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Audio Cable Connection Process for Audio Tests (Windows Embedded CE 6.0)

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1/6/2010

This topic describes the audio cable connection process that is required to run the audio quality tests.

Installing Audio Cables

To run the Voice over IP (VoIP) audio quality tests for Windows Embedded CE, you must install the required audio cables. The hardware engineer or hardware technician is responsible for constructing the audio cables. The VoIP audio quality tests can help verify specific aspects of audio quality on your Windows Embedded CE powered device.

The following two cables are required for the VoIP audio quality tests:

  • Loop-back cable: This cable feeds the audio output of the device being tested back into its audio input.
  • Computer connection cable: This cable connects the device being tested to the computer that plays or records test audio.

Note

The computer connection cable can be separated into two cables, one for each direction (computer-to-device, device-to-computer).

To construct the audio cables:

  1. Assemble the Loopback audio cable by following these steps:
    1. Obtain a headset plug for the device being tested. This may be a wired headset that is disassembled for the creation of the loopback cable.
    2. Create a resistive voltage divider between the right output audio wire and the ground wire: connect a 1 KB ohm resistor (R1) and a 10 ohm resistor (R2) in series between right output audio wire and ground wire. Optionally, replace one of these resistors with a potentiometer for easier configuration.
    3. Connect the microphone wire to the voltage divider, with a 220 uF capacitor (C1) in between them. That is, connect the microphone wire to one end of the capacitor and the other end to the net between R1 and R2.
    4. Add a 2 KB ohm resistor (R3) between the microphone wire and ground wire.
    5. Leave the left output wire unconnected.
      The following illustration shows the schematic diagram for the loopback cable:
      Loopback cable
  2. Assemble the computer connection cable by following these steps:
    1. For device-to-computer, no voltage divider is required because the output of the device is typically not biased. Connect the Left and Right audio wires from the connector on the device to the line-in left and right channels, with a 220 uF capacitor in between them for filtering.
    2. For computer-to-device, a voltage divider is required because the microphone is typically biased. Use only the Right output channel on the computer, and connect to the microphone wire on the device being tested. Notice that this step is similar to the previous loopback cable step. Create a resistive voltage divider between the right output audio wire on the computer and the ground wire: connect a 1 KB ohm resistor (R1) and a 10 ohm resistor (R2) in series between the right output and ground. Optionally, you can replace one of these resistors with a potentiometer for easier configuration.
    3. Connect the microphone wire to the voltage divider, with a 220 uF capacitor (C1) in between. That is, connect the microphone wire to one end of the capacitor and the other end to the net between R1 and R2.
    4. Add a 2 KB ohm resistor (R3) between the microphone wire and ground wire.
    5. Connect the ground wire from the computer line-in, computer line-out, and device connectors together.
      The following illustration shows the schematic diagram for the computer connection cable:
      Computer connection cable
  3. Adjust the resistor values to calibrate audio gain.
    • Play back and record a full-scale sine wave at -1 to -3 dB. For the playback procedure, see Audio Quality Test.
    • If the recorded peak of the sine wave from the audio file is less than -10 dB, increase the gain by decreasing the value of R2 or by increasing the value of R1.
    • If the recorded sine wave from the audio file is clipping, decrease the gain by increasing the value of R2 or by decreasing the value of R1.

See Also

Other Resources

Audio Tests