Something not identified in the previously posted info relating to the CVR:
Recording Channels
The recording system consists of four recording channels which simultaneously record audio as follows:
Channels 1, 2 and 3 have a narrow band and allow the recording of signals from the Captain, First Officer and 3rd Occupant via the Audio Management Unit 1RN.
Channel 4 has a wide band and allows recording of the ambient noises picked up by the area microphone and fed via a pre-amplifier to the input transformer for channel 4.
The three narrow band channels are recorded separately during the first 30 min. and are mixed together from 30 min. to 120 min.
This is written rather poorly, what is meant is that the most recent 30 minutes is individually stored, while data from channels 1,2 and 3 are mixed to a single track for 30-120 minutes.
So the recorder is not really a true 4 channel recorder for the 2 hours duration - the area mic is always individually stored, but we really have a hybrid 4/2 channel recorder.
This seems overly complicated and unnecessary given the low cost of solid-state memory. In fact the idea that the 'recorder' is manipulating (mixing and storing) the data once recorded seems astonishing to me... but then I suppose the design is some 20 years old or so (haven't checked that - it is a guess!).