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Topic: Frequency inversion for polyphase (Read 3815 times) previous topic - next topic
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Frequency inversion for polyphase

Can anybody tell me where is the code for Frequency inversion for polyphase in lame?
I can't find it in LAME.
Thanks!

Frequency inversion for polyphase

Reply #1 –
Why do you wanna know ?
I mean, it's one of the most simple tasks. In an MP3 encoder -- such as LAME -- this is done after the first PQF analysis stage and before the MDCT stage. How is it done ? The sign of every 2nd sample for every odd subband (counting from zero) is simply inverted. It's done to compensate for the "natural frequency inversion" of the PFQ analysis filterbank.

I'm not familar with the LAME sources, so I can't point you to the respective lines of code, sorry.

Sebi

Frequency inversion for polyphase

Reply #2 –
I'm newbie for Lame decoder.
I know that the formula for frequency inversion is really simple.
But while i go through the source code of lame, i can't see the implementation for that.
Maybe I miss something, i will trace that carefully.
Hope someone has good experience could throw some light on that for me.
Thanks!

Frequency inversion for polyphase

Reply #3 –
I guess we're even then. I didn't answer your question and you didn't answer mine.

Sebi

Frequency inversion for polyphase

Reply #4 –
Quote from: SebastianG on
I guess we're even then. I didn't answer your question and you didn't answer mine.

Sebi


Hi All,
        I am a novice in audio DSP. Can u please explain me what is "natural frequency inversion". I know how this is done, but unable to understand why this is done?

Regards
Abhijit