In previous articles we have discussed several popular audio codecs in use today. In this installment we address advanced audio codecs currently in use.
Vorbis Ogg ACM
Designers employing audio codecs face a minefield of possible patents that either limit applications or must be licensed just to use the technology. Early licensing complications led some developers to look for a “patent free” audio codec. Thus was born a movement to create and popularize audio formats and algorithms ostensibly free from patent entanglements. The most popular “patent free” codec is Ogg Vorbis, one of several multimedia codecs that are completely open and free available from Xiph.org. Even though the codec is freely available and was developed to avoid fundamental MP3 related patents, there is no guarantee that the Vorbis codec is in fact free from patent infringement. Engineers who chose Vorbis based on it having no applicable patents should undertake their own patent evaluation.
Like many innovations, Vorbis was developed as a way to avoid planned licensing fees associated with an industry standard. The Ogg Vorbis format is popular in open source communities. The technical rationalization is that its higher fidelity and completely free nature make it an excellent replacement for the wildly popular MP3 format. The Vorbis encoder produces 45 - 500 kbps streams depending on the quality setting. It is tuned for bit rates of 16 - 128 Kbps/channel but can also encode arbitrary bit rates chosen by the user. Vorbis is an inherently variable-bit rate audio codec employing the modified discrete cosine transform (MDCT) for converting sound data from the time domain to the frequency domain. Frequency-domain data is divided into noise floor and residue components, and then quantized and entropy coded using a codebook-based vector quantization algorithm. The decompression algorithm reverses the process.
Windows Media Audio (WMA) is Microsoft’s proprietary compressed audio file format. It enjoys a huge user base since it is part of the Windows software package. Initially a competitor to the MP3 format, but Microsoft has positioned the codec as a competitor to the AAC format used by Apple. It is part of the Microsoft Corporation Windows Media framework. WMA includes multi-channel-coding and achieves quality similar to that provided by AAC codecs. The basic lossy codec has been expanded to include Pro and a new lossless codec as well as support for variable bit rates. Microsoft Windows Media Audio (WMA) file format is claimed to be a higher quality audio format at smaller file sizes. The codec is bundled as the standard audio format in Windows 98/2000/XP virtually guaranteeing that a second audio format competing with MP3 will continue to exist.
RealAudio is a proprietary audio codec developed by RealNetwork. Designed to work well in low bandwidth environments, it is most often used as a streaming audio format.
For high bit rates, Real Media uses AAC and includes multichannel coding. RealAudio is popular among radio stations that stream their programming over the Internet in real time.
Comparison of Mobile Audio Codec’s
There are many methods to compare the quality of audio streams. Ultimately the gold standard for telephone quality is that achieved by in-person, face-to-face discussions. So, most researchers will use an audience to judge the quality. MUSHRA, used by the European Broadcasting Union (EBU) is often used as a reference. MUSHRA stands for “MUlti
Stimulus test with Hidden Reference and Anchors” was developed and proposed by the EBU Project Group B/AIM(Audio In Multimedia). MUSHRA is a subjective test where listeners in different EMU-countries compare different types of audio to a reference signal and
grade it according to a scale from 0 to 100. The 81-100 range is considered
“excellent”, 61-80 is “good”, 41-60 is fair, 21-40 is poor, and 0-20 is considered bad. The test involves different types of music such as classical, folk, jazz and pop music combined with broadcasting programs, both in a studio and a live environment with female and male voices. Using the test, there is little discernable difference between stereo cd-quality and HE-AAC compression at 48 kbps. HE-AAC (aacPlus) has the highest MUSHRA index of 80 compared to MP3PRO at 76. At 48 kbps RealMedia
Real 8, MP3 and MS Windows Media 8 codec’s receive lower ratings.
Audio quality in all consumer products has continued to improve as processing power has increased. Since the first GSM codec implementations, Digital Signal Processing power has increased by a factor of more than one hundred making more computationally complex codecs practical. Where the codec choice can be made on purely technical grounds, there is adequate information available on the popular codecs to make an informed choice. For circumstances in which the source material dictates the codec there is no real choice to be made.