Silicon Laboratories SI2494/39, SI2493/57/34/15/04 When to Use Audio Recording, Hardware Setup

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APPENDIX B—LINE AUDIO RECORDING

Recording and examining the audio signals on the phone line is one of the best debugging techniques for PSTN modems. Virtually all the relevant signals are in the audio spectrum and are easy to acquire using a standard PC sound card and accessory hardware and software that is especially designed for music creation and analysis.

The required hardware is a Radio Shack Catalog No. 43-228A “Recorder Control”. It can be used with any computer equipped with a 3.5 mm audio-input jack.

The resulting wave can usually be recorded in the field using the computer’s operating-system resources, but for analysis, a software package capable of showing the spectral contents as they change over time is recommended. The two most widely used ones are Adobe Audition, a commercial product, and WaveSurfer, which is a free open- source product that runs on Windows, Linux, OS X and Unix. This audio-recording technique does not replace sophisticated test equipment, but it is quite useful in showing up some faults in the line and in the modem's (DUT) negotiation with the device on the other side of the phone line.

When to Use Audio Recording

This technique is best used when the modem appears to connect normally against some servers but does not connect well when calling a specific server or modem. This implies the hardware is functional and the issues most likely involve the negotiations between the modems during connect and retrain. One way to rule out the possibility of a hardware problem is to call the server or modem where the connect issue is found using the Silicon Labs EVB module.

Times When Audio Recording May Not Help

Some signals are exceptions and cannot be monitored in this way due to the limits of the bandwidth examined. Examples are the dc voltage and currents that exist during both on- and off-hook conditions, precise details of the pulse dialing waveforms, and most EMI signals. EMI signals, which are not visible during the recordings, may produce in-band demodulated and cross-product signals in the modem.

Some in-band signals cannot easily be monitored this way because they are common-mode signals. While they may be less visible to the recording apparatus, they can be received by the modems in some cases. An example of this is a strong common-mode 50 or 60 Hz hum with its harmonics (a 50 or 60 Hz buzz).

Hardware Setup

The Radio Shack Recorder Control contains a transformer that bridges the phone line with a dc-blocking capacitor, plus a voice-operated switch output that starts and stops a recording device. We only use one of the output connectors since we are not interested in the VOX mechanism. Connect the audio output connector (a 3.5 mm O.D. connector) to the microphone input socket at the back of the computer. The RJ11 connector from the Recorder Control should be connected to the Tip and Ring of the phone line being monitored.

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