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3-3] How do I get rid of hisses and clicks on audio CDs?




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This article is from the CD-Recordable FAQ, by Andy McFadden (fadden@fadden.com) with numerous contributions by others.

3-3] How do I get rid of hisses and clicks on audio CDs?

(2002/02/25)

If you're interested in removing noise from audio captured from an analog
source, such as a record player or analog cassette tape, skip to section
(3-12-3). This section is about unexpected noise in audio from digital
sources, such as tracks extracted from a CD. (Start with section (3-2)
if you are new to "ripping" or copying audio tracks.)

The single most important rule of noise removal is to figure out where the
noise came from. Play the .WAV files off of your hard drive (if you're
doing direct CD-to-CD copies, extract a track and listen to it). If you
hear noise in the .WAV on your hard drive, the digital audio extraction
isn't working very well. You either need to extract more slowly, extract
from a different device, find a program that works better, or maybe just
clean the dust and grime off the source CD. For more information, including
a URL for recommended software and the CD-DA FAQ, see section (3-2).

Always start by inspecting the CD. If you borrowed it from a library,
don't expect it to be in pristine condition. With enough abuse, even CDs
will sound bad, and audio *extraction* is more susceptible to such errors
than audio *playback*. (This is what makes copy-protected CDs possible;
see section (2-4-2).)

If the problem sounds like repeated or skipped samples, rather than clicks
or hissing, the problem is probably jitter during extraction. See section
(2-15) for an overview, and then give EAC a try (section (6-2-12)).

A nifty trick for comparing two .WAV files is to use the "Mix Paste"
feature of an audio editor like Cool Edit. Extract a track twice, then
use Mix Paste to copy an inverted version of one file on top of the other.
The two sound files will cancel each other out wherever they are identical,
and have little spikes where they are different. This can be useful
for seeing if the problems are only on one channel or are happening at
regular intervals. You need to make sure though that both files start at
the same place though. If your CD-ROM drive doesn't always extract from
the start of the block, you will need to adjust the files so they line up.

Useful things to do with this include comparing two extractions from the
same drive, extractions from different drives, or extractions from the CD-R
you just wrote to the original .WAV file you used to write it.

If you just want to see if the files are the same, use the DOS File Compare
command, with the "binary" switch set: FC /B FILE1.WAV FILE2.WAV.

Some CD-ROM drives may put a click a few seconds into the first track being
extracted. This appears to be related to the drive spinning up. Try
starting the extraction, cancelling, and then immediately restarting.

It is possible, though still somewhat unlikely, that you are trying to
extract from a copy-protected CD. Section (2-4-2) discusses this in
some detail.

The rest of this section only applies if the extracted audio sounds fine on
disk, but has problems when played back from the CD-R.


If you're using track-at-once recording, you may get a short click or
silent "hiccup" at the start of each track. Hiccups are unavoidable, but
you should be able to get rid of the click by using different software.

If you're using disc-at-once recording, and are still getting a short click
at the *start* of every track, then your recording software is probably
writing the sound file with the headers still on it. You should either
use a smarter program, or remove the header manually (see the URL for
"StripWav", below).

If you are getting clicks in the middle of a track, they are either being
added when pulling the data off the disc or when writing it. If the .WAV
(AIFF on the Mac) file plays without clicks, then your CD recorder may be
failing somehow during the write process. Some people who got "static"
in audio recorded on an HP 4020i found that reducing the DMA transfer
rate to 2MB/sec helped.

One user was told by Yamaha tech support that crackling (similar to a dirty
vinyl LP) was a symptom of laser misalignment. If you've been writing audio
CDs for quite a while, but lately you've been getting "crackly" results from
tried-and-true media, this might be the culprit. Since it requires returning
the unit for repair, you should exhaust all other possibilities first.
(Side note: it's not clear how a laser gets "misaligned". They have to
adjust themselves constantly to stay in the spiral groove. It might be
due to poor focus, but that should be causing all kinds of problems.)

If you are getting clicks at the end of a track, it's possible that the
software used to create the .WAV file put some information at the very end,
which is legal but not handled correctly by some CD-R software. See
section (3-12-3) for tips on using Cool Edit to remove the data. If you are
finding that tracks extracted from CDs don't have clicks but tracks that
you have recorded or edited do, chances are the data size isn't a multiple
of 2352 bytes, and the last block is being filled with junk. This is
common on live recordings or when large tracks are cut into smaller ones.
Jeff Arnold's DAO will fill out the last block with zeros (digital silence)
if there is space left over, but most of the other programs will write
garbage that is audible as a short (less than 1/75th second) click. The
fix is to split the track on 2352-byte block boundaries.

A program called "StripWav" will remove .WAV headers and footers that
may be interfering with some applications. The program is available from
http://www.lightlink.com/tjweber/.


If you must use track-at-once, make sure you're writing it all in one
session. PC-based CD players may be able to see tracks in later sessions,
but the CD player in your stereo system almost certainly can't.

A distantly related problem can arise if you use "shuffle play" to play
random tracks from a CD-R. If the audio of track N begins immediately,
some CD players will slide from the end of track N-1 into the start of
track N, playing a short burst of track N before seeking elsewhere. This
can be avoided by putting a gap at the start of such tracks (e.g. with
"INDEX 01 xx:yy:zz" in a DAO cue sheet).


 

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