Re: [Discuss-gnuradio] isolate channels from wideband

[Daniele Nicolodi]

On 22/07/15 15:40, Tom Rondeau wrote:
> On Wed, Jul 22, 2015 at 4:57 AM, Daniele Nicolodi <address@hidden
> <mailto:address@hidden>> wrote:
> 
>     On 21/07/15 21:39, Tom Rondeau wrote:
>     > Here's my presentation from last GRCon:
>     >
>     > http://gnuradio.squarespace.com/grcon14-presentations#tut-rondeau
> 
>     Hello Tom,
> 
>     browsing through your presentation I see that on page 58 and 59 you
>     recommend to use firdes filter design tool and not optfir to build re
>     reconstruction filter.  However, I don't quite understand why the filter
>     generated by one tool is better than the other is this case.
> 
>     Can you please comment on it?
> 
>     Thanks! Cheers,
>     Daniele
> 
> 
> The shape of this filter matters greatly. The inband, transition, and
> stop band behavior all determine if the filter can be used for the
> reconstruction purposes. The image on slide 59 shows the specific
> transition between the pass band and stop bands. To match that with the
> PM (i.e., Remez) algorithm, you can't get the same stop band performance
> for that given transition. Plus the equal response in the stop band is
> bad when channelizing because all channels will alias at equal powers,
> whereas the roll off in frequency with the windowed (firdes) filter
> continues to decrease with f. Remez also produces a pass band ripple,
> which will also affect things. The ripple with the firdes is not
> equiripple like Remez promises, but it's much, much smaller.


Thanks Tom, very clear explanation.

Cheers,
Daniele