From: liz@poppyrecords.invalid.invalid   
      
   Cursitor Doom  wrote:   
      
   [...]   
   > No, it's not coming in bursts. As I said in my original post, it   
   > really does sound for all the world like an old 45 vinyl record that's   
   > been played with a worn stylus. Then a few minutes later it might be   
   > fine again - for a while.   
      
   That distortion is a combination of intermodulation and   
   noise-behind-signal.  Really severe tape squeal is the only thing I can   
   think of that sounds like that on a tape recording   
      
      
   > I made up a test tape of a 5khz sine wave using my Ferrograph deck and   
   > played it on the Uher. I could hear the distinct difference in quality   
   > cutting in and out periodically when I played it back. I then tried to   
   > make another test tape of the same tone on the Uher to play back on   
   > the F'graph, but didn't have sufficiently low source impedance from   
   > the generator in this instance so I had to use a radio broadcast   
   > instead. Not ideal, but the distortion is still readily noticeable on   
   > high quality audio recordings.   
      
   Have I understood this correctly?   
      
   1)   A tape recorded on the Ferrograph shows intermittent distortion   
   when it is played back on the Uher.   
      
   2)  A tape recorded on the Uher shows similar intermittent distortion   
   when played back on the Ferrograph.   
      
   3)  The distortion occurs at the same place each time the tape is played   
   back and isn't related to signal level.   
      
      
   By elimination: 3) rules out any problems in either playback amplifier,   
   2 ) shows the problem isn't in the Ferrograph recording amplifier and 1)   
   shows the problem isn't in the Uher recording amplifier.  The only   
   things left are the tape itself or one of the transports.   
      
   Years ago there were problems when one or two tape manufacturers   
   produced tape with a backing that caused squeal in pressure-pad machines   
   (which both the Uher and the Ferrograph are) but you would be able to   
   hear that by turning down the sound and listening to the tape going past   
   the heads.   
      
      
   I'm surprised the signal generator didn't have low enough source   
   impedance to feed a signal into the Uher; most audio signal generators   
   have an output impedance around 600 ohms and most Uhers have inputs of   
   around 600 ohms and 'high' (around 47k).   
      
   Why were you using 5Kc/s as your test tone?  The second harmonic of   
   that, which is the first spurious frequency created by distortion,   is   
   10 Kc/s which you may not be able to hear and which won't be recorded at   
   any tape speed below 7.5 ips.  (Intermodulation distortion would be   
   audible as spurious low frequencies if there were other tones present,   
   but you wouldn't get them from a single sinewave tone.)   I would have   
   chosen something around the middle of the audio band  e.g. 630c/s  so   
   the harmonics fell in the area of the greatest hearing sensitivity.   
      
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
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