iTunes and Windows Media file format comparison
People are often wondering what format to store their music collection in. Both iTunes and Windows Media Player now offer many option which are accessible from their preferences menus that allow to choose from various popular lossy or lossless formats. The encoding most often happens when user "rips" a CD into their library, but with iTunes it is also possible to convert an existing track into another format.
iTunes 8 offers encoding in MP3, AAC, Apple Lossless or WAV/AIF
Windows Media Player 11 offers MP3, WAV, WMA and WMA Pro.
To reduce the size of music library it is often appropriate to choose one of the "lossy" formats. The question is how much is removed from those files and will the difference be noticeable. There have been many "Listening tests" conducted which answer this, but another way of showing the qualities of those formats is using audio analysis software. We have used Rightmark Audio Analysier to compare 4 different formats. These test are not without flaws, because each format may be more suitable for encoding the test signals but less appropriate for music content. We have used a fixed bit rate with all the formats because variable bit rate would give much more misleading results with our test files. You can think of these tests as a way to tell which encoding is the most true to the original content.
File sizes
Even though the bitrate of the files is the same, the file sizes are different because of different encoding techniques and overheads. For example WMA has an added overhead of 7 kbps. While file size of AAC file is significantly smaller then other 3 formats.
MP3 128 kbps - 1,168,234 Bytes
MP3 192 kbps - 1,751,287 Bytes
AAC 128 kbps - 844,594 Bytes
AAC 192 kbps - 1,243,682 Bytes
WMA 128 kbps - 1,147,576 Bytes
WMA 192 kbps - 1,714,144 Bytes
WMA Pro 128 kbps - 1,146,544 Bytes
WMA Pro 192 kbps - 1,714,196 Bytes
Frequency response charts
Frequency response (at 40 Hz and 15 kHz), dB:
MP3 128 kbps: +0.23, -1.53
AAC 128 kbps: +0.36, -0.69
WMA 128 kbps: +0.31, -2.53
WMA pro 128 kbps: +0.06, -0.05
MP3 192 kbps: +0.36, -1.60
AAC 192 kbps: +0.16, -0.36
WMA 192 kbps: +0.02, -0.04
WMA pro 192 kbps: +0.02, -0.04
At 128 kbps WMA seems to be performing poorly, with a very smooth roll off at high frequencies. MP3 has a characteristic drop after 10 khz and a roll off at 16 khz after which what's left are unwanted anomalies in high frequencies. AAC has good response, with a sharp roll off at around 17 khz. WMA Pro has the best performance of the 4 formats, it stayed virtually flat until around 16-17 khz which is impressive for the file size.
At 192 kbps both WMA and WMA Pro performed almost identical, the difference can be seen on the zoomed in chart. The roll-off is at 17-18 khz. Performance at the low frequencies is also impressive stretching under 20 hz. AAC had a wider range cutting off at 19 khz while MP3 is still not very linear even at the higher bit rate but tries to cover the widest frequency range.
Noise level charts
Noise level, dB (A):
MP3 128 kbps: -400.0
AAC 128 kbps: -400.0
WMA 128 kbps: -105.3
WMA pro 128 kbps: -100.9
MP3 192 kbps: -400.0
AAC 192 kbps: -400.0
WMA 192 kbps: -100.1
WMA pro 192 kbps: -100.1
Noise level for MP3 and AAC is well below WMA, and is outside the view of the chart. WMA noise level at -130 db is still very low, and certainly inaudible.
Dynamic Range
Dynamic range, dB (A):
MP3 128 kbps: 93.3
AAC 128 kbps: 93.8
WMA 128 kbps: 92.5
WMA pro 128 kbps: 94.8
MP3 192 kbps: 93.8
AAC 192 kbps: 94.0
WMA 192 kbps: 95.8
WMA pro 192 kbps: 95.9
All 4 formats performed equally similar in this measurement.
THD (Total Harmonic Distortion) + Noise (at -3 dB FS)
THD, %:
MP3 128 kbps: 0.0025
AAC 128 kbps: 0.0081
WMA 128 kbps: 4.447
WMA pro 128 kbps: 5.160
MP3 192 kbps: 0.0025
AAC 192 kbps: 0.0029
WMA 192 kbps: 4.781
WMA pro 192 kbps: 4.785
THD test shows a very high percentage of distortion of WMA and WMA Pro. Both had near to 5% which is very high, and would be distinguishable by ear. While MP3 and AAC had expected levels, close to zero.
IMD (Intermodulation Distortion) + Noise
IMD + Noise, %:
MP3 128 kbps: 0.021
AAC 128 kbps: 0.179
WMA 128 kbps: 3.426
WMA pro 128 kbps: 4.223
MP3 192 kbps: 0.012
AAC 192 kbps: 0.044
WMA 192 kbps: 3.854
WMA pro 192 kbps: 3.855
Similar to THD test, there is also a high percentage of intermodulation distortion with both WMA and WMA Pro.
Conclusion
If you are using iTunes and want to save space then go for AAC at 128 kbps, if quality is important use a higher bitrate. If compatibility is important, use mp3 at the high bitrate (mp3 at the lower bitrate performs poorly).
On Windows Media Player I would recommend to use WMA Pro at low bit rates if space saving is needed. At higher bitrate the choice between WMA and WMA Pro is not as important. Avoid using standard WMA at 128 kbps as a lot of high frequency content will be lost.