FPGA Central

hi All
In MP3 encoder algorithm, after the PQMF filtering, there is a step in
which the odd samples in odd subbands are multiplied by
-1. After this the MDCT is applied. Does any one here have any idea why
this is done.
I think it will invert the frequencies, that is lower
frequencies become higher and vice versa. But why is it done I don't
understand. Thanks for any replies.

coolup wrote:
> hi All
> In MP3 encoder algorithm, after the PQMF filtering, there is a step in
> which the odd samples in odd subbands are multiplied by
> -1. After this the MDCT is applied. Does any one here have any idea why
> this is done.
> I think it will invert the frequencies, that is lower
> frequencies become higher and vice versa. But why is it done I don't
> understand. Thanks for any replies.
>

In effect, multiplying odd samples by -1 is the same as (efficiently)
multiplying the sequence by 1,-1,1,-1,...

That is, muliplying by a cosine wave whose frequency is fs/4, where fs
is the sample rate.

In short, it's a mixer.

--
Jim Thomas Principal Applications Engineer Bittware, Inc
[email protected] http://www.bittware.com (603) 226-0404 x536
Today is the last day of your life so far.

Jim Thomas wrote:
> coolup wrote:
> > hi All
> > In MP3 encoder algorithm, after the PQMF filtering, there is a step in
> > which the odd samples in odd subbands are multiplied by
> > -1. After this the MDCT is applied. Does any one here have any idea why
> > this is done.
> > I think it will invert the frequencies, that is lower
> > frequencies become higher and vice versa. But why is it done I don't
> > understand. Thanks for any replies.
> >
>
> In effect, multiplying odd samples by -1 is the same as (efficiently)
> multiplying the sequence by 1,-1,1,-1,...
>
> That is, muliplying by a cosine wave whose frequency is fs/4, where fs
> is the sample rate.
>
> In short, it's a mixer.
>
> --
> Jim Thomas Principal Applications Engineer Bittware, Inc
> [email protected] http://www.bittware.com (603) 226-0404 x536
> Today is the last day of your life so far.


Thanks for your reply Jin. I think the frequency of the
signal is fs/2, not fs/4.
But what I wanted was why is it done.
Why can't MDCT can be taken directly?