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Date: Wed, 3 Jun 2015 18:17:57 +0100
Message-ID: <CAA8k23S-WGTd1wuMBf-HVgOobmW_h2XsBYCYKeseX3yo1-cE7w@mail.gmail.com>
From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: USB stereo soundcard, CHEAP!?
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AFAIK there are no cheap and cheerful stereo mic input soundcards.      The
really cheap headphone dongles have stereo phones ouptut and mono mic -
specifically aimed at headsets for Skype etc.

If you are prepared to homebrew something, there is the PCM2900 USB Stereo
codec which needs very few extra components for a complete good quality
stereo soundcard.   Howeverm to get stero microphone as opposed to stereo
line in you'll have to "get at it" in teh driver.  By default it sets the
inputs as line level.  (It is the same Codec as used inte DG8SAQ Vector
Network ANalyser)
Actually there are a whole family of themPCM2900, PCN2902,  ..04, ..06 etc.
  I've only ever used the PCM2900. I think teh higher numbered ones are
improved, and the 02 is certainly cheaper from Farnell now.  The PCM290x
devices are available from many of the catalogue suppliers like Farnell and
RS

You don't say why you need stereo, but I'm guessing / assuming an SDR I/Q
input for Softrock or similar.  If so,  *AND YOU ONLY NEED A NARROW BAND* there
is a quick and simple bodge that works wonderfully with a mono Mic input.
But it is only applicable to bandwidths of no more than a couple of hundred
Hz, say 1400 - 1600Hz.   Certainly not a full SSB bandwidrth ..   For
example, it worksif you want to  receive a single beacon like the 5.29MHz
ones (which is where I did this for several years on a dedicated Softrock )
 or just about good enought to cover teh WSPR audio subband from 1400 -
1600Hz.  It works like this :

Take a resistor / capacitor pair whose RC combination give a break point at
the centre frequency of interest.  ie.  1/(2.pi.C.R) = Freqeuncy in Hz.
 Connect the resistor from say the I channel to the mono mic input.
Conenct the capacitor from the Q channel to the same point.    You will get
sideband cancellation  for a narrow band around the centre freqiuency.
 make the resistor adjustable to be able to set this freqeuncy to what you
need.   Eg for 1500Hz (WSPR band centre)   use a 10nF capacitor and 10.6k
(10k resistor and 10k variable).  The sideband rejection worsens either
side as you depart from the C Freq.

The interesting thing about this simepl RC network is that teh phase shift
from I and Q inputs to the mono summing point is completely independent of
any load resistance to ground there.  So the mic conenctor input impedance
doesn't mattter.    What does matter, however, is any residual output
resistance in the driver. Outputs from opamps are fine, but there should be
no significant additional series resistance there, certainly any present
shoudl be less than 5% of the Xc and R values making up the simple phase
shift network.

More complex wideer band I/Q all pass networks can be made up from chains
of OP-amps, and ther eis plenty of documentaion aroudn for that.

Andy  G4JNT


On 3 June 2015 at 17:43, DK7FC <selberdenken@posteo.de> wrote:

>  Hi all,
>
> On my remote RX i'm using a cheap USB-soundcard (attachment), which runs
> very well! However it has just a mono-input (Mic).
>
> What i need is such a soundcard with *stereo microphone inputs*. It has
> to be such a cheap and compact one, i.e. for example NOT a EMU0202 or
> similar stuff. 16 bit and 48 kS/s is totally sufficient. It would be nice
> if the mic-GND is at one potential with the USB-shield, just to avoid
> problems.
>
> Does someone know one and where to order it?
>
> 73, Stefan
>

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Content-Transfer-Encoding: quoted-printable

<div dir=3D"ltr">AFAIK there are no cheap and cheerful stereo mic input sou=
ndcards. =C2=A0 =C2=A0 =C2=A0The really cheap headphone dongles have stereo=
 phones ouptut and mono mic - specifically aimed at headsets for Skype etc.=
<div><br></div><div>If you are prepared to homebrew something, there is the=
 PCM2900 USB Stereo codec which needs very few extra components for a compl=
ete good quality stereo soundcard. =C2=A0 Howeverm to get stero microphone =
as opposed to stereo line in you&#39;ll have to &quot;get at it&quot; in te=
h driver.=C2=A0 By default it sets the inputs as line level. =C2=A0(It is t=
he same Codec as used inte DG8SAQ Vector Network ANalyser)</div><div>Actual=
ly there are a whole family of themPCM2900, PCN2902, =C2=A0..04, ..06 etc. =
=C2=A0 I&#39;ve only ever used the PCM2900. I think teh higher numbered one=
s are improved, and the 02 is certainly cheaper from Farnell now.=C2=A0 The=
 PCM290x devices are available from many of the catalogue suppliers like Fa=
rnell and RS</div><div><br></div><div>You don&#39;t say why you need stereo=
, but I&#39;m guessing / assuming an SDR I/Q input for Softrock or similar.=
=C2=A0 If so, =C2=A0<u>AND YOU ONLY NEED A NARROW BAND</u>=C2=A0there is a =
quick and simple bodge that works wonderfully with a mono Mic input.=C2=A0 =
But it is only applicable to bandwidths of no more than a couple of hundred=
 Hz, say 1400 - 1600Hz. =C2=A0 Certainly not a full SSB bandwidrth .. =C2=
=A0 For example, it worksif you want to =C2=A0receive a single beacon like =
the 5.29MHz ones (which is where I did this for several years on a dedicate=
d Softrock ) =C2=A0or just about good enought to cover teh WSPR audio subba=
nd from 1400 - 1600Hz.=C2=A0 It works like this :</div><div><br></div><div>=
Take a resistor / capacitor pair whose RC combination give a break point at=
 the centre frequency of interest. =C2=A0ie. =C2=A01/(2.pi.C.R) =3D Freqeun=
cy in Hz. =C2=A0 =C2=A0Connect the resistor from say the I channel to the m=
ono mic input.=C2=A0 Conenct the capacitor from the Q channel to the same p=
oint. =C2=A0 =C2=A0You will get sideband cancellation =C2=A0for a narrow ba=
nd around the centre freqiuency. =C2=A0make the resistor adjustable to be a=
ble to set this freqeuncy to what you need. =C2=A0 Eg for 1500Hz (WSPR band=
 centre) =C2=A0 use a 10nF capacitor and 10.6k (10k resistor and 10k variab=
le).=C2=A0 The sideband rejection worsens either side as you depart from th=
e C Freq. =C2=A0=C2=A0</div><div><br></div><div>The interesting thing about=
 this simepl RC network is that teh phase shift from I and Q inputs to the =
mono summing point is completely independent of any load resistance to grou=
nd there.=C2=A0 So the mic conenctor input impedance doesn&#39;t mattter. =
=C2=A0 =C2=A0What does matter, however, is any residual output resistance i=
n the driver. Outputs from opamps are fine, but there should be no signific=
ant additional series resistance there, certainly any present shoudl be les=
s than 5% of the Xc and R values making up the simple phase shift network.<=
/div><div><br></div><div>More complex wideer band I/Q all pass networks can=
 be made up from chains of OP-amps, and ther eis plenty of documentaion aro=
udn for that.</div><div><br></div><div>Andy =C2=A0G4JNT</div><div><br></div=
></div><div class=3D"gmail_extra"><br><div class=3D"gmail_quote">On 3 June =
2015 at 17:43, DK7FC <span dir=3D"ltr">&lt;<a href=3D"mailto:selberdenken@p=
osteo.de" target=3D"_blank">selberdenken@posteo.de</a>&gt;</span> wrote:<br=
><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1=
px #ccc solid;padding-left:1ex"><u></u>


 =20

<div bgcolor=3D"#ffffff" text=3D"#000000">
Hi all, <br>
<br>
On my remote RX i&#39;m using a cheap USB-soundcard (attachment), which
runs very well! However it has just a mono-input (Mic).<br>
<br>
What i need is such a soundcard with <u><b>stereo microphone inputs</b></u>=
.
It has to be such a cheap and compact one, i.e. for example NOT a
EMU0202 or similar stuff. 16 bit and 48 kS/s is totally sufficient. It
would be nice if the mic-GND is at one potential with the USB-shield,
just to avoid problems.<br>
<br>
Does someone know one and where to order it?<br>
<br>
73, Stefan<br>
</div>

</blockquote></div><br></div>

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