Converting Stereo to Mono taking into account Phase modulation

Started by Nadav December 16, 2008
On Jan 7, 9:35&#2013266080;pm, Robert Orban <> wrote:
> In article <46676447-fefc-4a6a-9a37- >>, > says... > > > > > > >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