audio quality. a number of services use the same bandwidth at different times. For example, CBeebies and BBC Four currently use the same space in their multiplex, with CBeebies broadcasting from 6am until 7pm and BBC Four from 7pm. some multiplexes allocate more bandwidth to services, providing a smaller number of higher-quality services. The modulation of the multiplexes can be varied to squeeze higher digital bitrates out of the same portion of the electromagnetic spectrum, but require a stronger signal for good reception. The modulation schemes used in the UK are, in order of bandwidth efficiency, each with a progressively higher bitrate, at the cost of progressively higher likelihood of signal degradation: QPSK (only used for tests in the Oxford and London areas) 16-QAM (no longer used as of digital switchover) 64-QAM (only used on the DVB-T multiplex) 256-QAM (only used on the DVB-T2 multiplex) 1024-QAM 2048-QAM (currently in development) By late 2009, multiplexes 2 and A used 64-QAM and were consequently more prone to poor reception, while the other multiplexes used 16-QAM. At switchover the transmission mode was changed from 16-QAM to 64-QAM on Multiplex 1 (PSB1), increasing the effective bandwidth of the multiplex. The switch to 64-QAM mode also provided extra bandwidth on Multiplexes C (COM5) and D (COM6). By late 2012, the digital switchover was complete, with all DVB-T multiplexes using 64-QAM. The switchover allowed the transmitters to broadcast at a higher power level, reducing the likelihood of reception errors when receiving 64-QAM encoded broadcasts. Multiplexes can make use of statistical multiplexing at the MPEG video coder whereby the bitrate allocated to a channel within the multiplex can vary dynamically depending on how difficult it is to code the picture content at that precise time, and how much demand there is for bandwidth from other channels. In this way, complex pictures with much detail may demand a higher bitrate at one instant and this can result in the bitrate allocated to another channel in the same multiplex being reduced if the second channel is currently transmitting pictures which are easier to encode, with less fine detail. Improvements in compression technology Developments in statistical multiplexing, improved compression technology, and, in some cases, an acceptance of lower quality or lower resolution broadcasts, allowed gradual increases in the number of services carried on digital terrestrial television multiplexes. As the number of homes receiving digital terrestrial grew, the interest in new