today I learned that DTMF is "dual tone" because each key generated one tone for the row it's in and one tone for the column that's a really... brute-force implementation, probably typical for 1963pic.twitter.com/vnDaxggVju
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Did they obtain a TÜV certificate for their piano?
No. But there are a lot of obscure stories from that time. For ex, there was also a law that prevented you from electronically "picking up". So for operating a BBS you had to build a small robot to physically release the plunger (handset permanently in acoustic coupler).
