This was a major advance which gave nearly everyone in the UK good quality stereo services.

The television SiS system used analogue companding and 10 bits per sample to squeeze sound information of sufficient quality into the limited capacity of the sync pulses.  There was less constraint on the PCM system for distribution of the radio services as plenty of linearly coded channels, using 13 bits per sample, could be accommodated within the capacity of a vision circuit.  (15kHz music circuits and 5.5MHz vision circuits were effectively the only choice of bearer circuit at the time.)
 

Telephone networks started to move towards digital technology and the standardised bitrate of 2048kbit/s (designed for 30 voice channels) looked attractive for conveying broadcast quality sound.  Given the potential cost of such circuits and the need for stereo, companding was required to put six mono channels (or 3 stereo) of music quality into a bitstream of 2048kbit/s.  BBC Research Department met this challenge by inventing "near instantaneous" companding, which gave better sound quality compared with instantaneous non-linear companding such as "A-law".  The system was developed by Designs Department and given the name NICAM - Near Instaneously Companded Audio Multiplex.  (NICAM as used on TV came later.)

Although 2048kbit/s circuits started to become available from the Post Office, the tariff was too high as it was related to multiples of relatively expensive low bitrate (e.g. 64kbit/s) data circuits rather than the inherent data capacity of broadcast quality vision circuits, for which the BBC had a long term tariff agreement.
 

An unusual form of mux/demux was produced by Designs Department which initially enabled 12 linear channels from the 13 channel PCM system to be multiplexed with one 6 channel NICAM coder.  This met an initial need for more channels (13 up to 18 mono channels that could be paired for stereo).  The mux/demux also offered the possibility of migration to 8 linear+12 NICAM and then 4 linear plus 18 NICAM.  However, in reality the transition was made straight from 12+6 to 24 NICAM in 1993.

Meanwhile the original Mark I NICAM equipment was made obsolescent by the design of Mark II NICAM which was far more compact.
 

With several bays of equipment involved at each site, obviously a great deal of system design and testing was required. Various backup arrangements were in place but there was no going back - it had to work - and it did.

Failure could have meant the loss of all the BBC's national radio services for a significant period.  So, no pressure then!