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AAC

 

Advanced Audio Coding (AAC) is a standardized, lossy compression and encoding scheme for digital audio. Designed to be the successor of the MP3 format, AAC generally achieves better sound quality than MP3 at similar bit rates. AAC is also the default or standard audio format for iPhone, iPod, iPad, and iTunes. Pro audio Converter offers the following AAC variants:

AAC
This produces the most common AAC LC (Low Complexity) audio format. This is the format you get when purchasing or exporting music from iTunes or quicktime.

MPEG 4 High Efficiency AAC
AAC HE (High Efficiency) is an extension of Low Complexity AAC (AAC LC) and is optimized for low-bitrate applications such as streaming audio. AAC-HE streams are compatible with AAC-LC decoders.

MPEG 4 Low Delay AAC
AAC LD (Low Delay) is optimised for playback speed and is often used for real-time applications like teleophony or or conferencing, e.g. iChat.

 

Pro Audio Converter offers the following parameters when encoding AAC files:

Sample Rate
Sample rate is the number of samples of audio carried per second, measured in Hz or kHz (1000 Hz). 44.1 kHz is the sampling rate of audio CDs and 48.0 kHz is commonly used for professional video. Higher sample rates result in higher quality audio with larger file sizes. Setting this to Auto will create an output file with the same sample rate as the input file. If the output file does not support the sample rate of the input file, Pro Audio Converter will use the greatest sample rate that is supported.

 

Bitrate Mode
Constant Bitrate (CBR)
This uses the same number of bits for every frame of your AAC file. Whether the musical passage is a difficult one to encode or an easy one, the encoder will use the same number of bits, so the quality of your AAC is variable. Complex parts will be of a lower quality than the easiest ones. The main advantage is that the final files size won't change and can be accurately predicted.

Variable Bit Rate (VBR) – In this mode, you choose the desired quality on a scale going from 1 (lowest quality/highest distortion) to 10 (highest quality/lowest distortion). The encoder tries to maintain the given quality in the whole file by choosing the optimal number of bits to spend for each part of your music. The main advantage is that you are able to specify the quality level that you want to reach, but the inconvenient is that the final file size is totally unpredictable.

Average Bit Rate (ABR) – In this mode, you choose a target bitrate and the encoder will try to constantly maintain an average bitrate while using higher bitrates for the parts of your music that need more bits. The result will be of higher quality than CBR encoding while the average file size will remain predictable. This is the default and recommended mode for encoding AAC files.

Variable Bit Rate Constrained
This mode is similar to VBR but limits the average bit rate variation. The lower limit is the user-selected bit rate. The Higher bit rate is adapted for difficult tracks and can generate larger files than the ABR mode. Recommended as a compromise between VBR and ABR.

 

Bitrate
The number of binary values which will be used to encode each second of audio - eg 128kbps (kilo-bits per second) = 128,000 bits per second. This bitrate is shared according to the number of simultaneous audio channels. Guide values:

 

Channels
This defines the number of audio channels contained in the output file. Setting this to Auto will attempt to create an output file with the same number of channels as the input file, if it is possible. You can also manually set it to output Stereo (2 channels) or Mono (1 channel).

 

Quality
The Good setting is optimized for the highest-speed encoding, for higher-quality choose Better or Best (optimal for 24-bit source). The tradeoff is between encoding speed and audio quality.