What is Digital Audio Restoration?

It's the preservation of audio recordings combined with the removal of unwanted artefacts.

For new recordings, no rules apply and you 'produce' whatever you like. However, pre-recorded sounds that we know and love fall into a different category.
It is the job of an audio restorer to try and maintain what the original producers and artists had in mind and to avoid introducing ideas of their own. Flaws that previous media have added, eg. clicks, hiss, grunge, glitches etc., form a layer of audio that might be best removed... but with great care. A working knowledge of what artists and producers originally intended is key. Restoring audio bears striking similarities to the 'Restoration' of a painting; if you try to clean too much, or change things to what you prefer, are you re-writing history?

What is a good source of audio for restoration?
The best source for any recording remains a good master tape in good condition, analogue or digital. Expertise with tapes, and the transfers themselves, do not come cheap.
Tapes are not going to last forever and in many cases music that was recorded on tape has a good chance of being damaged through misuse, bad storage, overuse or just plain loss and it's a sad fact that all tape, no matter how well kept, is gradually losing it's magnetic information due to cosmic bombradment..
If no suitable tape is available, your next obvious selection is... vinyl.

You mean with all those scratches, clicks and and glitches? Yes I do. Vinyl really could be your best source for an affordable, good quality reissue and if the tapes are extinct, it may be your only choice.
The majority of vinyl was made by highly skilled people working in specialist fields. Their hard work lives on in the humble groove of a vinyl record. With modern restoration techniques and a good ear, restoration of vinyl-sourced audio can be surprisingly high quality. Perhaps most important of all, a well kept vinyl record will have exactly the same sound in its grooves as it did the day it was made.

What are the risks of damaging the music?
Vinyl sources will always have noises that require removal and that's a one-way process; remove too much and you run the risk of removing music along with the junk. It's important to get your decisions right.
There are two main approaches to the digital audio restoration of vinyl-sourced audio; you must choose the one that best suits your needs. Neither one is perfect.

1) PASSIVE: For example CEDAR and similar dedicated hardware/software, employing fiendishly clever algorithms written by near-genius software engineers is used to remove the unwanted junk signal from the music. What gets removed is decided by the operator in conjunction with the software, but mostly it has to be said, by the software. The risks of passive restoration are that the algorithms used to decide which noises are 'junk' and which are music, might not be 100% what a human being would have decided. The temptation is always to use a setting that catches the larger flaws... which runs the risk of removing too much of the original information during less affected passages (think of a saxophone note, which is justa series of complex clicks). The algorithms are unbelievably clever but even so, occasionally flawed and it doesn't come cheap. 20,000 USD gets you the basics. Other cheaper software that purports to do all your restoration for you is either maybe disengenuous in it's claims (see my lawyer) or very difficult to master.

2) PRO-ACTIVE: Manually targeted digital restoration employing automatic software procedures where appropriate in addition to more specific 'hands-on' waveform editing. This requires experience and knowledge. It can yield great results but isn't an overnight learning curve. The risks of a manual approach are just that: the human being doing the work must pay attention. Most of the things CEDAR can do are achievable in this manual way but the time constraints are different. Manual work more often than not takes a while longer.

So why would you choose the human 'PRO-ACTIVE approach?
First of all let's be clear, it's not really human after all... it's just a human computer operator with a more hands-on approach than those auto-machines. But, importantly, it forces the restorer to choose what is best suited to the material.
I've worked on material ranging from nearly flawless mint Japanese pressings, all the way down to knife-scarred 45's. Each project is different. A machine can never know all this. Further, when doing the work, a skilled manual restorer becomes adept at allowing small anomalies to remain unremoved, such as the switch clicks of a King Tubby dub-mix for example. These are choices only a human can make. If your material includes many such elements, you probably should choose a manual approach.

What is your goal? What are the costs?
Whether you choose passive or pro-active techniques depends on what you want to achieve versus your budget, your source material and its condition, etc. Audio is usually unpredictable and an algorithm can't be a crystal ball. It's not as if humans have no faults but only humans can really decide if it 'sounds right'.
The most important factor ought to be quality - but I'm just biased that way! Costs are more difficult to explain. If you want an experienced engineer to operate with CEDAR, at a major mastering house, be prepared to spend a ot of money, in the high hundreds, to get an album restored properly. My own methods allow me to do a standard restoration job for rather less. Get in touch if you want to discuss costs.

Educated ears understand what you want.
Knowledge is power. 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. Likewise if someone knows more than you do, about the way it should sound, be prepared to accept that advice is a good thing.

Preserve the original recording.
Preservation is most important. If you haven't kept the music, or you've damaged it by your actions, what was the point? Use good equipment. Clean your records before transfer. Ensure it's all working properly!
Preserve the dynamic range. When someone asks me to 'make it louder' I just point to the volume control. If you want compression that's fine but let it be known that such compression is damaging the original recording and that the damage is irreversible. Just Google or Youtube 'Volume Wars', you'll see what I mean. There are two negative sides to such compression; 1) you get less dynamic 'peaks' in the music, causing fatigue and monotony. 2) It raises the noise floor.
NB: On 'Dynamic Range' it is often pointed out that vinyl has a poor dynamic range. That's only true in the context of a comparison with digital. The comparison is driven by the total noise-floor difference. Vinyl's noise-floor is a lot higher than digital.
This does NOT mean that digital is louder, it means that digital can go quieter. In practise this issue only concerns music which contains much near-silence or actual silence (except tape hiss). So, if your music is loud all the way through, it won't be a problem.

Noise is good... sometimes.
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 possible to remove all noise from a recording. Believe me I've heard it done. This is a bad thing! In general, tape hiss that is constant, that forms an inseparable part of the original sound, is acceptable to the ear; you can 'tune it out'. Groove noise often overlaps this tape hiss so you need to be careful when reducing it.
Clicks and pops, however, are far more disturbing; a big click can swamp a listeners attention for a few seconds depending on what the background sound is. A really loud scratch can even physically cause your ears to shut down momentarily. Luckily, clicks are easy to treat because they occupy so little space in time. However, If you forcibly remove steady state 'broadband' noise from a music recording, you run the risk of removing some of the music as well because low level musical information sits inside the hiss and you never know it's gone until you lose it. Circumstances often arise where leaving noise in results in a more musical sound.

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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