Audio Quality Investigations
There are a number of investigations running presently to look into this issue. NATS Engineering are taking it very seriously. We have to have specific instances reported to us (date, time, frequency, position) to correlate results and look for any patterns that may emerge.
A characteristic that has been uncovered is that a large proportion of aircraft receivers are not set up as well as they could be and where NATS uses two transmitters with off-set carrier frequencies to get as much coverage for a sector as possible, the receiver gets "confused" as to which transmitter to listen to and refuses to lift the mute. This results in apparent 'dead' spots for particular aircraft types using these receivers. There has been a safety bulletin covering this issue describing the symptoms and possible mitigation. Work at a European level is being done to change the receiver specification to eliminate the effect, but will take some time to put into place.
On 8.33kHz use: because a single transmitter is used (at any one time) on a frequency, the reported audio quality has improved, despite a slight reduction in the bandwidth. It can be used to overcome some effects related to the issue I describe above.
Audio level seems to be simply down to microphone position. End to end system checks using calibrated test equipment has shown that the audio levels are correct, but when a microphone is correctly positioned, the problem goes away.
If you would like me to keep you posted here as to progress on these issues, I am pleased to do so as it seems to be more avidly read than any other means of communication!!