, and crackle); 3) using software to adjust the volume and equalization; and, 4) exporting (saving) the file in the desired format (WAV, MP3, FLAC, etc.). The information detailing these four steps is mainly to be gleaned from various websites, and also from the helpfiles of the many applications devoted to this activity.
Contents |
[edit] Hardware
The first step involves hardware. One must have a record player. The output of a stylus and magnetic cartridge combination on a basic unamplified turntable is of a very low volume so that the signal must be amplified with a preamplifier to bring it up to line level before being routed into the line-in jack of a computer's sound card. If one has a "receiver" (a radio-preamp-amplifier combo), one can run a cable from either the receiver's cassette "record" jack or from the headphone jack directly to the sound card's line-in jack. Some manufacturers are nowadays offering preamplifiers dedicated to this purpose: they will run directly from the turntable to the sound card, bypassing the receiver or amplifier completely. Some of these products are equipped with algorithms built-in to counteract the "RIAA compensation curve" (also known as RIAA equalization) applied when the recording was originally made. Whatever the hookup, one must ensure that the recording volume is not set too high when recording through the sound card, or digital clipping (audio), a bad kind of distortion, will result. A low average volume can easily be corrected later on during editing - however, too low a volume setting can result in greater amount of noise (especially the inherent sound-card or system noise) relative to the usable audio and this noise will become prominent at the time of normalisation of the audio. The VU meter should not be bouncing into the red zone or approaching the "0" clipping point, but just touching the maximum or 0dB point. That said, a few clipped locations are usually acceptable since these are extremely small in width and do not cause any audible difference – however, this is subjective and at the recordist's discretion. One must also be sure that all equipment is grounded together, or subtle hums will very likely result. The computer should have sufficient power and memory to record an entire LP without causing any "drop-outs" — tiny gaps in the audio stream lasting just a fraction of a second.
[edit] Software
The software used to process the resulting digital files ranges in price from thousands of dollars to freeware. Some of these applications are simple, and some are very complex. Many are general purpose waveform editors that also happen to include record restoration features or plugins, and others are dedicated to the sole purpose of record restoration. Moreover, some applications are designed for easy fast processing with the push of a few buttons, and others require a time-consuming but perhaps more exact manual approach to editing out damage. Most applications present a waveform display, but a few are basically noise and click-pop filters that provide no visual display at all. All record restoration applications for Windows work directly upon WAV files, but a few will also directly open files in other formats, such as MP3.
Record restoration software normally handles 2 different categories of noise separately. First, there is the constant background noise that goes on through the entire recording that is the result of the sound the stylus makes in the groove when no music is playing, plus whatever subtle drones are generated by the electronics involved (such as turntable rumble or 60 cycle hum). In addition to band-stop filters (also known as "notch filters"), low-pass filters, and high-pass filters for filtering out hum and noise, many applications allow the user to take a "noiseprint" of a small section of waveform when the stylus is tracking but no music is playing; the filtering is then accomplished specific to this noiseprint. Second, there are the transient bursts of damage, mostly clicks and pops, caused by scratches or record defects, and crackle caused by lots of minute defects grouped close together. The software must filter this kind of click-pop damage conservatively, because a click or a pop can look very much like a legitimate percussive effect, such as a light snare drum rim-shot. If the automatic filtering software is getting every last click, chances are good that it could also be filtering some percussion instruments. After an automatic click filtering, it is reasonable to expect a few clicks to be left over, and these must be removed manually by isolating them one-by-one in the waveform. These residual clicks may then be corrected by attenuation (reducing or muting the volume of the anomaly), interpolation (replacing the waveform "spike" with a less offensive section, either a straight line--linear interpolation--or a calculated facsimile deduced from what the wave looks like on either side); substitution (replacing a damaged waveform segment with a similar section from elsewhere); channel substitution (where damage occurring in only one channel of a stereo waveform is replaced by a similar good segment in the other channel); and simple deletion, which is usually not noticeable for small samples. Some applications also have a "pencil tool" with which one can actually redraw the waveform.
[edit] Volume and equalization
After the noise and clicks and pops have been removed, one may adjust the volume. This is usually done by a process called audio normalization whereby the loudest tone in a track is amplified right up to some specified point, usually the point of digital clipping, and the rest of the waveform is amplified accordingly. In another form of amplification called "hard limiting," the loudest passages are attenuated drastically after they hit a certain limit, while the quieter passages are amplified. The result is a compressed waveform that sounds considerably louder, though it may not be what the original recording engineers intended. In all of these volume adjustments, one should respect the original dynamics of a piece, and of the variation in dynamics among different tracks in the same LP.
In addition to adjusting the volume, at this point one may desire to adjust the frequency profile of a piece with the "graphic equalizer" that is normally supplied with a wave editor. Some might feel that a track needs a slight treble boost, or reduction, or a big boost in the bass department. One should satisfy one's own perception of what sounds best for any particular track. An application usually lets you "preview" a piece before applying the equalization effects.
[edit] Export and save
After all this is done, the file (or files) are ready to export (or save) in whatever form the user desires. Almost all wave editing applications have the default ability to save files in WAV form, and some can also save files as MP3, FLAC, or in other formats. Many CD-R burning applications can then take these files and burn them onto a blank recordable disc in a form that can be played on a common CD player (using the standard CD-DA format).
