This equipment in the metal case was used to send high quality audio from the Rotterdam area (the pictures are from the Rotterdam Telephone Museum) via telephone lines rented from the ptt telecom (now privatised and called KPN).

The other end of the circuits terminated on the incoming lines area at Hilversum studios.

The red terminals are for the “muzieklijn” (music line, capable of 15 KHz bandwidth) and the black ones for the “spreeklijn” (talkback circuit) for voice comms with the production staff. the music line would be a mono circuit, two of these devices (and two music lines) would be required for stereo (though when this device was in common use not all outside broadcasts were stereo anyway).

This is what is inside (complete with a label so small items were not forgotten!).

the telephone has local batteries and a magneto generator to signal to the switchboard at Hilversum (or another telephone connected on the same line) as the “spreeklijn” may not have been an automatic telephone circuit that could dial external numbers. there is also an socket for an extension telephone (old PTT type), but if it was a magneto line to Hilversum local PBX switchboard (there were various reasons why the PTT preferred this) then when the operator answered the engineer could ask for a connection to an external telephone number (there are two phone numbers on the labels, one is for a control centre in Hilversum and the other in Rotterdam, those numbers will not work today as the digits are changed).

There are lots of U links and test points for connecting the programme audio and checking the circuit levels and voltages (the PTT were fussy about what maximum levels could go down a telephone line or the sound would end up on other peoples telephone circuits, and they certainly do not want 230V ending up on their telephone wires either)

In most of Northern Europe these devices were rendered obsolete in the 1980s by ISDN codecs, although similar ones appeared to be still in use by the BBC until the early 2000s (they contained a modified Post Office test telephone and often a mixer and PPM meter). British Telecom just provided a box for them which converted the lot to ISDN and charged Auntie a shit ton of cash for this device :laugh_at:

the original pics were here (I didn’t want to deeplink them and cane the museums bandwidth)

Welkom bij telefoniemuseum Rotterdam