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I am hoping this is a pretty easy one. There is already DSP for EQ, Crossfeed, and such.

Just wanting a polarity flip. Just a simple request, but it would also be nice just to flip L and R independently.

Not sure if Sox or some other add-in has something that can do this.
Are you asking for Polarity (+/-) reversal or for L/R channel swap?

Whats the usage scenario?

My 2cents .... Polarity (+/-) for absolute phase of system. :-)

As an aside re L/R channel swap.. does mpd have a balance control option ? My system has no balance control but the room requires one...(odd shaped and vaulted ceiling)... or possibly adjust L/R volume independently.. ? or is this available in alsa..hmmm .. will go look ...meantime your comments re a balance control would be appreciated.
There is no balance setting in MPD but ALSA maintains individual channel volumes. You would have to use amixer or alsamixer to change the volume of an individual channel.
I second the request for a button to flip polarity. For an explanation, why this is needed, see e.g.
Despite being easy to demonstrate (interchange the two speaker cables at each of your speakers), this effect is not widely known and even denied by some people.

A few amps have a dedicated switch for this (like the TACT Millenium), unfortunately mine does not. A great number of recordings suffer from reversed polarity (which allegedly sounds better when listening with headphones), see

Daphile, which is only available for x86-based machines, has such a button, although hidden deep inside the menu system.

In sox the reversal can be accomplished by multiplying the volume with -1:

sox -v -1 <in-file> <out-file>

Ideally, the library would store the information about the polarity of each track and play it accordingly without further user intervention, apart from telling the library once that this particular track needs to be inverted.

Best regards,

-- Jürgen
Are you referring to polarity or phase, and how is it possible to know which one or if both were used, or neither were used in the mix/mastering of a recording?

I'm referring to polarity, i.e., turning the waveform into its mirror image wrt. the time axis. The original wave form and the inverted one would sum to zero, which need not be the case for a phase shift of 180 degrees.

The only way to determine the correct polarity is to listen to both variants, and for this purpose a direct switch would be most helpful (instead of having to reverse the plugs in both speakers).

More concretely, when say a base drum is hit, the loudspeaker cone initially is supposed to move outward. In case of reversed polarity it will move inward instead. Due to the way sound is perceived in the human ear, the difference is audible.

Of course, when a music piece is mixed from several tracks, these may already display different absolute polarities, so for the end result it may be difficult to say which one is the right one. Also, cross-overs in 2- or 3-way speakers can interfere by reversing polarity in some frequency ranges but not in others. Nevertheless, when somebody is singing, one polarity setting should enable the listener to better localize the singer on the sound stage.

[For some recordings it is also beneficial to adjust the phase of one channel separately. Between 1968 and 1970 a number of albums were mastered using the HAECO technique, which allowed to produce a decent mono record from a stereo mix by just combining the two stereo channels. By phase-shifting the right channel by 90 degrees, the components present in both channels are not amplified unduly, however the quality of the stereo mix is diminished. Cream's "Wheels Of Fire" was a prominent victim of this technique, other albums can be found on the net. Fortunately, this effect can largely be reversed with suitable software, and there is no real need to have this available during regular playback.]

-- Jürgen

(09-24-2018, 11:28 PM)Tim Curtis Wrote: [ -> ]Are you referring to polarity or phase, and how is it possible to know which one or if both were used, or neither were used in the mix/mastering of a recording?

Interesting stuff but in any case, I don't know of any way to do Polarity reversal in ALSA, the low-level audio layer that is used by moOde but if someone knows how to code this I'll be happy to investigate.

At people are discussing how to flip the polarity of one channel for headphone use (strange idea), but a "mrfibble" has posted his alsa-configuration in order to achieve this. Perhaps this could be a starting point?

-- Jürgen
Hi Jürgen,

It could be a starting point except that the author simply posted a blob of ALSA script with no explanation of how the code is achieving polarity reversal. This makes it vary difficult to adapt unless one is an expert in ALSA which I am not.

I'd suggest posting something like "Can Polarity Reversal be done in ALSA?" in the diyAudio Forum link below. Maybe one of the experienced ALSA guys for example Charlie Laub, will offer some help. I'd be happy to participate in the thread if it gets some attention.

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