Transmit Audio Cable

Posted on Feb 6, 2014

The TX audio cable can be a little more complicated then the RX audio cable. Normally, the TX audio cable must have a circuit to attenuate the voltage leaving the sound card`s LINE OUT jack, otherwise the radio`s transmit circuit will be overdriven. Note: Use the LINE OUT jack, do not use the SPEAKER jack (which is found on some older sound cards)

Transmit Audio Cable
Click here to download the full size of the above Circuit.

. If your card or laptop only has a HEADPHONE jack, it can be used, but you will need to lower the sound card`s TX audio volume. Quality will usually not be quite as good as a LINE OUT. The exact attenuation will depend on both your radio and sound card. An approximation is a 100:1 attenuation (40 dB) which will reduce the sound card output level (max. 2 Volt p-p) down to the level your radio normally would expect for microphone ( 20 -40 milliVolt p-p ). For example, a 50:1 attenuation works better for me, since I use my my radio`s data jack which has a maximum input voltage of 40 mV p-p. Don`t Overdrive Your Radio! If a sound card (or a TNC) overdrives a radio`s input circuit (i. e. it is too loud), the radio may distort the true packet tones as it tries to limit the signal to a maximum deviation of approx. 5 kHz. The attenuation circuit described on this page attempts to prevent such overdriving and distortion. Interestingly, a 1200 baud packet station that is properly adjusted with 3. 25 kHz of deviation (+/-. 25) should sound softer than channel noise! Option #1: Instead of the two resistors, you could use a 4K7 (4. 7K) Ohm variable resistor (potentiometer/pot). The pot would actually give you more control over the attenuation, although a pot is bulkier. Option #2 : Or here`s a schematic of a circuit that combines a pot with a fixed resistor to give the pot a less delicate range of settings and make is easier to adjust:...

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