Editing compressed audio files

When editing audio from lossy formats, it is important to use a programme which does not re-encode the audio when saving your edited file. Re-encoding is also known as transcoding. It is like making a ‘copy of a copy’ and affects the quality. According to Wikipedia:

The process of lossy-to-lossy transcoding introduces varying degrees of generation loss… The key drawback of transcoding in lossy formats is decreased quality. Compression artifacts are cumulative, so transcoding causes a progressive loss of quality with each successive generation, known as digital generation loss. For this reason, transcoding is generally discouraged unless unavoidable.

The side effects are particularly problematic when re-encoding MP3 files, according to listening tests performed on the Hydrogen Audio forums. An enthusiast explains:

Each time, there was the same kind of distortion. It’s a form of ringing, very typical of lossy encoding, and which ruins the quality of background noise or ambiance. I was often amazed by the huge difference existing between the encoded file and the re-encoded one. I didn’t imagine that re-encoding could have such impact on quality.

Most programmes including Audacity & Total Recorder re-encode the audio! Whilst these are indispensable, it is recommended to use them to analyze your compressed audio only. This screen capture shows me doing the ‘dirty deed’, saving a purchased MP3 file using Audacity!

I don’t understand what programme I’m supposed to use! Explain it properly!

Sure. Here is a real example. I have a one hour recording of tropo recorded using my Degen DE1121 or Creative Zen which encode to 128 kbps MP3 and 160 kbps WMA compressed formats respectively. I connect the device to my USB drive and copy the file to the hard drive. I want to get rid of the static and keep the useful part of the file so I can share it on the blog.

Opening up the programme Audacity, I listen to the file using headphones and view the waveform to find out what bits are garbage (this step is optional). I make note of the only sections of the 60 minute recording that I want to keep. Let’s say I only want 01:30 to 6:45 and 15:00 to 24:15. Armed with these times, the actual editing of my MP3 clip performed in MP3 Direct Cut or MP3Splt. In the case of a WMA clip, editing is performed in WMA Splitter or ASF Tools.

Why should I bother with specific programmes for editing MP3/WMA files? Why can’t I just convert my compressed audio to a lossless format?

Is it true that you can simply convert the original WMA/MP3 clip to uncompressed Wave format and edit that file using Audacity. Unfortunately, this technique only avoids re-encoding if you never save it in a compressed format. Because uploading huge Wave files or lossless formats such as lossless WMA or FLAC is extremely painful for most people, I don’t consider this method really practical. You will still end up needing to convert the Wave file to a lossy format (re-encode) when you need to share it on the internet! However, this is an excellent technique for storing compressed audio on your hard drive. As long as you keep saving the file in Wave format only, this method will not induce any loss.

Do I need to worry about this if I record directly from my computer?

No. My understanding is that by recording directly to uncompressed Wave or lossless format, editing a file does not usually result in any loss of quality. If you are lucky enough to have your laptop’s sound card hooked up to your tuner to record, any editor will suffice. It’s not convenient for me to lug around my laptop every time I want to record. For this reason, I always utilize the convenience of recorders that induce some minuscule loss of audio quality (Compact cassette, Video cassette, DVD-RAM recording, DVD-RW recording & portable MP3/WMA recorders).

Free programmes to edit your Dolby Digital (AC3), MPEG2 Audio Layer 3 (MP3), Windows Media (WMA) or Ogg Vorbis (OGG) compressed audio recordings without re-encoding:

MP3 Direct Cut

Mp3splt for MP3 & OGG

WMA Splitter

ASF Tools for WMA

AC3 Splitter

This author has been professionally trained in live sound mixing. This includes recording experience (for broadcast) using early compression techniques such as Adaptive Transform Acoustic Coding (ATRAC). He has been using MP3 Direct Cut & MP3Splt for several years. Both programmes are extremely stable on an Intel Celeron 2 GHz laptop running Windows XP Home SP2 2002 edition. Asf Tools has been used successfully for several weeks. Feedback on the software cited (especially programmes that I haven’t fully tested yet) is greatly appreciated!

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