FM vs MPX-FIL on source selector.

[Rod Beauvex]

I have a small Lafayette LR310 stereo reciever that has positions on its source selector for FM and MPX FIL, and there doesn't appear to be much difference between them. They both recieve in stereo, and seem to tune the same. The only difference is that the highest frequencies seem more muted on the MPX FIL setting, which seems kind of redunant as I have a High Filter button. Could anyone shed some light on the purpous of these two different settings?

 

[danj]

Could it be short for "multiplex filter", a filter for the FM tuner 19kHz pilot tone that messes up the tracking on some Dolby circuits? If so then it might have an audible effect if the filter slope is pretty broad. Hopefully it would be designed to not noticeably affect frequencies below 15kHz. In any case, you don't need to use it unless you are recording FM onto a tape deck, and then only if the pilot tone causes the Dolby to audibly mistrack. Most tape decks have a MPX FIL button and that's what I would use instead of the one on the receiver. I have never had to use the Multiplex filter and I purchased my first Dolby-equipped deck in 1974. Most tuners have adequate internal supression of the 19kHz tone.

I've never seen a receiver with a switchable multiplex filter built in. Does that receiver include a Dolby "B" encoder/decoder? If so, that explains the filter switch.

 

[Rod Beauvex]

I figured Multiplex Filter is what it meant too. It is not equiped with Dolby FM, although I have seen recievers with that.

 

[Old_Tech]

The FM Multiplex filter is used to help reduce noise generated in the FM audio detection circuits. I dare say in most tuners it is nothing more than a low pass filter as you have seen the high frequencies are affected somewhat.

The "MPX Filter" on better tape decks is the same type of filter and should be used if there is MPX noise when recording FM. It should not be used during playback.