Tuesday, April 27, 2010

SSB Audio Compression

This morning George KW7GAM asked for reports about how his audio sounded on 3740 kHz with and without compression. Click here for the audio recording made from my transceiver. The screen shot above shows the audio waveform as displayed in Cool Edit 2000, with text labels added to identify the two compression states. Click on the image for a larger view.

The appearance of more black space in the right-hand, uncompressed portion of the audio waveform supports the expectation that average audio power is lower for that state. The software allows for selecting a portion of the waveform for statistical analysis. Here are some data for the two portions of the overall waveform:

Compression ON OFF
Minimum RMS Power: -37.89 dB -39.54 dB
Maximum RMS Power: -10.5 dB -11.23 dB
Average RMS Power: -17.56 dB -19.63 dB
Total RMS Power: -16.48 dB -18 dB

The two histograms below also show the difference. The graph for compression ON (lower left) shows a greater number of samples near the high amplitude (right-hand) side of the graph, while the graph for compression OFF (upper right) shows more samples spread across the amplitude range. The X-axis of these graphs provides the amplitude scale in dB relative to some reference not explicitly stated by the program.

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