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Old 2005-08-14, 10:21 PM
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Re: An urgent message about digital noise reduction

Originally Posted by rerem
Actually-a big peeve of mine is torrents put out with major clipping. Some of these were recent vintage,from DAT so analog hiss is no factor,but somewhere along the line the waves got major clipping. A few clips are fixable-but if the whole thing is maxed-there's no hope. I actually have run into that more than a botched NR job. Almost as annoying-but at least fixable are those where the whole show needs a 300% volume boost-and I've DL'd plenty of those. The way I try to do it-up what is first rate. If something seems scarce-yet what I have really is flawed...odds are a better version exists,and often it appears on a bt site. Later-after I do the next puter,with a better soundcard-I'll hunt through my analogs and see what still has not appeared online. Already several have shown up-and in better quality. A few probably won't appear until I digitize them.
rerem - I think that this is a very good point to make. I have run into many recordings that are ruined by clipping, and really no way to recover the music, only some "peak-rounding tools" that yield far from perfect results.

There are several different causes of clipping. In analog recordings, the pre-amp (like any amplifier) has a linear range relative to input level, and beyond that a non-linear range where distortion is introduced. A good piece of analog recording equipment should have a VU meter that calibrates 0 dB to the top of the linear range. If recording levels go just a little above the calibrated optimum, distortion is introduced. Low levels of distortion might actually sound pleasant to some people's ears, adding a kind of warmth, but if the levels get too high then the sound becomes nasty and the recording is ruined in those parts. One might refer to this as "analog clipping". I think "analog clipping" is also referred to as "overdrive" or "over-saturation", and it leaves the recorded waveform rounded but distorted compared to the actual waveform of the input. I do not know of any "cure" for this, maybe someone out there can point to some tools that can help recover from this type of problem?

More pertinent to our discussion here is digital clipping, where 0 dB represents the maximum level that can be represented with any given n-bit digital recording.

Digital clipping occurs when you have exceed the maximum sound level that can be represented (in binary, either 16 1's for 16 bit formats or 24 1's for 24 bit formats). Digital clipping completely flattens the waveform, which introduces some very nasty distortion, so it absolutely must be be avoided.

Digital clipping can occur in the digital recording process, in A/D'ing an analog recording, or in the process of "remastering" a digital recording.

One example is A/D'ing an analog cassette - if your preamp levels are set too high you can introduce clipping. One of my A/D units, a Motu 828, has a warning light that stays lit if I clip even a single sample. It is a 24bit-48 kb/s unit, and I figure that I am just "wasting bits (not using the full dynamic range) unless I set my input level "just high enough" so that it is at the maximum level that will not introduce clipping. In practice, this means I set my levels and A/D, then tweak my input levels up and re-A/D, repeating until clipping occurs, then tweak the levels back down "just enough" and do the final A/D conversion so that no clipping occurs. This is the only way I can figure to take advantage of the full dynamic range that 24-bit offers. If the recording is clean and well-balanced (no drop-outs, no warble, no speed correction needed, etc), then the only only other operations I will perform is to remove DC bias from the untracked (large) wav file, and normalize the entire show at once "by peak" (not compression!).

I think that what some people do not realize, when they are attempting to remaster a show, is that many operations increase the levels and can potentially can cause digital clipping. For example, any kind of active filtering (which can include some NR algorithms), dynamic equalization, and poorly-implemented compression are some examples (to add to the discussion, maybe somebody can point out other operations that change levels?). If you plan to perform these types of operations, leave yourself some "digital headroom". Most decent audio remastering software will have a utility to check for digital clipping, so this should be run after any other operation is performed, and at the end of your remastering effort before tracking, just to be sure.

As a related aside, I hate it when folks normalize "by track" instead of "by show". The relative dynamics between the tracks from a show should be preserved, because some songs are meant to be "soft and sweet" while other songs are meant to be loud. If you normalize by track, you are subverting the true intentions of the artist or band when they performed the show, and if you didn't document exactly what you did, there is no going back, so your version of the show is in a sense "polluted" forever.
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