Silicon Laboratories SI2493/57/34/15/04, SI2494/39 manual Audio-Recording Pitfalls

AN93

Audio-Recording Pitfalls

To facilitate communications protocol debugging, it is imperative that audio recordings be made properly. The two most common conditions that degrade the quality of audio recordings are:

"Waveform clipping due to excessive recording level

"Time-varying levels due to use of AGC (automatic gain control)

Figure 62. Recording Made at Excessive Level

The above recording was made at an excessively high level. One can see clipping in the time domain and numerous distortion products in the frequency domain.

Figure 63. Recording Made with AGC Noise Reduction

The above recording was made with AGC "Noise Reduction" still enabled, as can be seen from the gradual level drop in the time domain graph at the start of V.22 negotiations. It also shows evidence of a microphone being used instead of the Radio Shack adapter. This is visible in the frequency-domain graph as horizontal striations (an undulating frequency response) during the scrambled portion of the V.22 communication. One can also see third- harmonic distortion.

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