The Optimal Solution for Converting WMA Streaming to AAC for iPhone App

When developing an iPhone application that requires streaming audio in Adaptive
Conversion Audio Compression (AAC) format, one crucial step involves transcode
ing raw WMA (Windows Media Audio) streams into an AAC stream. This process is critical
to ensure seamless audio playback in the app. Here we discuss various methods and
solutions, with a focus on quality and feasibility.

Introduction to WMA and AAC

WMA and AAC are both audio compression technologies. WMA is proprietary to Microsoft,
while AAC is an open standard supported by Dolby, Fraunhofer, and Sony among others.
Direct transcoding between these formats is not feasible due to differences in technology
and intellectual property rights.

Direct Transcoding with FFMPEG

For those familiar with FFMPEG, the versatile multimedia processing library, you can use
it to decode WMA and encode it to AAC. FFMPEG supports WMA decoding natively and
AAC encoding through the libfaac library. This can be done on modern computers in real time.

However, be aware of the following caveats:

Audio Quality: The transcoding will be naive, meaning the raw audio from WMA will be
decoded and then encoded into AAC. The audio quality might suffer significantly due to the
loss of detail in the transcoding process. Platform and Location Considerations: Depending on your development platform and location,
you might need to build or find a version of FFMPEG with WMA and AAC support due to patent
concerns. Quality Comparison: Studies from the past, although not up-to-date, criticized the audio quality
of FAAC compared to other AAC encoders. While quality comparisons can be subjective, they
hint that other encoder options might yield superior audio results.

Re-encoding After Decoding

The second approach, which is most likely to yield the best audio quality, involves
converting the WMA stream back to its raw PCM (Pulse Code Modulation) format using a
tool that supports WMA decoding. Once the audio is in PCM, you can encode it into
AAC. This method sacrifices one generation of audio quality, but it ensures higher fidelity
than a naive transcoding process.

Considerations:

For Internet radio-quality streaming audio, the bitrate is often lower than the standard
CD-quality represented by iTunes at 128k, so audio quality will suffer even further. Returning to the source audio and directly compressing it with AAC would be the most
preferable method for maintaining the highest possible audio quality.

Conclusion

In summary, when converting WMA streaming to AAC for an iPhone app, direct transcoding
with tools like FFMPEG might work in a pinch but can compromise audio quality. The reliable
method involves using a tool that can decode WMA to PCM and then re-encode this raw audio
back into AAC. Although it's a little more complex, this process should ensure the best audio
quality for your app's users.