AMC, or AM companding, is currently used on all high-power BBC LF and MF transmitters to reduce electricity consumption. The BBC-designed equipment carrying out the companding is the AM6/34A. In essence, AMC system reduces the carrier power at high modulation levels, the argument being that the modulation will then mask any increase in background noise and interference. As a bonus, the AGC system in the receiver will tend to compensate for the changes in carrier level, hence making the overall system more transparent. At present, companding of 3 dB is applied.

With the BBC anxious to cut costs and establish its Green credentials, the hope was to increase the companding to 6 dB. Hence a joint project was set up between the BBC and Arqiva (the transmission providers) to evaluate the feasibility of this, using a combination of laboratory tests and field measurements. The field measurements involved the transmitters at Washford and Droitwich. This report describes the work carried out.

The important conclusions are as follows: - The existing AMC system introduces appreciable distortion artefacts because of its very short attack time. This distortion becomes worse as the amount of AMC increases. - The distortion artefacts can be rendered negligible by lengthening the attack time, and 50 ms is recommended in this report. The effect on electricity consumption is negligible. - Although the longer attack time causes the transmitter power to overshoot during the attack, no harm results; in fact, the effect on system performance is beneficial. - When background noise is present at a level equivalent to that at the edge of the service area, the degradation introduced by 6 dB companding corresponds to a loss in transmitter power of less than 1 dB, and is negligible in practice. - Extensive field-survey measurements of both sound quality and field-strength confirm that the increase in AMC does not have any detectable ill-effects. - The long-term measurements made at Droitwich indicated electricity savings of £86k for the Radio 5 transmitters alone, and £155k if the transmitters for Radios Ulster, Scotland and Wales were to be included. This assumes an electricity cost of 10p per unit. - Although worthwhile, the above savings are rather less than predicted from the laboratory work and the initial trial at Washford. The differing programme content could be a factor. It is also possible that a change in transmitter alignment could retrieve part of the loss. Modifying the AM6/34A Compander is straightforward. The amount of companding is set by means of an internal handbag link, and the increase in attack time involves adding two capacitors. The Compander already has provision for these components.

This publication is part of the Distribution Core Technologies section