Sean, all analogue magnetic tape recorders use some kind of high frequency
bias, even your cheapo walkman (as long as it records).  With out the bias
the recording would be very poor.  Have a look at
http://www.vex.net/~pcook/rec.audio.pro/analog.html (section Q4.2) and it
explains.

Mike Rigby-Jones

> -----Original Message-----
> From: Sean H. Breheny [SMTP:shb7@CORNELL.EDU]
> Sent: Wednesday, October 06, 1999 7:11 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: OT: Wire audio recordings
>
> This discussion is confusing me more and more as it continues ;-)
>
> At 06:44 PM 10/5/99 +0100, you wrote:
> >The bias is to overcome the hysteresis (I'm not sure about that
> >spelling!) in the recording medium. As you say, modern recorders use
> >about 100KHz for the bias, the recorded audio is added to this, and
>
> If this is so, then why when you look at the specs do they give the
> frequency response of the recording head as something like -3dB at 19kHz?
>
> Also, if I understand magnetic recording, you NEED hysteresis to make a
> recording (in order for the tape to effectively retain the information,
> the
> tape must be magnetized, which means that you are on one of the flat parts
> of the hysteresis loop). The audio itself must overcome the hysteresis and
> reverse it (if needed) in order to be strong enough to produce a good
> recording. I don't see how adding an additional 100kHz signal helps this
> process. I would assume that the hysteresis loop would be a lot more
> linear
> at 100kHz (and have a LOT less slope than in the vertical region of the
> low-freq. loop).
>
> I just want to make sure that we're talking about the same thing: I'm
> talking about the walkman that I buy at the corner store for $25, not some
> high-end specialty audio recorders or digital decks.
>
> >amplitude modulates it. The bias is usually a fairly high signal level,
>
> How does adding two signals together (in a roughly linear system) cause
> one
> to modulate the other?
>
> >up to 100 volts peak to peak, and the audio is very low, only a few
> >100mV. This makes it very hard to see the modulation on an oscilloscope,
> >sometimes you can just see it with the gain turned up and the vertical
> >shift fully one way :-).
>
> I don't know where in a normal tape deck there would be a 100Vp-p 100kHz
> supply!! I am thinking more and more that we are talking about different
> things.
>
> >--
> >
> >Nigel.
>
> Sean
>
>
>
> >
> >        /--------------------------------------------------------------\
> >        | Nigel Goodwin   | Internet : nigelg@lpilsley.demon.co.uk     |
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> >        | England         |                 Ju Jitsu                   |
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> >
> |
> | Sean Breheny
> | Amateur Radio Callsign: KA3YXM
> | Electrical Engineering Student
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