标签: frequency

Before magnetic tape recorders appeared on the scene, their predecessors worked by rapidly pulling a wire over a recording/playback head. To be honest, I'd never even heard about these little beauties until a guy called Nick Gent told me the tale of how his grandfather had built his own wire recorder and made recordings of the family, including Nick's great grandparents and even his mother as a child. Thus it was that, about a year ago as I pen these words, I wrote an article about magnetic wire recorders ( click here to see that original article). Well, it's amazing how these tales keep on growing in the telling, because I was just copied on an email sent to Nick from a guy called Jim Wood who is in the process of constructing his own magnetic wire recorder from the ground up. If there is a right man for the task, that man is Jim. I've since discovered that he and Mark Drake co-founded a company called Inovonics back in 1972. Jim's company is best known for producing high-quality broadcast audio processors, RDS/RBDS encoders, and AM/FM modulation monitors. In his email to Nick, Jim explained that he was Googling "Wire Recorder" when he ran across my article on EE Times and read the story of Nick's grandfather. This made Jim think of an old advert from a 1949 issue of Wireless World magazine (shown below), which advertised parts for experimenters to construct their own wire recorders ("More thrilling than Radio—More gripping than Television"), and which caused Jim to wonder if Nick's grandfather may have been inspired by something like this: Nick responded that Wireless World was indeed quite likely to have been the inspiration for his grandfather, who was avid reader of hobby electronics magazines. Nick also noted that his grandfather probably paid a few visits to Park Radio—the shop advertising the wire recorder parts—because he lived only 20 miles from that establishment. But we digress... It seems that Jim has been a passionate sound recording enthusiast for the better part of 60 years. When Jim was about 13, his dad bought him a used wire recorder, which became one of his most prized possessions. Even after Jim had graduated to tape later in the 1950s, the massive speaker in his wire recorder console cabinet was relegated to subwoofer duties for several years more. Recently, Jim embarked on a project: to build a top-quality wire recorder employing present-day components and techniques. He is using a Webster Chicago mechanism, but is building all of the electronics from scratch. Below we see a shot of Jim's test bed, which he's using to develop the record and playback amplifiers and equalisation circuits: And in the photograph below we see a more detailed view of the breadboarded electronics with annotations explaining the various bits and pieces: Jim says that, thus far, the major hurdle in the design has been accomplishing good low-frequency response. Due to the high wire speed (24 inch (convert to mm except for displays)es (convert to mm except for displays) per second) and the small geometry of the record/reproduce head core, the initial response exhibited severe "head bumps" and egregious deviation from flat low-end response, as illustrated below: Since then, Jim has changed the erase/bias circuitry from a linear amplifier to a power oscillator, which gives a slightly quieter recording. He's also implemented what he describes as "some fancy 'dip' filters" and performed a lot of SPICE simulations to achieve the required equalisation, which is illustrated in the graph of the final overall frequency response shown below: As an example, Jim attached a few seconds of an MP3 recording he made of a transfer from a vinyl LP to his wire recorder ( click here to hear this recording). In his email to Nick and me, Jim modestly noted "It doesn't sound bad at all." I don't think Nick agrees with this assessment, because he immediately responded "The quality of that recording is stunning!" I have to admit that I'm with Nick on this one. Jim says his job is demanding a lot of his time right now (I know that feeling), but when he gets back to this he plans to put it all together in a nice box of some sort and then "do some 'unique' location recording." Jim also says that when the whole thing's finished, he plans on "doing a proper write-up of the project," which I hope he will share with us.