[sci.electronics] Defeating MPX

frank@snowy.rice.edu (Franklin Tae-Sung Kang) (06/11/91)

Does anyone know what the MPX filter on a tape deck consists of?
I have a cheapo JVC tape deck I use to make recordings for my
car stereo, but since I don't record FM I'd like to disable the
MPX filter in the hopes that the high frequencies will improve
from CD recordings.

My guess is that it's just an op-amp filter, but I'd like
some input from someone in the know.

Thanks in advance

Frank Kang
frank@owlnet.rice.edu

strong@ee.rochester.edu (John Strong) (06/13/91)

How about simply turning the filter off when making CD recordings?
There should be a switch that allows you to do this. The multiplex
filter is supposed to attenuate noise from FM signals that are
being decoded (for stereo separation). If there is no switch to do
this you will need a schematic of the unit to eliminate this area
from the signal path.

John Strong

agc@briar.philips.com (Aldo G. Cugnini) (06/13/91)

In article <1991Jun12.191926.24744@ee.rochester.edu>,
strong@ee.rochester.edu (John Strong) writes:
|> The multiplex
|> filter is supposed to attenuate noise from FM signals that are
|> being decoded (for stereo separation).

Not quite.  The filter is usually intened to remove the (residual)
19kHz pilot tone from the audio output.  If this signal is strong enough,
it will cause the Dolby encoder to malfunction.

The filter is usually a 19kHz notch filter, but in a cheap receiver could
be just a lowpass filter.


agc