Converting Stereo to Mono taking into account Phase modulation

On Jan 7, 9:35&#4294967295;pm, Robert Orban <donotre...@spamblock.com> wrote:
> In article <46676447-fefc-4a6a-9a37-
> ffe1bb30e...@w1g2000prm.googlegroups.com>, robert.ad...@analog.com
> says...
>
>
>
>
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> >Some 30-year-old history;
>
> >the way mono-to-stereo was often done in pre-DSP days was to apply the
> >mono signal to a series connection of allpass filters. You then form;
>
> >left = mono + alppas(mono)
> >right = mono - allpass(mono)
>
> >Therefore, where one channel peaks, the other dips, so the overall
> >tonal coloration was unaffected.
>
> >The "art" was to pick the frequencies and Q's of the allpass filters.
> >My former employer (Dave Blackmer of dbx fame) was convinced you
> >needed at least 12th-order, judiciously spread across the audio band.
>
> >Bob Adams
>
> I published an article about this in the April 1970 issue of J. Audio
> Engineering Society. Probably the most interesting thing about it was a
> proof that the sum of the power spectra of the two pseudostereo channels
> is equal to the power spectrum of the input. This proof was generalized
> to being true if the original mono signal is passed through any allpass
> filter before the output matrix. In other words:
>
> left = mono*allpass1 + mono*allpass2
> right = mono*allpass1 - mono*allpass2
>
> which reduces to the case Bob described above when allpass1 = 1.
>
> For pseudostereo this is important because it creates a pseudostereo
> process that does not produce tonal coloration compared to the original
> mono. There are also interesting applications to power-complementary
> filterbanks.
>
> Bob Orban- Hide quoted text -
>
> - Show quoted text -

Thanks for the reference Bob; I'm sure your article is where it all
started. I was a mere high-schooler in 1970 and had not yet entered
the world of reading AES journals!


Bob