Optimal low pass filter for DSD

[vladserg]

I am using DSD (1.41, 1.5, 1.6) with Winradio G305E and Wouxun KG-UVD1P with discriminator tap, listening to apco 25 C4FM.

The best result from Winradio i get when using 8 kHz FM demodulator bandwidth, post-mixer filter length=31 and audio filter length =31, (although audio filter is OFF). I'm getting very few errors (1-4 bars) if i use Winradio's native vitrual audio bridge to feed audio to DSD (11025 hz sample rate).

If I Use Virtual Audio Cable software (11025 or 44100 or 48000) to feed the same audio from Winradio to DSD i got worse results for some reason... I'm getting errors pattern like this:
=
=
=
====
===========R====
=
==
=
====
=================

The same strange error pattern i'm getting from common radio discriminator tap. Disciminator itself have 330 omh source resistance, then 10 kiloohm in series, then 10 nF capacitor to the ground, then to sound card. I found 10nF to be optimal, without R-C filter i'm getting a lot of errors.

If i connect the same discriminator tap to different computer, i got the following error distribution:

=
=
=
=========================
===============
======
==
=
=
=
=
=
=
=========================
===============
=======
=

I suspect that this is a problem with low pass filtering inside sound card, too low sampling rate, or maybe sound card frequency oscillator offset. Could anybody suggest how to solve this problem?

Maybe DSD should have higher input sample rate and optimal (or variable) low pass filter inside it? It is very hard to make optimal filter outside DSD.

 

[vladserg]

I have tried another two sound cards, with similar result - periodical error distribution with different period.

I have also tried third order low pass filter between TAP and DSD, but this does not help. So it seems to be a problem in DSD itself. Have anybody encounter the same problem?


Another strange fact - DSD does not show high error values when i run it with "-s -z 1". But it crashes after several hours in this mode (All windows versions 1.4, 1.5, 1.6).