Training Manual - Part Two: Using The Desk (and an entry about the Point 13 Mod that's much too long)

The Point Thirteen Modification was a thing of beauty.

Back in the day, before that new fangled ISDN came on the scene, your BBC Local would lease high quality "music lines" from BT to connect it with the local football grounds and any district studios or radio car receivers. These lines were one way, allowing contributions from the OB, but with no return feed from the studio. So you also leased a control line, a two-way circuit with much lower bandwidth which also allowed you to ring down the line to get the attention of whoever was at the other end. It was considered polite to always try talkback down the control line before ringing - you didn't want to ring in someone's ears.

So, on your Point 13 modded desk the music line would appear as an OS numbered between 1 and 12 which you would select on the top ring of the OS selector. This gave you the incoming circuit on a fader, but no comms. To get the control line associated with that source you'd also dial up point 13 on the bottom selector. Now you could send cue to the OB (no need for CF because there were no pesky digital delays to worry about) and chat away on the talkback as needed.

But some sources didn't have control lines, such as NCA studios at other BBC Locals, or GNS two-ways from London. For these you needed to ring a phone in the contributing studio. If you diverted that call to the desk you could select the TBU on your bottom OS selector and use that as a temporary control line.

And that was the brilliance of the Point 13 Mod.