concatenated FFT's?

I read about something like the following before, but don't know what
search terms to feed Google in order to find it.

If I have only the FFT results, length N, for 2 successive frames of
data, naively I can get an FFT of length 2N by first doing two IFFT's
and then an FFT on the concatenation of their output.  Is there a more
efficient method?


IMHO. YMMV.
-- 
Ron Nicholson   rhn AT nicholson DOT com   http://www.nicholson.com/rhn/ 
#include <canonical.disclaimer>        // only my own opinions, etc.

Ronald H. Nicholson Jr. wrote:

>I read about something like the following before, but don't know what
>search terms to feed Google in order to find it.
>
>If I have only the FFT results, length N, for 2 successive frames of
>data, naively I can get an FFT of length 2N by first doing two IFFT's
>and then an FFT on the concatenation of their output.  Is there a more
>efficient method?
>
>
>IMHO. YMMV.
>  
>
Ronald,

Yes, there is!  In fact, the FFT Butterfly is no more than an efficient 
method of combining two length-N FTs to deliver a single length-2N FT.

All you need to do is put the two length-N FFTs end-to-end, and treat 
that block of 2N samples as the input to the last stage of a length-2N 
'decimation-in-time' FFT.  Arrange the frames so that the more recent 
frame  is the one that receives the phase-rotation.

Regards,
John