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Topic: Is & Ms (Read 1928 times) previous topic - next topic

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  • xash
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Is & Ms
I am working on the AAC Encoder
I know about the coding gain and the usage of Intensity stereo and Mid/side stereo from ISO/IEC ref code.
But in some cases i am getting an artifact due to these.
can anybody plz tell me what are the factors need tobe considered while doing the Encoding so that my quality is not affected much.

I mean the factor which will tell me when to IS & or MS and when i can't


  • Ivan Dimkovic
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  • Developer
Is & Ms
Reply #1
This is up to encoder design - i.e. various algorithms could be used to determine whether particular sfb should be coded as LR, or MS  or IS

For perceptually loseless methods, like M/S stereo  you can use PE check + channel difference check, like method used in LAME MP3 encoder.

For Intensity Stereo (IS) a more complex method must be used to find the best trade-off. One effective algorithm would be:

1. For each sfb calculate IS and LR(MS) MDCT
2. Reconstruct IS bands (> 4 kHz)
3. Measure noise introduced in reconstructed IS compared to LR
4. If the noise does not exceed some threshold, use IS. If does, use LR or MS

The best place for this algorithm is psychoacoustic model. Note that IS mode is usable in coding modes where compression is bigger than 1 bit per sample (for AAC).