What is Digital Audio Restoration?

The nature of sound is as complex as it is possible to perceive.

Every original recording in its first stages is very raw. Replaying that initial recording will get you closest to the sounds that were recorded but that isn't necessarily what the performers or producers want you to hear.
Subsequent editing and mix-down work produces a 'Master' which is agreed to be the 'sound' that will be heard on the final product. This is usually stored carefully and used to make copy 'masters' for production of LP's, CD's or other formats. Normally these masters are suitable for further use with little preparation. However, many older analogue tape recordings either no longer exist, or are damaged or unavailable for other reasons. That's where vinyl sourced audio restoration becomes a valuable source of historical audio material. In many cases, the only source.

Sourcing audio from vinyl has historicall been seen as a compromise. It's a true that many vinyl 'rips' are done badly.
See Why Audio begs to differ. The audio quality obtainable from a well made vinyl disc, mastered from original master tape, can be outstanding. For reasons of degradation of the master, it may sometimes even be the best possible source. Audio stored on a vinyl disc does not degrade with age in the way that a magnetic tape will.

Arguments will rage forever ... audio is a subjective world, and restoring audio bears striking similarities to the 'Restoration' of a painting with its built-in dangers. 'Audio restoration' can be thought of as the art of identifying and removing extraneous audio damage, while keeping the audio that should be preserved. Warts and all.
The goal is to reduce or remove sonic artifacts that have been introduced after the manufacture of the record or tape, preserving the music as much as is possible, as the producers intended it to sound. Anything else is re-writing history.

To the untrained ear, the effects of audio restoration might not be obvious, but they are profound. Taking stuff away from a sound-source is a one-way process; you can't put it back so it's important to get your decisions right. Recent developments in computer technology mean audio engineers are now able to employ a dizzying array of noise reduction techniques. There is much ongoing argument and plenty of room for disagreement. However, with a sound [sic] philosophy and a dedication to detail, the results can be truly amazing.

There are two main approaches to the digital audio restoration of vinyl-sourced audio;
1) PASSIVE: Machine-based (for example CEDAR) digital restoration employing stand-alone hardware or software that the audio is 'passed through', or
2) ACTIVE: Manually targeted digital restoration employing any of the above in addition to more specific 'hands-on' waveform editing.
The two methods have some cross-over and both are useful in their own ways. They often work best in some kind of combination.
Above all, an Audio Restorer must concede that there is no 'magic bullet' fix-all technology. You need to be flexible - because music is flexible - and there is NO SUBSTITUTE for attention to detail.

Working manually can avoid some common pitfalls of automated systems. A classic example would be a saxophone note, in reality composed of a series of natural clicks, some algorithms will mistake these clicks for damage and ruin them. Audio is ususally unpredictable and an algorithm can't be a crystal ball. A hands-on 'active' human approach handled by a skilled audio restorer allows for value-based decisions and responses to the random nature of audio and the often temporary blemishes that accompany most analog formats. It's not as if humans have no faults but only humans can really decide if it 'sounds right'.
The factors which detrmine your choices are cost, time, quality or any combination. The most important factor should be quality!

See Why Audio maintains that there are four basic principles behind a succesful Audio Restoration:

Educate your ears to understand what you want.
Knowledge is power. Lack of knowledge will surely cause damage. I'm not being a snob, it's a fact (LOL!). You don't need to be a bat with the skills of a Symphony Orchestra Conductor but an audio restorer should be able to separate flaws from the music with some ease, and that requires experience. Nothing else will really do.

Preserve the original recording.
Preservation is the most important ingredient of a successful restoration. If you haven't kept the music, or you've damaged it by your actions, what was the point?
Oh, and preserve the dynamic range. When someone asks me to 'make it louder' I just point to the volume control. Just Google or Youtube 'Volume Wars', you'll see what I mean.

If it ain't broke, don't fix it.
For example if a recording is clean except for a few small clicks, edit only the clicks. Leave the surrounding audio alone. Go find the clicks and zap them, don't 'sweep' with automatic systems. This approach pays dividends musically.

Noise is good.
The late John Peel, when asked to defend his love of 'noisy' vinyl over CD, famously replied 'Life has surface noise'. Amen to that, sir.
It is now possible to remove all noise from a recording. Believe me I've heard it done. This is a BAD thing, doing so has quite tragic results for the music. Consider: People say the sound of vinyl is 'warmer' or 'less fatiguing', and many similar terms. There is one simple fact: Vinyl introduces low level constant noise to a recording even in it's most pristine form, (most vinyl isn't pristine anyway) yet vinyl is also very good at delivering the original tape or source noise that forms part of the recording. This creates a conundrum for those faced with the task of removing noise as part of a restoration.
Humans react in different ways to 'clean' sounds and 'dirty' sounds. Vinyl can be said to be 'dirty' because it has a relatively poor signal to noise ratio and is prone to certain distortions, however if you preserve the accuracy of volume changes in a vinyl recording (the dynamic range), you can accurately recreate what was originally recorded as a dynamic, emotional experience. It's only when things get really quiet that noise becomes an issue. A skilled restorer needs to know when to admit defeat in the struggle to get rid of broadband noise because forging on regardless can do irreparable damage.

Here is See Why Audio's cautionary note: One should always consider the true nature of what is being restored. Tape masters have hiss. Leaving it in more faithfully preserves the recording. Vinyl records also have groove noise which was introduced after the recording, so getting rid of it should, in theory be fine... but only if you can keep the tape hiss intact! See how this gets tricky? Inevitably there will be overlap between these sources of noise and it is impossible to be 100% accurate about noise reduction. Nor is it always necessary or desirable. Noise, especially the noise that was part of the original recording environment and medium, can give a recording a sense of identity that makes it easier to place in the memory or as a historical artifact. Nothing is as simple as it first seems. Circumstances often arise where leaving noise in results in a more musical sound because low level musical information sits inside the hiss and you never know it's gone until you lose it.

These are the basics of Audio Restoration - According to See Why Audio. You will be sure to find alternative opinions elsewhere :-)

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