Audacity is a very powerful sound recording and editing application.  To a certain extent it can also be used to change the format of sound files, although it is not the most efficient program for this mode of operation.  Most of what follows has been gained by my using Audacity over a number of years.  I have used this application to simply record streamed audio content from the internet plus, to converting some cherished vinyl records and cassettes to digital format.  I've even created mobile phone ring tones too.  Many Linux podcasts are created and edited (or not!!) using Audacity as well.

Please remember, you may have that treasured vinyl album but check to see if the recording has been released on cd first.  First of all the source for the cd will normally come from the original master tape, thus providing by far and away a higher quality experience and, it is definately more easy to buy a copy than do what follows!  Also take note of your local laws dealing with the personal copying of copyrighted material.

Before we start, let me just say that the quality of a recording is dependent upon the weakest link in the chain.  Yes, you can modify, filter or edit a sound file but if the source introduces alien elements it can sometimes be impossible to remove or counter those effects.  So, if your electronics are humming away with the music, it is far better to remove the source of the hum instead of filtering it out.  (Most hums in modern electronics are caused by earth loops with more than one point of contact to earth to the audio system.  Please remember your electrical safety though if you start slashing away at different earth cables connected to your system.)

I will describe the recording of a vinyl record and converting it to a digital format as this covers most, if not all the techniques that I have used with Audacity.  First of all you need to set up the recording level.  Too high and you could drive the system into clipping (not very nice on the ear); too low and some of the original recording detail will be lost in the noise that all electronic audio systems suffer from.  Luckily the gap between these two extremes is very wide in digital systems but, you should try to keep the maximum levels of a recording  just below the clipping point (say to about between -6 and -3dB).

To set the volume level, using either a software level control or a physical control on the source, you can simply make a trial recording or, click on the area of the Record Bar Level Meter which brings it to life, whilst playing the disc.  You will notice two vertical lines appearing across the right hand side of the dancing red bars.  The right most line indicates the single most highest peak in the input signal.  The next line shows a recent highest peek providing a short term peek level.  In addition to this, there is a darker solid red block within the meter bar.  This shows the dynamic range of the signal.  Remember that you may find that you could increase the source level to a point where distortion could be caused in analogue circuits by driving them too hard before digital conversion.  Always listen to the result, through the computer and on to good quality headphones or loud speakers.  (I use Rogers BBC LS3/5a studio monitor speakers driven by an old JVC HiFi amplifier.  Yes, I know, it is a bit extreme for a computer audio system but, hey ho!)

Where possible start the recording with Audacity before starting the source as we will want to sample the system noise before the actual sound starts.  When recording a tape or a record disc also keep recording until the side is completed to the end.  Especially with discs, it is a good idea to keep notes, logging the time of any cracks or crackles that occur during the recording.  There is a time log at the top of the main window, below the Menu and top tool bars.

Once the recording has been completed, save it as a project into a folder specifically created for the task.  Mine initially go into /home/ray/music/recording... etc.  Whenever you make changes to this file, use the Save As option and apply an incremental number to the file name; just in case you need to abort some editing and return to an earlier version.  You can always dispense with these incremental files once you are satisfied with the final result but, I normally retain the original for a while, just in case.

Adjusting the Volume
Before we do anything else we will check the peek sound level in the recording.  We do this because some techniques we use in cleaning the sound up can increase the volume in some parts and, could drive the signal into clipping.  So we need to ensure there is a margin for safety and reduce the overall volume if needed.  First select all the track, from start to end then use the Amplify tool under Effects (Effects/Amplify) to see what amplification would be needed to get it to peek at 0dB.  If it shows that some amplification would be needed, above 3dB for this just press cancel for now.  If there is evidence of clipping you may need to rerecord at a lower level.  If, like the example below, and the Amplification is between 0 and -3dB use this tool to reduce the sound level.  Just enter say -5.0 dB into the box 'New Peak Amplitude' and press OK.

It is best to check the sound level after each task as some can have a dramatic effect upon the dynamic range of the recording.

Noise Filtering
In the background of any recording there will always be some noise, be it hum from your mains system in the electronics or, rumble and pressing noise on the disc and other noise from your turntable, even from a tape.  These sounds will generally be constant throughout the recording and Audacity has a handy tool to attenuate (filter) these sound so they are not so obvious.  It can be found at Effect/Noise Removal...

First of all select a section of the recording before your 'music' starts.  To do this select a point just before, or just after, the 'music' then, with the shift key depressed select another point encompassing as much of the constant noise as possible.  Go to the Noise Removal window and click on, 'Get Noise Profile'.  Now return to the recording and Select All, (in the Edit Menu).  You then return to Noise Removal, (I have generally found the defaults in Step 2 to be satisfactory but please play with them and try them out), and click on OK, (which should by now not be greyed out).  You should then find the quiet sections of your recording to be far quieter but, check at both ends of the selection and, if there are separate tracks those blank sections as well.  You can always back track if things don't work out well.  You may also find that some sections may be louder than before, well peeks in the signal anyway, so check that you have not incurred any clipping using the Amplification tool.

Click Removal
There is a Click Removal Tool under the Effects menu (and please give it a try) but I prefer to manually edit clicks out of a file.  This is where your note book comes in handy or, you can simply play the recording and listen for clicks and crackles.  There are two ways of dealing with clicks, chopping them out or over drawing them.  The first way is more crude and may not be so satisfactory, it all depends upon where the click is and its duration.  The second method requires you to redraw the audio signal.  A steady hand and good mouse control is needed here!

When you hear a click stop the playback and select the area about the click and press >E.  This expands the field of view over the selected area.  You may need to carry out this process a number of times until just the click can bee seen and, you can see the lines of the signal with dots on them.  (The dots represent the points where the analogue signal has been sampled).

There are a number of tools I find useful when editing files, they are the pencil or Draw Tool, to drag the signal to a different point and the Selection Tool, used to select the point along the signal upon which you wish to work.  (This is the default mode in Audacity.  The Draw Tool will only work when the individual signal sample points are shown.)  To select a section with the Select Tool first choose your start point, then hold down the <SHIFT> key whilst you select the end point, just like in selecting the area to be viewed, earlier.  One point though, when you use the Selection Tool it selects across both the stereo channels but the Draw Tool just effects the track in which you are working.  Using these tools you can sometimes completely remove a click from an audio track but, when a bad click disturbs a track I have only been able to reduce the severity of the disturbance.  But at least, this is better than nothing.  With Stereo recordings you will need to redraw the signal in both the Left and Right channels.

Volume Adjustment
Once you have completed your editing of a recording, the last major thing to do is to adjust the volume, Effect/ Amplification.  Because the conversion of some audio to different formats can change the volume by a little I always allow 1dB as a buffer so never go along with the default setting of 0dB as the maximum signal level also, some music is not meant to be loud and a lower level may be required.  Whatever the level is chosen, apply it to all of the recording, even if it is to be split up into separate tracks when you save them.

Looking for Gaps.
You can find Analyze/Silence Finder a useful tool to locate gaps within a recording.  I've found it useful when there has been a short break in an audio stream from the internet and I can quickly locate these points within a third track, laid down under the audio tracks, with markers flagging up any gaps found.  You can also use it to quickly find silent gaps between tracks on a record or, embarrassing pauses during a podcast recording!

Saving the Recording
Although you can save the file as a project in Audacity, once edited, it is best to convert it to a more popular file format like MP3, OGG, WMA etc.  It is always best, if the original came with separate tracks to save them in the same way.  You will also have a chance to save Meta Data about the track at the same time.  From File/Export select what file format you wish to use and any other variable (like sample rate).  Choose where the file is to be saved and the name of the file.  The Export routine then provides you with a chance to write in any Meta Data providing any details about the recording.  It is always a good idea to save this as a separate meta file as well, so you can use it as a standard data version or, when exporting further tracks from an album.  (See the buttons for Load, Save and Set Default in the Template section.)  Once complete press the OK button.  The actual saving/exporting process can take some time so always have a good book nearby!

Other little tips
Fade In and Fade Out
When you are chopping up a recording into different sections, or topping and tailing a track before you save it you may find it produced a click when it starts or ends.  This is brought about by the recording not starting, or finishing with a signal at zero, so there is a sudden jump from zero to the level of the first signal sample when the file is played.  There are two tricks I use to overcome this problem.

1. When selecting an edit point within an audio recording, after the point(s) is selected press <Ctrl>Z, this forces Audacity to find the nearest zero points and shifts the edit point to there.  Sometimes, because stereo tracks are independent of each other there is no common zero point a compromise is made so, this cannot always be 100% effective.
2. I normally select a short introduction or end section of the track and use Fade in or Fade out in the Effect menu.  This allows you to quickly fade in the track at the beginning or fade out at the end.

Seeing Signal detail
When you are editing a track the detail of the signal is too low to see on the Chart area.  To increase the sensitivity of the chart Left Click on the Charts Vertical Axis and the scale will reduce so you will be able to see greater detail.  Right Clicking the same area returns the scale back towards normal.

Time Scale length
I have already told you that <Ctrl>E expands the time scale to that selected but <Ctrl>1 reduces the time period shown thus magnifying it, <Ctrl>2 sets it to normal and <Ctrl>3 increases the time period zoomed out on to the screen.  This is useful when you are zooming in on a click within the recording.

Selection
Keying <Ctrl>A selects the whole recording.

Silence
Sometimes you need to either insert a silent section or, reduce a selected section to silence.  Use Generate/ Silence... to achieve this.  I normally add a second of silence to the end of my recordings as some players (including Audacity itself) stops playing when it reads the end of a track, even though some of the 'music' is still in the buffer and yet to be played.

Please consider this to be work in progress and, if you have any questions just yell.  If it is a good question (and I know the answer) I will include it somewhere in the text.  This also goes for any comments anyone has to make as well!

Have a look at Audacity in Linux Mint for a tutorial on the initial setting up of Audacity.