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Date: Tue, 8 Mar 2011 20:42:46 +0000
Message-ID: <AANLkTiksDowcKxyJ86R=p-4SqvwcbAQBjQrGY1efm_jN@mail.gmail.com>
From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
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Subject: LF: VLF Stability and soundcard locking
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It would appear that a typical soundcard isn't stable enough for VLF use
when stations are running 1mHz bandwidth signalling, and using any locking
option to other off-air signals is a bit restrictive in what software you
can run.

It may be worth looking at a custom codec driven from a locked frequency
source.   The PCM2900 (which costs only a few pounds from Farnell, and they
supply the data sheet) is a self contained USB codec which needs only a
handful of additional components for a USB soundcard with stereo line input
and output.     For normal operation it needs a 12MHz crystal, but an
external 12MHz source can be used which could be locked to, or derived from,
a high stability reference.  Even a TCXO on its own with typically 1ppm
accuracy / stability would be better than any normal 20ppm crystal

I've built one of these codecs, but have yet to confirm the resulting
sampling rate(s) are *exactly* what they should be.  By the time it has gone
through USB packetising and Windows'  drivers I hope full synchronicity is
maintained right through to the user software.   This may only be the case
for sampling rates like 12000Hz - it all depends what windows does.   Exact
conversion is needed for this solution to be worthwhile, not just a close
approximation.

Andy
www.g4jnt.com

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<div>It would appear that a=A0typical soundcard isn&#39;t stable enough fo=
r VLF use when stations are running 1mHz bandwidth signalling, and using=
 any locking option to other off-air signals is a bit restrictive in what=
 software you can run.</div>

<div>=A0</div>
<div>It may be worth looking at a custom codec driven from a locked freque=
ncy source.=A0=A0 The PCM2900 (which costs only a few pounds=A0from Farnel=
l, and they supply the data sheet) is a self contained USB codec which nee=
ds only a handful of additional components for a USB soundcard with stereo=
 line input and output.=A0=A0=A0=A0 For normal operation it needs a 12MHz=
 crystal, but an external=A012MHz source can be used which could be locked=
 to, or derived from, a high stability reference.=A0 Even a TCXO on its ow=
n=A0with typically 1ppm accuracy / stability would be better than any norm=
al 20ppm crystal</div>

<div>=A0</div>
<div>I&#39;ve built one of these codecs,=A0but have yet to confirm the res=
ulting sampling rate(s) are <em><strong><u>exactly</u></strong></em> what=
 they should be.=A0 By the=A0time it has gone through USB packetising and=
 Windows&#39;=A0 drivers I hope full synchronicity is maintained right thr=
ough to the user software.=A0=A0 This may only be the case for sampling ra=
tes like 12000Hz - it all depends what windows does.=A0=A0 Exact conversio=
n is needed for this solution to be worthwhile, not just a close approxima=
tion.</div>

<div>=A0</div>
<div>Andy</div>
<div><a href=3D"http://www.g4jnt.com/">www.g4jnt.com</a></div>
<div>=A0</div>

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