# PQMF Filters

Started by November 9, 2005
```When I checked for PQMF filters in Wikipedia , I got the following
information.
It says that signal in odd subbands is stored frequency inverted. Does
anybody know how the frequency is inverted?

A polyphase quadrature filter, or PQF is a filter bank, which splits an
input signal into a given number N
(mostly a power of 2) of equidistant sub-bands. These sub-bands are
subsampled by a factor of N, so they are critically sampled.

This critical sampling introduces aliasing. Similar to the MDCT time
domain alias cancellation the aliasing of polyphase quadrature filters
is canceled by neighbouring sub-bands, i.e. signals are typically
stored in two sub-bands.

Note that signal in odd subbands is stored frequency inverted.

```
```coolup wrote:

> When I checked for PQMF filters in Wikipedia , I got the following
> information.
> It says that signal in odd subbands is stored frequency inverted. Does
> anybody know how the frequency is inverted?
>
>
> A polyphase quadrature filter, or PQF is a filter bank, which splits an
> input signal into a given number N
> (mostly a power of 2) of equidistant sub-bands. These sub-bands are
> subsampled by a factor of N, so they are critically sampled.
>
> This critical sampling introduces aliasing. Similar to the MDCT time
> domain alias cancellation the aliasing of polyphase quadrature filters
> is canceled by neighbouring sub-bands, i.e. signals are typically
> stored in two sub-bands.
>
> Note that signal in odd subbands is stored frequency inverted.

Frequency invertion is due to the down sampling. For example when you have
your signal divided into two subband (cutoff at 0.5pi) then the "upper"
band will be frequency inverted because of the downsampling. Upper band
aliases completely.

--
----
Jani Huhtanen
Tampere University of Technology, Pori
```
```Thanks for replying Jani
If Frequency Inversion is happening because of downsampling, Why it is
not happening in
"Even" subbands? In MP3, only for odd subbands freq inversion
compensation is done.

```