# Average power of SSB telephony – experimental verification

Average power of SSB telephony used 80 year old research by (Holbrook and Dixon 1939) to come up with a ratio of peak voltage to RMS voltage of a voice waveform, and from that derive the ratio PEP/Pav..

(Holbrook and Dixon 1939) explored the subject measuring the voice characteristics of many talkers (as there is variation amongst talkers) to come up with an average characteristic.

Whilst in its day, obtaining instantaneous samples of voice was a challenge, it is trivial today and if you can’t believe the numbers given, try your own experiment (but realise it is for your own voice rather than the general population).

Many modern PC sound applications are capable of the measurement, I will demonstrate it with the feed Windows application Audacity with the stats.ny addin.

Above is a screenshot of a 6s recording of my voice made without stopping for breath. The statistics window shows a peak of -8.9dBFS and RMS of -27.4dBFS, giving a peak voltage to RMS voltage ratio of 18.5dB.

On repeated trials it is within tenths of a dB. If you try the experiment, keep your voice level constant, don’t stop for breath, don’t pause as you might in reading sentences as all these will result in an overestimate of instantaneous peak voltage to RMS voltage. Make sure the peak is well less than 0dBFS, otherwise you will underestimate instantaneous peak voltage to RMS voltage.

(Holbrook and Dixon 1939) gave the graph above which characterises the ratio of instantaneous peak to RMS voltage of voice telephony for different numbers of channels in a multiplex and different expectation of overload or clipping.

My measured instantaneous peak voltage to RMS voltage of 18.5dB reconciles well with (Holbrook and Dixon 1939) approximate operating limit (the dashed line) for n=1 channels.

Remember that PEP is 3dB less than the instantaneous peak voltage indicates, so in the measurement  PEP/Pav=18.5-3.0=15.5dB. The chart suggests PEP/Pav=18.0-3.0=15dB.

If you have seen figures of Pav/PEP of 20% (-7dB) or PEP/Pav=5 (7dB) bandied around for uncompressed SSB telephony without experimental evidence or explanation, you might question the credibility of the source.

## Conclusions

This is a measurement at source of my voice and the recorded data shows that no clipping took place.

Different voices may produce different results.

Lower Peak/Average is almost always a sign of non-linearity (eg peak clipping, compression etc) and warrants further tests of system linearity.

## References

• B D Holbrook and J T Dixon. Oct 1939. Load Rating Theory for Multi-Channel Amplifiers” in Bell System Technical Journal, Vol. 18.