Xref: utzoo rec.audio:8722 sci.electronics:4149
Path: utzoo!yunexus!geac!syntron!jtsv16!uunet!ncrlnk!ncrcae!wte
From: wte@ncrcae.Columbia.NCR.COM (Bill Eason)
Newsgroups: rec.audio,sci.electronics
Subject: Re: Extracting Mono from Stereo
Keywords: mono stereo signal processing
Message-ID: <3837@ncrcae.Columbia.NCR.COM>
Date: 24 Oct 88 19:48:07 GMT
Article-I.D.: ncrcae.3837
References: <341@ivucsb.UUCP> <343@ivucsb.UUCP>
Reply-To: wte@ncrcae.Columbia.NCR.COM (Bill Eason)
Organization: NCR Corp., Engineering & Manufacturing - Columbia, SC
Lines: 71

NOTE: SORRY IF THIS IS A DUPLICATE; SYSTEM WAS SHUT DOWN DURING MAIL PROCESS
AND I WASN'T SURE THIS EVER GOT OUT OF THE PLANT.  - WTE

In article <343@ivucsb.UUCP> todd@ivucsb.UUCP (Todd Day <todd>) writes:
>In article <341@ivucsb.UUCP> I (Todd Day) write:
>_I have a very interesting signal processing problem for those signal
>_processing gurus.

>_I want to use a DSP chip to extract mono from stereo.  Does anybody
>_know of any algorithms to accomplish this or can anyone direct me to
>_any literature that might describe this?

>_I've looked into the math behind this.  It is not a simple as it first
>_appears!  Now, it's very, very easy to get rid of mono from stereo...
>_just subtract left from right, and the mono disappears.  However, you
>_can't just subtract this (R-L) from (R+L) to get only mono.  What
>_you always end up with is some combination of the mono plus one of
>_the other channels (2M - 2R, etc.)

<...stuff deleted...>

Could you not take this (2M - 2R) and add 2R?  I built a little
five op amp circuit not long ago to try to increase stereo separation 
by taking k*(R-L) and -k*(R-L)==k*(L-R) and adding these back into R 
and L, respectively.  The resulting output is then:

		R' = R + kR - kL  =  (1+k)R - kL
		L' = L + kL - kR  =  (1+k)L - kR

	 (R' - L') = (1+2k)R - (1+2k)L
		   = (1+2k) (R - L)

	 (L' - R') = (1+2k) (L - R)

It seems that approximate volume is maintained in each channel if R and L
are of about the same magnitude, since kR is subtracted when kL is added,
and vice versa.

I realize this doesn't solve the phase delay problems between R and L,
but my idea was to first get (R-L), the difference between the right and
left channels, and magnify this, just to see if it would work.  It requires:
	1) an inverting op amp circuit on L
	2) a "summer" op amp to add R and -L with variable gain, k, determined
	     by variable feedback resistor value
	3) an inverting op amp on (R-L)
	4) a summer for R + (R-L)
	5) a summer for L + (L-R)
I used cheap LM324 op amps and had one wiring problem that I never fixed, but
it seemed to work to some extent anyway.  Maybe I'll go home tonight and try
fixing it....  

Finally, don't overlook the obvious: you can get (2M - 2R), but you still 
have R, don't you?  I admit I'm no DSP guru and that my little project 
was entirely analog, but would the same principle not hold in the Z-transform 
world?  Hope this helps.

>_Your help would be greatly appreciated.  I'll summarize replies.
>Your smart-aleck answers are not appreciated.  I won't summarize these. :-)
 
  
>   /| Todd Day (805)968-9352 |\            "Hurt me, punish me.
>+-+ | The Audio Club at UCSB | +-+      Make me listen to mid-fi!"
>+-+ | 926 B   Camino Del Sur | +-+   {}!pyramid!comdesign!ivucsb!todd
>   \| Isla Vista, CA   93117 |/              todd@ivucsb.UUCP

Bill Eason   NCR Corporation   
-- 

Bill Eason   (803) 791-6419    ...!ucbvax!sdcsvax!ncr-sd!ncrcae!sauron!wte
NCR Corporation		  ...!rutgers!mcnc!ece-csc!ncrcae!sauron!wte
E & M Columbia   3325 Platt Springs Rd.   West Columbia, SC  29169