Mastering your audio tracks with a home studio
Audio mastering is one of the essential arts you need to learn to make your tracks sound good on CD, vinyl or even as MP3.
People often confuse mastering with mixing, but the two are different in many ways : Mixing is balancing the levels between instruments and getting the individual instruments to sound good, where as mastering is the final step where you want to polish the overall sound and maximise volume. Often the reason that commercial CDs sound so much louder than your own mixes is that they use compression and clever limiting techniques to maximise the levels, boosting the overall sound. However, with the right tools and some patience you can do this in a home studio.
If you're looking for a far more detailed explanation of what mastering is and why it is needed then read these excellent articles at Digido.Com.
Tools You Will Need
To master a track you will ideally need a decent sound editor (preferably one that will work at high bit and sample rates and will accept DirectX and/or VST, such as SoundForge, WaveLab or Adobe Audition (formally CoolEdit Pro). There are also stand-alone packages dedicated just to mastering, such as T-Racks. However, if you haven't got one of the 'big three' then check out HitSquad : Shareware Music Machine for a list of free and shareware audio editors. Ideally you will also have some high quality plugins for your audio editor for mastering with (as the ones that come with audio editors are sometimes lacking) - the best ones I know of are made by Waves. At the very least you will need to be able to normalise your audio, EQ it and compress it. Ideally you should also have access to plugins that will remove DC offset and also a multi-band compressor, parametric EQ, stereo imager and a limiter.
The First Steps
The first thing you need to do before mastering audio is make sure you are happy with your mix! It's very difficult, if not impossible, to fix errors in your mix when you are mastering, so make sure your mixdown is as good as it can be before exporting it to an audio file for mastering. Preferably listen to it on good monitor speakers (after resting your ears) at different volumes to make sure that everything sounds balanced and that your bass frequencies are prominent but not too 'boomy'. It's also a good idea to listen to your mix on cheapo PC speakers too, so you know how it will sound on more basic setups - again make sure that everything still seems balanced on crappy speakers.
Once you are happy with the mix the next step it to export it as an audio file. Before doing this, though, make sure your mix isn't clipping - when you are working in the digital realm you NEVER want your maximum level to exceed 0dB (unless you are Iggy Pop). Don't worry if your mix sounds a little quiet, that can be solved in the mastering stage - just don't be tempted to let it clip.
OK, now you need to actually export your track as audio (normally this will be as a .wav file on PC or as an .aiff file on Mac). Often you will get a choice of what resolution to export your audio at ie. the sample and bit rate. Generally it's best to choose the highest bit and sample rate your audio software will support - 24bit/96Khz is usually the best quality, if you are given the choice. Do this even if your audio card doesn't support playback or recording at these sample rates (this might sound counter-intuitive, but trust me it will still work!). It's probably beyond the scope of this short piece to explain why you should always master at high sample-rates, but basically it's down to reducing errors caused by floating point precision - the higher the precision the less chance of errors creeping in. However, your final mix should still be "CD Quality" 16bit/44.1Khz.
Next you want to EQ your audio file - this will be a matter of personal taste, but now is the time you can boost the bass or add a little more 'air' to the mix. It's also a good idea to roll-off any inaudible, low frequency bass sounds - usually a high-pass filter set to roll of frequencies below 60hz will do. This will help clear up your bottom end and avoid things sounding muddy, especially on systems with sub woofers. If you have any plugins such as Waves MaxxBass now would be a good time to use them.
Next you should look at compressing your mix - this reduces the peaks and allows you to increase the overall amplitude, or loudness, of your mix. If possible use a multi-band compressor which allows you to add different levels of compression to different frequencies. If you're not sure about compression then read more about it's uses here and here. Just remember to avoid clipping, as digital clipping is nasty! If you have access to one a limiter is very useful (a limiter is basically a 'brick wall' compressor that stops a signal ever going beyond a defined threshold - typically set this threshold to -0.3 dB for CD mastering). A great limiter is the Waves L1 or L2 Ultramaximizer.
Remember you should put your limiter last in any audio chain. Also be aware that once you have raised your overall level to very near 0dB that you should NOT do any more processing on the audio else you risk introducing clipping (believe it or not, even subtracting EQ can actually increase levels).
Beside EQ and compression there are other tools you can use too, depending on the effect you wish to achieve. Sometimes adding a very small amount of reverb, especially one that defines a real 3 dimensional space, can help bring your mix together. This will have the effect of situating your track in a virtual sound-scape. For instance, a touch of room reverb might be useful to bring a rock mix together, whereas for more ambient, experimental works a larger reverb could help expand the sound.
Just don't over do it, especially if you've already used a lot of reverb when mixing, otherwise your mix will sound mushy. If you can limit the reverb to just the higher bands this would be preferable, as you don't want your bass and kick drums to sound 'boomy'. Another useful tool is a stereo-imaging tool that can help give your mix a wider stereo field. This is useful for mixes that sound a bit too mono, or when you are making vast electronic music soundscapes. Again, easy does it, as too much can make your mix sound strange and the bass sounds weak.
One final thing you'll have to take into account if your mastering digital music is dithering and re-sampling. No, I'm not referring to the usual procrastination and uncertainty you have when mixing, but rather the process of getting your final master into the right format for burning to CD. As you probably know CD Audio uses a sample rate of 44.1Khz and is 16 bits deep - so what do you do to make sure your pristine 24bit/96Khz master sounds good at this lower rate? Well, this is were dither and re-sampling come in. Basically this is the process of reducing the resolution of audio by aliasing the waveform - it's rather similar to when you shrink a photograph in Photoshop and you use bi-cubic resampling to remove the jaggy edges you'd otherwise get. Good dithering plugins introduce a small amount of noise to help smooth the audio 'jaggies' when the bit depth is reduced (this is often referred to as noise shaping).
Now, adding noise to your master doesn't sound like a good thing, but believe me this is virtually inaudible and really does help maintain the quality when you dither down. It's something you need to if you're mastering for CD, but make sure you do it at the final when you are fully happy with your mastered mix.