Back in the old days of reel to reel, if you slowed a tape down recorded
from the fm stereo tuner you could clearly hear the 19Khz tone which was in
fact halved in frequency. I had until recently thought that cassettes
struggling on frequency response and doing the same there would not work.
but it does. A fault on an old Technics dbx decks motor control meant it ran
around half speed and on many Maxell tapes you could still hear that tone
when it was slowed down. Both of these show me that the so called mpx
fltering in tuners and decks are not really that good.
Brian

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Brian Gaff wrote:
--------------------------
> Back in the old days of reel to reel, if you slowed a tape down recorded
> from the fm stereo tuner you could clearly hear the 19Khz tone which was in
> fact halved in frequency. I had until recently thought that cassettes
> struggling on frequency response and doing the same there would not work.
> but it does. A fault on an old Technics dbx decks motor control meant it ran
> around half speed and on many Maxell tapes you could still hear that tone
> when it was slowed down. Both of these show me that the so called mpx
> fltering in tuners and decks are not really that good.
>

** The purpose of the MPX filter is not so much that someone might be able to hear a faint 19 kHz tone on tape playback, but the far more audible frequency resulting from a beat frequency with the record bias / erase oscillator.
A bias oscillator running at say 32kHz will beat with 19kHz to produce 13kHz on the tape.
Some cassette decks had a switch to shift the bias frequency up a number of kHz to mitigate this.

...... Phil

I never encountered that, but quite a few portable cassette radios did
manage to put birdies all over the medium wave if it was in record, and
there was often a control on the back to move the birdie off the frequency.
Mind you in some of the cheaper models they never used ac bias with he
effect of extremely noisy and low dynamic range recordings.
Brian

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The Sofa of Brian Gaff...
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Blind user, so no pictures please
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"Phil Allison" <pallison49@gmail.com> wrote in message
news:81568204-5724-4ced-9f99-0d2cd736c180n@googlegroups.com...
> Brian Gaff wrote:
> --------------------------
>> Back in the old days of reel to reel, if you slowed a tape down recorded
>> from the fm stereo tuner you could clearly hear the 19Khz tone which was
>> in
>> fact halved in frequency. I had until recently thought that cassettes
>> struggling on frequency response and doing the same there would not work.
>> but it does. A fault on an old Technics dbx decks motor control meant it
>> ran
>> around half speed and on many Maxell tapes you could still hear that tone
>> when it was slowed down. Both of these show me that the so called mpx
>> fltering in tuners and decks are not really that good.
>>
>
> ** The purpose of the MPX filter is not so much that someone might be able
> to hear a faint 19 kHz tone on tape playback, but the far more audible
> frequency resulting from a beat frequency with the record bias / erase
> oscillator.
> A bias oscillator running at say 32kHz will beat with 19kHz to produce
> 13kHz on the tape.
> Some cassette decks had a switch to shift the bias frequency up a number
> of kHz to mitigate this.
>
>
>
> ..... Phil