The following is representative of the way that I clean up audio source files. It is intended simply as a guideline for those new to the process; many other programs and approaches can be used and different sorts of faults on the original call for different tools and techniques. Programs referred to in the text are generally available from links here.
If the source is monaural, begin by recording it monaurally. If it has limited frequency range, do not sample at too high a rate. Many of the steps which follow are disproportionately difficult in redbook format (44.1 KHz, stereo); if you can work at 22.05 KHz monaural, life will be much easier. I usually record with CDWAV (linked here). It permits me to capture as I wish and to split the tracks from a tape or LP while saving them. I also use CoolEdit for the purpose since I like its level indicator or GoldWave for quick manual editing. I rarely use Spin Doctor for poor sources because it permits recording only in redbook.
If my source does require full fidelity and stereo, then Spin Doctor lets me pre-clean the file during capture. That is usually good enough to eliminate the need for denoising and declicking the whole recording, but manual intervention is likely still to be necessary for an LP source. Because of the nature of the work I will be doing, automatic splitting is rarely useful. In addition, if a single source tape or LP has several tracks, it is usually easier and better to do the automated denoising and declicking before splitting the tracks. In that way, the general sound will not vary from one track to the next.
The audio should be captured at a low enough level so that overload is not a threat. In general, if you are going to clean the source, it starts out poor enough so that capturing a few db low will not harm the result after normalization. If the source has a serious problem - such as a scratch - I like to remove the extreme spikes by hand before running anything else. Without question, GoldWave is the best way to manipulate the waveform by hand. Where CoolEdit requires moving each sample or very brutal treatment (snipping or zeroing), GoldWave allows you to redraw the waveform with the mouse. Another advantage of GoldWave (especially Versions 4 and above, in beta as I prepare this) is that it will open virtually any file and provide you with access to edit any header and footer which sneaks into the audio stream. Those are the usual sources for clicks at the beginning or end of a track and various programs will introduce or eliminate them. Note that even StripWave will not necessarily handle footers if they prove to be a problem. However, I use Strip Wave when one program's output will not immediately load into another - as when a CDWAV file is to be read by DART Pro.
Step two is to remove the clicks, ticks and pops of modest amplitude. Having tried several programs for the purpose, I have settled on DART Pro as the most effective and easiest to use. In particular, the Test option in DART Pro 32 is consistent and very informative. The similar capability in CoolEdit is available only in some of its restoration tools and is much more awkward to use. Do not overclean your track! In general, if you try to eliminate all the transient noise, you will distort the music. Any automated denoising should be adjusted to leave the music at nearly full quality even if that means some noise is left behind. In particular, count on removing some leftover ticks after a noisy track is passed through DART Pro. Again, GoldWave is the way to smooth out those residuals.
My choice for these operations is CoolEdit. If there is a persistent tone in the track, spectrum analysis will identify it and a notch filter will cut it down to size. But before beginning any track-level operation, I like to normalize it - again, in CoolEdit. Many tracks are normalized to 100% so that the peak level is just 0 db - maximum signal. However, I usually normalize a highly compressed track to only 80-90% so that its average level balances well with a fully normalized track that has a wide dynamic range. Similarly, I cut the level for a track which is quiet by its nature; I don't want a serenade and a march to play back at the same level.
After reducing fixed tones - whines, hums and the like - I listen to the track for frequency balance and for hiss. In general, I am willing to put up with more hiss than others prefer, but if I must cut it I use previews in CoolEdit to be sure I don't take out too much sound in the process. Then I judge the frequency balance of the result and decide on any general curve I want to apply. Again, I tend to change the signal as little as possible and I use the very flexible options in CoolEdit for that job. (GoldWave is similarly powerful and flexible.)
CoolEdit offers excellent tools for resampling, mixing and splitting tracks and similar manipulations. In general, I finish off the track in that program and use the Edit menu option to change the file parameters. For example, I may produce a redbook WAV for CD-DA; a 22KHz, stereo, 16-bit track for MP3; and a sample at 11 KHz, monaural and 8 bits for posting at my WWW site. GoldWave will reformat for me, but not as well and without all the flexibility of CoolEdit.
This process sounds tedious and for best results from poor sources, it is. However, it can also be quite rewarding. It may take an hour or more to extract a listenable result from a well-worn disc recorded in 1903, but it may then become a valuable resource not only for my pleasure but also for scholars and music lovers for generations to come.
E-mail me at cdrecording@mrichter.com
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