This section is essentially redundant with information in three other parts of this primer: Digital Audio Extraction (DAE), Disc At Once (DAO), Snap, Crackle and Pop and It Doesn't Sound Good. The purpose is not to beat the message into you, but to indicate the process by which a problem is analyzed.
These noises usually arise on tracks extracted digitally (ripped) from a CD-DA. They come from faulty DAE or occasionally from a damaged source disc. While a better reader is the right answer, other choices include reading from your writer, slowing down DAE, and recording through an analogue connection. You can hear such clicks on a WAV file extracted to your hard drive. Note that the low signals generated by CD-R compared with pressed discs make some sources of clicks more likely; as a result, a setup you find acceptable with commercial originals may be too noisy for you if you are ripping from a CD-R.
The source here is usually data being read as audio. There are different ways that the WAV envelope can be written and some software assumes that some of those options are not in use. In that case, they pick up some bytes of data as though they were audio and encode them into the CD-DA. If you look at the first milliseconds of the WAV (from whatever source) in one or two WAV editors (linked from my page of URLs) you can see the noise as a spike; some editors will give you a strange error message in this case because they think that the WAV file is a raw format. In that case, they may or may not show the glitch. If you get that message, that editor will save the file without the click. In any event, some audio players should reproduce that click if you listen carefully to a WAV file.
A track must begin at a block boundary, where each block is 1/75th of a second. If the last block of a track is incomplete, it will be filled in by something else. Depending on the software, it may be filled with zeroes or with whatever bytes happen to lie next on the disc from which you are writing. In the latter case, you are likely to get a click. A click from this cause will not be audible on the WAV file you are going to write since the block limitation is encountered only when writing to the CD-R. Most good tools for splitting tracks enforce the block boundary, but the more powerful ones (such as CoolEdit and Sound Forge) let you choose a size different from the nominal. If you do not use 1/75th second for the sector size, you can produce this problem with even the best software.
A second cause is similar to that of a click at the beginning. The WAV envelope around an audio track may include a footer as well as the header and that footer may be read as audio data. If so, you may again be able to hear the click in some WAV players.
The third source for a click at the end of a track comes from a click at the beginning: if some bytes of header are read as audio and if the audio data started as an integral number of blocks, then the last block being written has only as much audio as got pushed out of the first block by the unwanted data. The result may be a click or may be silence; you can identify it easily in a WAV editor by the fact that it usually lasts for 100 milliseconds or more where other sources are usually much less than a millisecond.
This is an oddity that occurs only when you record a continuous program without using DAO. Some software allows you to violate the spec requirement of two seconds between TAO tracks. You may even be able to run it down to what appears to be zero - but it isn't. In the short intertrack gap which is produced, the signal is zero. If the continuous program had sound at the track split, the brief, sudden silence may be audible and may seem to be a click. If you tried a 'zero' intertrack gap, this is a likely cause and can be fixed only by writing in true DAO.
So you get clicks - and want to know what to do about them. The first step is to listen to them to find out where they occur. The second is to use information such as that above to identify the cause. If that fails, ask in the Adaptec mailing list or the newsgroups for assistance - but be sure to report what you learned from your own analysis.
Once you find the cause, what do you do about it? The best choice is to eliminate the cause, not the effect. If your mastering software does not like the header from your WAV editor, change one of the programs until you have a compatible pair; if your reader doesn't do good DAE, use a different reader or a different speed. Another choice is to clean up the results. Many of the sources can be fixed with a program like StripWave, which eliminates most problems with the envelope. You can use a WAV editor to remove those clicks manually. You can do generalized cleaning in powerful WAV editors or standalone programs such as WAVClean, DART Pro and DCART. Spin Doctor (part of Easy CD Creator) has a simplified cleaning capability as well.
Are there other causes? Sure, but I don't know enough about them to write them up here. Fortunately, they are also uncommon. Frankly, the specifics of dealing with clicks is less important to me here than exposing the process used to track down errors. If you post - to tech support, the mailing list, the newsgroups or by e-mail - that you have clicks on your tracks, you haven't said enough for anyone to help. The same is true when you report that you can't burn a data disc. You must do your homework before you can get any help. Unless you bring in a personal consultant, you're the only one who can isolate the cause - or at least reduce the possible causes to one or a few of the many potential sources.
E-mail me at cdrecording@mrichter.com
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