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Mastering for mp3?
What’s The Idea Behind
Mastered for iTunes Anyway?


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Mastering for a specific format - ie: mastering for mp3 or mastering for iTunes - is an idea akin to the 70's era of recording, mixing, and mastering.

That idea says one did whatever one needed to during those procedures, however odd or counter-intuitive they may seem, so that the resulting vinyl pressing matches the artists' vision.

Mastered for iTunes

Apple's ‘mastered for iTunes’ philosophy basically follows what one should be doing when converting to mp3, or any lossy format.

For the typically loud, pop genre master many clients still request today (in 2012), when outputting to an mp3 delivery format, the output limiter should having a lower ceiling set at - for example -2.0 dB, as opposed to the usual setting for CD, .wav, or .aiff files of -0.1 or 0.0 dB (or -0.3 dB which is what I generally use. Yup, I guess my masters are slightly quieter!)

Why mp3’s Sound Bad! Can You Make It Better?

The reason for setting the limiter that way is that when rendering to a lossy format the resulting file generally will have peaks going past the limiter's ceiling - in this case -2.0 dB.

Let’s say you render to a 192k mp3 with the limiter settings I described above. You will find that when analyzing the file for the highest peaks you'll often see values of -0.9 dB, or -0.25 dB, etc. obviously 'dis-obeying' the limiter absolute of -2.0 dB.

If I don’t set the limiter ceiling at -2.0 dB the resulting lossy format files will have clipping distortion, hitting 0 dB.

That’s right!!

Here it is again, in case you missed this:

  • A straight conversion to mp3 type formats of a typically loud master file will clip and distort!

A Little Extra Work Is Needed

Of course, setting the output ceiling of the limiter at -2.0 dB, requires some extra work in re-adjusting the levels to the input threshold. That’s so the final RMS levels of the mp3 or iTunes format, match that of the CD/wav/aiff version. If this isn’t followed, we’ll have a file that's 2 db quieter than the master CD file.

  • It goes without saying that the better quality source file we start with, the better the end result will be for the inferior format.

Of course, if we're starting out with, say, a 96 kHz 24 bit file, we'll need to choose an excellent resampler and dithering plugin to maintain that quality as much as possible. I have excellent results with WaveLab’s Crystal Resampler. Voxengo’s r8brain PRO is another highly regarded resampling software.

Mastering For mp3: We Can Still Make It (A Bit) Better!

Another procedure I follow when mastering for mp3, is to low pass filter the file at around 15 kHz (with a high quality filter of course) since many lossy formats don't have content above that.

I might do some extra mid-side EQ and 'soften' the side levels by 1 dB, or so, at that frequency range, if the stereo image is rather pronounced in that region to minimize any artifacts that might result.

  • These artifacts can be clearly heard when monitoring the side channel of the resulting lossy file. I might redo a file several times using different procedures to minimize these artifacts as much as possible to get optimum final results.

This is the reason I charge an extra fee for providing additional mp3 versions for an artists project should they request it. A little extra time is involved to make them sound the best they can be.

One Final Point:

Naturally, the procedures I’ve outlined for mastering for mp3 won't create a precise match with the master file.

But...

  • they definitely won't clip; and
  • most artifacts will be minimized.

This ensures a better mp3 listening experience for all my clients' listeners.





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