AAC to OPUS
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| - Information about AAC |
| Id |
| 1 |
| File extension |
| aac |
| File category |
| Audio |
| Stands for |
| Advanced Audio Coding |
| Developer |
| Bell Labs, Fraunhofer IIS, Dolby Laboratories, LG Electronics, Sony Corporation (Combined Efforts) |
| Overview |
| AAC is the descendant of the MP3 format. It provides better sound quality using a similar bit rate as of MP3. Besides this, it uses lossy data compression but it has high standards of audio quality than of MP3. We can find this format on a number of platforms and Apple's iTunes is one such example that uses it as default audio format. Size of its file is much smaller which is approximately one-tenth of MP3 file. It also neglects the redundant information that is not even processed by the human ear and is often not noticed. Although, this format was introduced as MPEG-2 Part-7, yet, standardization was done by ISO/IEC as a part of the MPEG-2 and MPEG-4 families. It is also termed as MPEG-2 NBC (Non- Backward Compatible) because it is incompatible with MPEG-1 audio formats i.e. MP1, MP2, MP3. |
| Technical description |
| Modified discrete cosine transform(MDCT) algorithm is used by AAC which ensures that it provides high compression efficiency. MPEG-2 PART-7 has three default profiles namely Low Complexity (LC), Main Profile (Main) and Scalable Sample Rate (SSR). These profiles come with Perceptual Noise Substitution which enhances coding efficiency and provides excellent audio quality. Furthermore, it uses Auditory Masking to ignore the surplus information. This codec has come up with advanced features which are not available in the predecessor. These include additional sample rates ranging from 8 to 96 kHz, 48 Channels, better efficiency with a simpler filter bank, higher coding efficiency for stationary signals and higher coding accuracy for transient signals. |
| Links |
| wikipedia.org |
| - Information about OPUS |
| Id |
| 9 |
| File extension |
| opus |
| File category |
| Audio |
| Stands for |
| Ogg OPUS |
| Developer |
| Xiph.Org |
| Overview |
| It is a format for audio files which compresses the audios using lossy compression. It neglects the redundant information which can not be processed by the human ear. Internet Engineering Task Force (IETF) has standardized this format. It has superseded the Vorgis and Speex. It offers low latency i.e. there is a less delay in the signal when it enters the system and when it emerges out, this results in real-time interactive communication like video conferencing. Due to its good audio quality, it is widely used by in various listening tests. |
| Technical description |
| OPUS has good compatibility with low-end embedded processors as it is less complex. Ranging from 6kbps to 510kbps, it can support both variable and constant bitrate coding. Also, the frame size from 2.5ms to 60ms is supported by this codec. It works smoothly and maintains stability (by avoiding the distortion) while mixing of packets from distinct streams. Speech, Hybrid and pure-CELT are the three different methods used for encoding the audios of varied frequencies. For compressing the Speech up to 8kHz, SILk is used which is an audio compression developed by Skype Limited. The next mode, Hybrid uses CELT to compress the audio files above 8kHz frequency or for wider bandwidth and frame sizes 10ms and 20ms are supported by this mode. CELT(Constrained Energy Lapped Transform) is an open and free audio compression format which is especially used in communication that requires low latency. For general audio, the last mode pure-CELT is used. CELT is used for frame sizes which are less than 10ms. |
| Links |
| wikipedia.org |
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