Does Converting MP3 to AAC Improve Quality?

MP3 files are notorious for their lossy (irreversible) compression, but their efficient size makes them popular for storing our tracks and recordings. AAC was designed to be the successor to MP3, promising us a better lossy format. So even if you’re not an audiophile, this begs the question: does converting MP3 to AAC improve quality?

Converting MP3 files to AAC does not improve audio quality. AAC encoders cannot infer information about the original audio from an MP3 file that MP3 decoders can’t. While uncommon, the only reason to convert an MP3 file is for playback by an AAC-only player.

If you have the option between the two, the AAC and MP3 formats have crucial differences that you should be aware of. You may nonetheless want to perform such a conversion for other reasons.

Does Converting MP3 Files To AAC Lose Quality?

AAC (Advanced Audio Coding) files use an encoding scheme that typically offers higher audio fidelity than the equivalent MP3 (MPEG Layer-3) formatted audio. The added cost is typically only for a fractional increase in file size. Despite this, while re-encoding MP3 to AAC is possible, it’s rarely advantageous.

MP3 effectively discards more information than AAC at lower bitrates, leaving less that can be recovered when decoding an MP3 file. MP3 decoders cannot infer the lost data, which makes the compression ‘lossy’ and the original data unrecoverable, reducing the audio quality.

An AAC encoder can be given this decoded data, but it would only discard more information, using a different method to do so, further distorting the decoded waveform. In the best-case scenario, the AAC encoder would redo the MP3 compression as efficiently as possible. In reality, they do not share the same encoding scheme, so this is impossible.

This incapacity is not specific to AAC re-encoding. Even converting to lossless formats like WAV or FLAC would be fruitless, as they will encode the same information as the MP3, just compressed using a different method. Since MP3 was designed to take full advantage of the reduced quality to reduce file size, unlike lossless formats, you will end up with a much larger file of the same quality.

Can You Hear The Difference Between AAC And MP3?

With decent enough audio equipment and a low enough bitrate, the difference is perceptible in AAC’s favor. MP3 files at these bitrates are often described as muddier, whereas AAC better retains more vibrant sound.

Over 192kbit/s, but especially at 320kbit/s (the highest supported by MP3), the quality approaches lossless. Hence the difference in quality becomes negligible.

At these higher bitrates, the quality of the decoder becomes the most crucial difference. Unfortunately, higher bitrates demand more computational resources to decode; inefficient implementations on low-end devices can cause performance degradation and choppy audio.

Why Does AAC Sound Better Than MP3?

The core distinction lies in how each format encodes the waveform. Both encoders apply the Modified Discrete Cosine Transform (MDCT), but MP3 combines this algorithm with the Fast Fourier Transform (FFT). In addition, AAC integrates a variety of other improvements and is much more modular in its encoding, allowing far more flexibility in the implementation of AAC audio codecs.

The AAC encoding algorithm was developed after MP3, in no small part because of its predecessor’s shortcomings. MP3 often achieves slightly smaller file sizes for a given bitrate than AAC but sacrifices audio quality considerably at lower bitrates. Hardly an advantage for MP3 back then, and the cost per gigabyte of disk space has only fallen from $10,000 in 1990 to $0.10 in 2010.

What Are The Pros And Cons Of AAC Over MP3?

If you have the choice between the two, for example, when choosing the output format of recording software, which should you choose? Under what circumstances?

Pros Of ACC Over MP3

  • AAC preserves audio quality at lower bitrates, such as equal to or less than 128kbit/s, significantly better than MP3.
  • AAC supports a broader range of sample rates (8 kHz to 96 kHz as opposed to 16 kHz to 48 kHz).
  • AAC employs MDCT (Modified Discrete Cosine Transform) to encode audio frequencies. MP3 uses a hybrid of the MDCT and the FFT (Fast Fourier Transform), which is relatively inefficient and complex.
  • AAC supports arbitrary bitrates.
  • AAC allows up to 48 audio channels, whereas MP3 supports up to five (plus one for bass).
  • AAC’s legal status is much clearer in the USA than MP3, over which Texas MP3 Technologies and Audio MPEG have patent claims. These patent claims are invalid in the EU.

Cons Of ACC Over MP3

  • MP3 is occasionally supported even when AAC is not.
  • While AAC files are legal to stream and distribute freely, a license is required to make AAC audio codecs. However, this restriction is legally circumvented, such for the FFmpeg and FAAC free software projects.

These days, AAC is usually the superior choice over MP3, especially where it is considered the standard audio format, including:

  • Devices such as iPhones, iPods, and iPads
  • Consoles such as PlayStation 3, 4, Vita, Nintendo Wii, DSi, 3DS
  • Particular series of Nokia phones
  • Other more niche devices and platforms.

Rest assured, Windows and Android users: media players are available that also support AAC, such as VLC. AAC support won’t be a hurdle for most general-purpose operating systems.

What’s The Highest-Quality Audio Format?

The audiophiles’ favorite is the FLAC format (Free Lossless Audio Codec). As the term ‘lossless’ suggests, FLAC does not discard any information, and the original waveform can be reconstructed from a FLAC file. As you would expect, you pay the price in file size, which is typically around three to five times larger than an AAC track at the same bitrate.

In theory, all lossless formats have identical audio quality, as they reversibly encode the recorded waveform at a given sample rate. Ideally, your lossless file encodes the originally recorded waveform, thus free of compression artifacts and loss of detail. In terms of quality, FLAC, ALAC, WAV, AIFF, and others are identical in terms of fidelity, although their compression schemes – or lack thereof – differ.

How To Determine A File’s Audio Format?

It’s good to know what format you are using first. Often, the file extension is sufficient to differentiate between MP3 and AAC. MP3s virtually always have a “.mp3” file extension, though AAC is much more complicated.

(Tip: Make sure your file browser is displaying file extensions when checking formats.)

AAC-encoded audio will have any of the following extensions:

  • .aac
  • .mp4
  • .m4a
  • .m4b
  • .m4p
  • .m4r
  • .3gp

But besides ‘.aac’, none of these files are strictly AAC-encoded. This is because MP4 (MPEG-4 Part 14) is a ‘multimedia container format’ containing video, audio, subtitles, or images – often multiple. 3GPP is based on MPEG-4 Part 12, which is very similar to MP4.

MPEG-4 containers can contain a wide variety of audio formats. So, in order to determine the encoding, while there are various options depending on your platform, the following is free and straightforward if you can use a command line:

  • Install FFmpeg for Windows, Mac, or Linux
  • Run “ffprobe ./path/to/my_file.mp4” (or ‘.m4a’, etc.)
  • Search for whether the text “aac” appears among the program output.

Conclusion

Converting between audio formats is rarely advantageous unless required by a particular application. Like infamous JPEG compression artifacts: they can only get worse.

It can be disappointing that the data lost to MP3’s inferior coding scheme can never be recovered. At the very least, you needn’t fret over converting your entire library of audio in pace with modern audio codec development. MP3 will be with us for the long haul, for better and worse.

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