Digital Audio Restoration Technology, or DART, is designed to be used for the restoration of archived audio recordings. This is a timely tool, as lots of people are starting to cull through their cassettes, LPs and such, saving the gems on disk. DART can get rid of that tape hiss, LP crackle, and even those clicks and pops caused by scratches on your albums. In fact, DART can do more than just noise reduction. It allows you to record and edit audio, and perform spectrum analysis. As advertised, it includes several tools for eliminating both impulsive (clicks ‘n’ pops) and broadband (hiss ‘n’ hum) noise.
February 25, 1999
DART allows you to record in mono or stereo, at an 11.025, 22.05, or 44.1 kHz sampling rate. Meters and the usual transport controls are provided. Each sound file is displayed in its own window, in which you can move around and cut n paste sections to your heart’s content. In fact, the tools for selecting parts of a soundfile, and setting markers, are quite precise. They should be, in order to do the sort of surgical operations required to clean up noisy recordings. You can zero right in on that click or pop.
Standard editing functions include Scale, Maximize (called Normalize on some other packages), Fade In/Out, Crossfade, and of course Reverse. You can also do Spectral Analysis to show a graph of the frequency content of a sound. DART provides 2 different Spectral Analysis methods, a parametric (Fourier) method, and a non-parametric (model-based) method.
He or she who would clean up old records n tapes ought to have plenty of EQ in their toolbox, and DART comes through with a 10-band graphic EQ, and 5 filters: highpass, lowpass, bandpass, bandstop and notch. The notch filter, which lets you zero in on a narrow frequency range, is handy for getting rid of things like 60-cycle hum. Unfortunately, the frequency controls on the filters are not very precise, and must be set in discrete increments. There is no true parametric EQ, so I’ll put that on my wish list for the next upgrade.
The big show here is, of course, the noise reduction, and it takes place on DART’s Restore menu. The DeClick, DeNoise, DeHiss and ReTouch tools target different types of noise. One of the most popular kinds of noise is tape hiss. All analog machines add it, and old demo tapes that were tracked, mixed and duplicated on cassettes can sound like Victoria Falls. Broadband noise such as tape hiss can be reduced with the DeNoise or DeHiss tools. To use DeNoise, you must first make a "noiseprint". You select a short section of your source recording that contains nothing but the noise that you want to eliminate (perhaps right before or after a song). From this you generate a noiseprint. When you run the DeNoise command on your sound file, everything that sounds like the noiseprint is removed, and only your signal remains. If you can’t get a proper noiseprint, then you use the DeHiss tool, which uses a standard algorithm to reduce broadband noise. Both DeNoise and DeHiss allow you to adjust the intensity of the processing, and provide 4 other parameters that can be adjusted to fine-tune your noise reduction. DeNoise can be used to eliminate any kind of hiss, hum, or indeed anything that can be isolated and noiseprinted, even the sound of a hard drive chugging along! If you use your imagination, you could obtain some powerful effects by using one signal as a "noiseprint" to process another signal.
Now, your broadband noise is pretty much continuous throughout a recording, but your impulsive noises are short little spikes, like clicks and pops. Scratches and dust cause ‘em on LPs, and really old LPs can have a crackly sound going on the whole time. Actually, on some of the old solo jazz piano recordings, it was kind of cool, because it sounded like there was a drummer stirring the soup with brushes. Anyway…Careless digital recordists can end up with nasty little clicks and pops, too. These nasty little clicks, caused by brief clipping of the digital signal, sometimes crop up when you exchange material with other studios, because of differently calibrated DAT machines, meters and such. Get rid of ‘em with the DeClick tool. A device called an outlier detector searches the sound file for noise pulses, then the program reconstructs the distorted samples. DeClick has 4 parameters that can be fine-tuned.
The ReTouch tool allows you to use the renovation program in the "manual" declicking mode, i.e., to overrule decisions made in the automatic detection (DeClick) mode. By editing the "detection files" (created by DART during declicking) you can set, cancel or modify detection alarms to remove overlooked noise pulses and fix any mistakes that may have been made by the automatic process.
Like most audio editing packages, DART has a system for keeping track of files, which takes a while to understand. Some of the things you can do with DART are very subtle, and you must keep careful track of which file is which, especially if you are doing multiple passes of processing on a file. DART allows you to keep track of groups of files, as "groups" or "soundtrees". When you process a file, DART will automatically generate a new name for the output file, each new version being assigned a number. The original File.wav becomes file00.wav, and later file01.wav and so forth. Before you get rolling with DART, make sure you understand how the file naming system works. Fortunately the manual is thick and well-written. There is also an online help system.
DART is a handy item to have in your toolbox. Another popular editing package, Sound Forge, does include a noise reduction tool, which works like DART’s DeNoise command, but DART provides a much wider range of audio restoration tools. Like many audio pros, I have a closet full of recordings on cassettes, old reel-to-reel tapes, LPs, wax cylinders (well not really). LPs have a certain sentimental value, but not so cassettes, and I plan to phase ‘em out by the turn of the century. This means going through the archives, selecting the material that deserves to survive, and sourcing it into the ‘puter. DART makes those old recordings a lot more palatable. Highly recommended.
