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 several separate steps performed in the following order:
1. Cleaning the record, to prevent unwanted audio artefacts from being introduced in the capture that will necessitate correction in the digital domain (e.g. transient surface noise caused by dirt), and to prevent unnecessary wear and damage to the stylus used in playback. 2. Transcription of the record to another format on another medium (generally a digital format such as a wav file on a computer);
3. Processing the raw sound file with software in order to remove transient noise resulting from record surface damage (clicks, pops, and crackle cause by surface scratches and wear);
4. Using software to adjust the volume and equalization;
5. Processing the audio with digital and analogue techniques to reduce surface/wideband noise;
6. Saving the file in the desired format (WAV, MP3, FLAC, etc.).
The source of the information for these steps is available from various websites and the help files for the software employed in the process.
The first step involves cleaning the playing surface of the records (unless they have been stored in archival, dust-free conditions since they were last cleaned). This can involve anything from turntable-based, vacuum equipped, professional cleaning machines that use proprietary chemical formulations and cost four figures, to improvised methods involving home-made equipment and/or cleaning solutions consisting of isopropyl alcohol, distilled water (unpurified tap water should not be used, as it will probably leave limescale deposits on the record surface) and a surfactant to aid drying. Isopropyl alcohol should only be used to clean vinyl records: it will cause permanent damage to shellac, master and one-time recordings (acetate, wax and lacquer).
The second step involves transcription of a record using a suitable turntable and a suitable cartridge-stylus combination. More often than not, a magnetic cartridge and stylus combination is used because of its superior sound characteristics and signal-to-noise ratio over other pickup systems. The output of a magnetic cartridge is of a very low volume (typically ~5mV) so 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.
Three main types of phono-preamplifier exist for the process of record restoration and playback:
- those that apply RIAA equalization or RIAA de-emphasis on playback to counteract the equalization used when the recording was originally made. These are generally not suitable for 78rpm records and early microgroove recordings.
- those that include a switchable frequency turnover filter to match the various turnover frequencies used by the many record manufacturers between 1925 and ~1960.
- those that apply no equalization (also called "Flat" phonopreamplifiers). These require audio software to apply the correct equalization to the digital recording during the restoration process. As such, this type of premplifier is suitable for all record formats regardless of equalization employed by the mastering process.
Regardless of the preamplifier employed, one must ensure that the output volume is not set too high when recording through the sound card, or digital clipping may result. A low average volume can easily be corrected later on during editing (although with some loss in dynamic range) - 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. Ideally, the VU meter should not exceed around -2 or -3dB to allow for some signal headroom. However, some clipping due to transient responses caused by scratches or cracked records are usually acceptable since these are extremely small in width and do not usually cause any audible difference. One must also be sure that all equipment is grounded appropriately together, or subtle hums will likely result from the formation of ground loops. Similarly, the computer should have sufficient power and memory to record an entire record without any "drop-outs"— (tiny gaps in the audio stream lasting just a fraction of a second).