The issue of interface of computer to radio has been a mystery to many folks
for years. It would appear that they just don't seem to understand the
methodology of audio levels and impedances.
Typical, sound cards have microphone level inputs. These can be very easily
overdriven by feeding a speaker level signal or headphone level signal from
the radio into the mike input of the sound card. No amount of reducing gain
will correct the fact that the first audio stage is being overdriven and
thus the signal has acquired a significant amount of distortion.
Secondly, most sound cards have speaker or line level output levels.
Feeding either of these into the microphone input of a radio will most often
overdrive the first input stage and again, no amount of reducing the gain in
the radio will do anything but lower the overall distorted level. Once the
signal is distorted it is always distorted regardless of level.
Correctly, if one chooses to feed the speaker output level or line output
level of a radio into the microphone input of a sound card then some
external means to attenuate the signal before the input of the sound card
must be employed. This can be as simple as 2 or 3 resistors configured in a
L network or T network to attenuate the signal some 20 to 40 dB. The same
approach works if one chooses to connect the speaker output or line output
of a sound card direct to the mike input of the radio. Again, a resistive
attenuator providing some 20 to 40 dB attenuation gets the levels down to a
manageable level.
Ideally, obtain a sound card for the computer that has line level inputs and
line level outputs. These can usually be connected direct to the input and
output of the radio if it provides for line level signals. Here again,
caution should be used if one elects to use the mike input and speaker
output or headphone output levels of the radio.
In summary, correct level and correct impedance must be observed in order to
maintain good signal-to-noise ratio and low distortion as well as maintain
reasonable control over audio levels. I find all too many cases of hum and
buzz are the result of improper wiring technique, improper levels or
improper impedance usage. To that end the use of "matching transformers" is
a band-aid, not a cure. If you must use a transformer, do understand that
they may be the source of induced hum.
Sound cards which provide for line level inputs and outputs can be easily
and successfully connected to the back of the Omni VI Plus without any
external devices or attenuators using only cables with the proper mating
connectors.
73
Bob, K4TAX
----- Original Message -----
From: "Mike" <gu3whn@cwgsy.net>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Sunday, January 15, 2006 3:41 PM
Subject: Re: [TenTec] RTTY on the Omni VI
> It is, of course, possible to drive the rig on digital modes without any
> attenuation into the line in or mic socket. It is my experience that many
> new ops do not appreciate the -28db IMD level required by PSK31,
> recommended
> by Peter G3PLX, to ensure a clean splatter free transmission. This is why
> I
> suggested in an earlier mail that it pays to keep an eye on the input
> levels. Relying on the soundcard settings alone makes adjustments more
> difficult but obviously not impossible.
>
> Will close on this one.
>
> 73 de Mike GU3WHN, Ex 9V1OB, DA2XS
>
>
> ----- Original Message -----
> From: "jack" <jfriend31@comcast.net>
> To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
> Sent: Sunday, January 15, 2006 8:48 PM
> Subject: Re: [TenTec] RTTY on the Omni VI
>
>
>> very well said, Bob.
>>
>> i have the Orion and have it directly connected to my computer and sound
>> card. it works fantastically with MixW and LOGic7. no needed interfaces
>> of
>> any kind for digital modes including CW.
>>
>> jack ak7o
>>
>>
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>>
>>
>
>
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