> I think that you need to qualify the statement with "as long as you
> don't saturate the sound card by using a wide filter."
Not to mention saturating the receiver IF or causing the AGC to reduce
the receiver gain to the point that a weak signal falls below the noise
floor, etc. There are a lot of benefits for narrow filters - as long
as they're not too tight.
73,
... Joe, W4TV
On 3/30/2012 4:08 PM, Kok Chen wrote:
>
> On Mar 30, 2012, at 12:45 PM, Jay WS7I wrote:
>
>> Joe wide filters work better, trust Chen, but you have to use his
>> software.
>
> Wait, wait... (oops, that is from another show, but on NPR radio, not TV :-).
>
> I think that you need to qualify the statement with "as long as you don't
> saturate the sound card by using a wide filter."
>
> Once the sound card clips, it is "game over" for any software demodulator.
>
> However, as long as the sound card does not clip, it is better to trust the
> software to do the proper filtering, where it can apply optimal Nyquist
> filters (Match Filters are also Nyquist, incidentally).
>
> The problem with arbitrary filters that don't satisfy the Nyquist criteria
> (and this includes crystal I.F. filters) is that they generate Inter-symbol
> Interference (ISI). A Baudot bit from a station is smearing into its *own*
> next bit's position, causing errors under low SNR conditions even when there
> is no QRM.
>
> By using a wide filter, you avoid adding in the group delays from crystal
> filters, and achieve the design criteria of the software author.
>
> When there is enough QRM, you will need to sacrifice demodulator sensitivity,
> however. So it is not a case of never using narrow filters.
>
> 73
> Chen, W7AY
>
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