From: "Paul Keinanen" <keinanen@sci.fi>
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: Shrinking sounds - fiasco
Date: Fri, 23 Jun 2000 15:48:12 +0300
Message-ID: <j7m6lskbblceu38662f5575ignmc33tp5d@4ax.com>
References: <3617AC3245C2D1118A840000F805359C01AB8CD9@pdw-mercury-1.dera.gov.uk>
In-reply-to: <3617AC3245C2D1118A840000F805359C01AB8CD9@pdw-mercury-1.dera.gov.uk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

On Fri, 23 Jun 2000 10:27:59 +0100, Talbot Andrew
<ACTALBOT@dera.gov.uk> wrote:


>1)	Take the off air signal recording at (say) 8000 Hz sampling rate

>2)	Digitally mix down to some arbitrarily low frequency 

>3)	Filter to the necessary sub-Hz bandwidth centred on the signal

>4)	Decimate this filtered data to a much lower sampling rate by
>just taking one sample out of every (say) 256 . 

>For anyone contemplating writing software to do this, note that stages
>3) and 4) can be taken together - if using an FIR filter, the filtering
>process needs to be done for each decimated sample only - this saves
>considerable processing overhead for the long filter tap lengths
>necessary


If you plan to combine steps 3) and 4) by doing first decimating and
then narrow band (sub-Hz) filtering, shouldn't you low pass filter the
signal produced by the mixdown at 2) to have a bandwidth less than 15
Hz for the final 31.25 Hz sample rate to meet the Nyquist criteria ?

Paul OH3LWR