There is much to consider when trying to decide which software to buy for restoring records. Besides a general knowledge of how much money you have in your wallet, you must first of all have an idea how much effort that you want to put into this kind of work. There are some applications that are very simple--you load the wave file into the software, push a few buttons, and the rest of it is pretty much done for you. Some of these tools get surprisingly good results. Other applications are much more demanding, and a few of these are downright intimidating. Although it is generally true, I can not say that the more challenging applications are also the most expensive. Some of them are quite expensive, but one of the more complex applications is Audacity, which is freeware. Anyway, I have compiled a list of features that you should consider when deciding which product will be best for you.
NOISE REDUCTION. Here I am going to differentiate between:
(1) Ongoing background noise, which tends to remain at more-or-less constant levels throughout a track (and often throughout an entire side of an LP) which is the result of the sound the stylus makes in the groove when no music is playing, and a variety of hums, hisses, and rumbles contributed by your system's electronic circuitry and mechanical operation; and (2) Transient noise (clicks and pops and the like) which appear here and there and are the result of record damage (e.g., scratches, scuffs, record defects) in specific spots. Anyway, software must and does handle these 2 types of noise separately.
(1) Ongoing background noise. There are two approaches to reducing ongoing background noise:
(a) The first and superior method may ask you to find an audio sample of the background noise your recording is making. This is a simple process of finding and selecting a spot where no music is playing, either before the music starts, between tracks, or after the last track ends. The software will then filter the noise according to this "noise fingerprint."
(b) Another method will simply filter frequencies commonly known to constitute noise, such as 60 cycle hum and its harmonics, or frequencies above a certain level that often constitute hiss. Many good softwares incorporate both methods (a) and (b), and provide adjustments so that you can tweak the results.
(2) Transient noise (clicks and pops). Once again, there are two general approaches to filtering clicks and pops:
(a) The first method is an automatic processing of the entire waveform that filters out clicks and pops in accordance with certain predetermined settings (and most applications let you tweak these settings). These settings are critical and touchy because if they are too "liberal," they will endanger legitimate percussive transients in the music (such as the many gentle clicks in latin music) and distort other musical sounds, such as saxophones, trumpets, and some female vocals. This automatic processing is a feature possessed by pretty much all of the softwares dedicated to record restoration, although the flexibility allowed the user in tweaking the basic settings can differ considerably.
(b) Unfortunately, the automatic processing discussed in (a) is just about never 100% perfect. After processing, you will usually still hear clicks and pops that the software is unable to remove. Indeed, if the software does succeed in removing all of the clicks and pops, it is reason to be suspicious that perhaps legitimate musical transients have also been removed along with the bad noise. Therefore, if one is to be thorough, a manual method of removing residual clicks is needed. That is, the user must be able to manually listen his way through the piece and zero in on the unwanted pops one-by-one, and then fix or delete them. Some of the lower-end applications do not offer this option. In my opinion, this is a serious drawback. Among the applications that do provide some kind of manual-fix option, there are various possibilities. Some offer a "pencil tool" with which one can modify the waveform by drawing out a click's sharp spike. Some use a simple highlight and delete (and for very small fixes, this deletion is not noticeable). An application may look at the waveform on either side of the click and substitute accordingly. Of course, if you cannot see the click, you cannot fix it manually, and strangely enough, this can be a problem. Some very noticeable and annoying clicks simply cannot be seen on the standard waveform display. For this reason, applications that allow a "spectographic" display option, in which clicks show up as bright easily seen lines, have a very strong advantage over those softwares that do not have this capability.
CHANNEL MIXING.
The ability to mix together the two recorded channels, and to selectively transfer the contents of one channel to the other channel, is an extremely useful tool. This is particularly true of mono recordings, where the waveforms for both recorded channels should be identical. However, it is also useful for stereo recordings, when there is a flaw in only one channel that can be repaired by substituting the corresponding section from the other channel (provided the two channels are sufficiently similar so that such a patch is not noticeable). Many of the lower-end applications lack this capability.
VOLUME CONTROL.
Almost all of the applications we are considering have some form of control for adjusting the waveform's volume. This usually goes under the name of "Normalization," but similar features may be called "amplitude adjustment" or "hard limiting." The general idea is to maximize volume without "clipping" distortion, while respecting and preserving natural variations in dynamics that occur within a track, or between individual tracks within an LP. It is also very helpful if there is some way of amplifying the channels separately, in case one channel or the other is too weak for one reason or another.
Some applications feature RECORDING capabilities. Although this can be convenient and might at first seem essential, it is not really as critical a feature as it might first appear. Your soundcard probably came with recording software that will suffice, and freeware and shareware recording applications are commonly available. Moreover, unless you have a very powerful computer, some applications use so much in the way of system resources that recording with them can result in drop-outs.
Another feature that at first might seem essential is the ability to LOAD AND PROCESS MP3 FILES. Actually the applications that "can process mp3 files" are (I suspect) actually converting them to WAV files before working on them. You can do the same for any application by using a simple freeware converting tool like dBPowerAmp. However, it is nice if not essential to have your main application do this conversion for you. Similarly, the ability to EXPORT (SAVE) FILES IN MP3 FORM is a very handy feature.
A feature that I have found to be extremely useful is the ability to display a waveform in a SPECTRAL VIEW. This makes it very easy to find clicks and pops that can sometimes be very hard to find with the standard waveform view. Unfortunately, I have not found very many applications with this capability. Two aps that do have this feature are Adobe Audition (expensive), and (in its new version) Wave Repair (inexpensive). As I become aware of other applications with this capability, I will note them here.
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