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Help Date: 21 June 2004

 

 

Sound Card Interface

An interface is the collection of wires and circuits used to connect your radio to the your computer's sound card. The interface needed for Packet Engine Pro is the same as those used for other sound card digital programs, such as PSK31, so you may already have a suitable interface.

Typically an interface will have three (3) cables:

The TX and PTT cables will need some circuitry/electrical components to match the radio and computer -- see below. Note the stereo channel capabilities of a sound card allow you to interface with two (2) radios at once!

Kits/Ready-made Interfaces

Many vendors sell ready-made interfaces or kits. Do a web search for "sound card interface" or "PSK31 interface". The quality, time savings, and relatively low cost of such interfaces make them worth considering.

Build Your Own

You can also build your own interface. You will find designs on this page or you can look for others by doing the "sound card interface" search on the internet.

NOTE: You may find simple interface designs which use a direct (non-isolated) electrical connection from the sound card to the radio. These designs have two problems:
  • A difference in voltage potential between the radio and computer may damage one or both devices
  • The ground loop that results may introduce noise to your signal.

Some operators use these direct designs without problem, but it is better to use an interface design that electrically isolates the two components, as in the designs shown below.

If you build it yourself - General Info:

  • Make sure the cables are the right length- long enough to reach from computer to radio, but as short as possible to limit possible radio frequency interference (RFI) absorption.
     
  • Use a shielded cable to inhibit RFI. Use a cable with a single insulated wire and a braided shield. Small coaxial cable should work well. The shield can act as ground line but only if you are using an isolation transformer. If you aren't using a transformer, attach the shield at the radio ground only; do not attach the other end to the computer/sound card ground to minimize RFI entering the computer (you do not need to make a completed electrical circuit; the other device provides the ground).
     
  • Sound cards normally have stereo jacks, so you will need two 1/8" (3.5mm) stereo (3 conductor) male mini-plugs with tip, ring and sleeve (ground). Do not use a mono (2 conductor) plug. 
     
  • Transformers on the RX and TX cables should be 600 Ohm 1:1. The exception is that if you are getting the audio from the radio's speaker jack (8 Ohm impedance), then you should use a 1000:8 transformer. If you are using a 1000:8 transformer, it is important to connect the 1000 Ohm primary coil of the transformer to the sound card and not the radio. Hint: computer stores may give you discarded internal modems that have 1:1 transformers that you could use.
     
  • The connection at your radio varies by the radio. Consult your radio manual for its pin out assignments. You can also do a search on the internet for your radio type and the words "TNC pin out". Or do a search on "ka5ztx" or "k4abt" and look for "radio to TNC diagrams". Remember that all three cables will eventually attach to the radio connector, so plan for this.
     
  • For handheld radios, the TX line and the PTT line will both go into the radio's Microphone jack. Usually this jack accepts a 3/32" (2.5mm) stereo (3 conductor) or mono (2 conductor) sub-mini plug. If your radio's user manual says to use a stereo plug instead of a mono plug, be sure you do. In addition you will probably have to insert a resistor on the PTT line (to keep the TX audio's AC signal off of it) and a capacitor on the TX line (to keep the PTT DC voltage off of it).

    The RX line comes from the speaker jack on the radio and requires a 1/8" (3.5mm) mono (2 conductor) mini-plug. If your radio's User Manual says to use a stereo plug instead of a mono plug, be sure you do.

RX Cable

This cable brings received audio from your radio (microphone, data, or speaker jack) to the sound card. Use the radio's data jack if available, since it usually has constant volume and squelch. If you use the microphone or speaker jack, remember that RX audio will be affected by the radio's volume and squelch knobs.

1. Using the sound card's  LINE IN jack -- It is best to use the LINE IN jack of the sound card, if possible, since it is a better voltage match to the radio's output. If use the MIC jack of the sound card instead, you should use a simple circuit to reduce the voltage (see second drawing below).


 

2. Using the sound card's MIC jack -- If you use the sound card's microphone (MIC) jack instead of the LINE IN jack, then you will probably need to add an attenuation circuit on the RX audio line so that you do not overdrive the sound card. The exact attenuation will depend on the output voltage of your radio's RX line and the maximum allowed input voltage on the MIC jack. (Some examples: most Sound Blaster cards have a max. input voltage of 200 mV. The Kenwood TM-V7A outputs 500 mV on the 1200 baud RX pin in its data jack.)

Here's a schematic for a 10:1 attenuation circuit:

If you use MIC input, you should not use the Microphone Boost or the 20dB option. This option is not found in the PE Pro Volume settings, but it may be found as a button in the Windows' Recording Volume Setting window under the Microphone slider (you may need to use the Option: Advanced menu option to have it display). This option will increase the MIC input by 100 times, which could distort your packet audio. You should check that it hasn't been selected by default.

NoteIf you are planning a two port/ two radio cable, you can't use it and the MIC input jack. Typically, the ring in the MIC jack is used to provide a bias (+5V) voltage for electret microphones. It is not used for audio (right channel) input, so it couldn't be used for RX audio input from a second radio.

Also note that while a mono plug could be used in a MIC jack, it is better to use a stereo plug; just leave the ring unwired. A mono plug will short circuit the ring (voltage) to the sleeve (ground), although the card's circuitry is designed to cope with this (a resistor severely limits current flow).

Hint: When you're done, label the sound card end of your RX audio cable, either LINE IN or MIC, so you don't confuse it with your TX audio cable (LINE OUT).

.

TX Cable

This cable brings audio to be transmitted from the sound card's LINE OUT jack to the radio's microphone or data jack. It needs some components to reduce the sound card voltage so that it matchs the maximum allowed voltage on radio's TX circuit. The exact attenuation will depend on 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 (often a max. of 20 milliVolt p-p ). For example, a 50:1 attenuation would be better if your radio's data jack which has a maximum input voltage of 40 mV p-p.

1. Here's the first option for an attenuation circuit:

 

 

 

 

 

 

The value of capacitor C1, if used, is often specified in the radio's user manual in the section on MIC/Speaker accessory plug wiring.

2. Here's a second option that combines a pot with a fixed resistor to give the pot a less delicate range of settings and makes it easier for you to adjust the total attenuation.

 

PTT Cable

For the TX audio to be transmitted, you need a way to open the radio's PTT (Push to Talk/Transmit) circuit.

If your radio has a VOX (Voice activated transmission) feature, then the TX audio will trigger the PTT. Two things to be careful of with VOX are that 1.) you must guard against room noise being transmitted when the VOX opens, and 2.) you may need a longer TXDelay because VOX may take longer to key up. If you don't have VOX, you can build a VOX circuit that does the same thing (do an internet search on "Tone Keyer") and there is at least one commercial tone keyer, Tigertronics SignaLink.

The other alternative is to build PTT cable with a 'gate' circuit that opens the radio's PTT circuit. Packet Engine Pro provides for this by placing a voltage change on the RTS pin of your computer's RS-232 serial (COM) port or parallel (LPT) port when Packet Engine Pro has audio to transmit.

Here's a schematic for a circuit which uses a phototransistor, or optocoupler, as the gate. The optocoupler is a special transistor that electrically isolates the radio from the sound card. A simple transistor does not do this.

  • R1 = Resistor, e.g. 1K2, to reduce current on the IC pin 1
  • IC = Integrated Circuit; this sketch shows an IC, such as a 4N33 or PS2603 Optocoupler, which uses a Darlington pair transistor. (Note: to identify pin #1, look for a small embossed circle on the top of the IC above pin 1; or looking into the notch in one side of the IC with the pins down, pin #1 is to the right of the notch.)
  • D1 = Diode, e.g. IN4001, would shunt any potential reverse voltage that might damage the sensitive diode/emitter in the IC. (Note: the band printed on the diode marks the cathode end, which attaches to the Serial Port/IC Pin 1 line in the sketch above. The opposite/anode end attaches to Ground.)

Note that this circuit is used on the RTS pin if you are using the sound card in single port mode (left channel only). If you were using the card in dual port mode to control two radios, then a second circuit would attach to the DTR pin of the RS-232 port.

RS-232 Connections for PTT circuit - If you have a DB9 Serial port (COM), the RTS (Request To Send) pin is # 7 and ground is pin #5

If your serial port has a DB25 COM connection (rarer), RTS is pin #4 and pin 7 for ground is pin #7. Note this is the diagram of a DB25 serial (COM) port not a DB25 parallel (LPT) port.

To use the parallel port (DB25 LTP) for PTT control, use LPT's Data Bit pins 2 or 3 for the PTT line and any pin 18 thru 25 for the GROUND.

Hints:

  • To test your completed circuit, hook up the cable to your radio and then use a 9 volt battery to simulate the computer signal. Connect the plus (+) side of the battery to the RTS and the negative (-) side of the battery to ground pin. Your radio should transmit if the circuit is correct.

  • Note on handheld radios: Most handheld radios use the microphone jack to feed both the TX audio signal (AC voltage) and the PTT signal (DC voltage) into the radio. To make sure the TX audio signal goes into the radio and not back out the PTT line, you may need to add in a resistor in the PTT line on the radio side (AC will follow the path of least resistance and stay off the PTT line). See your handheld radio's user manual or consult the "TNC pin out diagram" web sites.

  • No serial port? Some computers now come without a serial port but with USB ports. A serial-to-USB adapter can be used to make a USB port emulate a serial port, and you can attach your PTT connection to them. Some users have reported problems with some adapters, but it is likely you can find a brand that will work.

Using Two Radios with One Sound Card - Because sound cards have stereo -- two channel -- audio capabilities, you can use one sound card to interface with two radios. To do this, you'll need a more complicated interface with a second set of RX, TX and PTT cables. Note: this feature will not work if you use the MIC jack for RX audio. It will work if there is a LINE IN input jack.

For the RX audio line, you will need to build a second cable -- with any necessary circuits -- that connects to the ring of the sound card jack (the right channel of the sound card). You will need to fit the RX audio line from both radio's into the stereo jack.

You will need to do the same thing for the TX audio line -- a second cable with another attenuation circuit that attaches to the ring of the sound card jack (right channel).

For PTT control, you will need to build a second PTT cable with a gate circuit that gets its signal from the DTR pin (Data Terminal Ready) in the RS-232 COM (serial) port or one of two Data Bit pins in the RS-232 LPT (parallel) connector.  Here is where to find the DTR pin:

  • DB9 COM (serial) port - PTT #2 is pin #4  (ground is pin #5)
  • DB25 COM (serial) port - PTT #2 is pin #20 (ground is pin #7)
  • DB25 LPT (parallel) port - PTT #2 is pin #8 or 9 (ground is any pin #18 to 25)

Remember that you will need to fit the PTT lines from both radio's into the RS-232 connector. Or you could make a more elaborate arrangement that lets you detach a cable when not in use (DB9 shown in this example; ground pins not shown but must be used):

You can find more information on this subject by doing an internet search for "sound card packet".

 

Copyright 2004 SV2AGW George Rossopoulos . All rights reserved.