Decorrelation of Rear Speakers
Hallo Everyone
I have a 5.1 setup with three front speakers and two rear speakers, i am looking for a way to decorrelate the rear speakers from the front. So far from my reading and going through the blogs what i understood is i can use allpass filters to do the task as they keep the magnitude same and introduces random phases to signal. Now i have a cascade of 8 all pass filters, each of them is a single biquad, implemented from max msp, at different frequencies and hoping for a decorrelated signal. Does any one have an idea how many allpass filters are needed and at which frequency steps they should be placed
what i know is to have a fully decorrelated signal i need to have a correlation coefficient of zero between two signal, any idea how can i measure the correlation from max msp.
does any one have any idea about this, can you please help me
I am an audio engineer with over 40 years of experience. I've mixed 8 major motion pictures... most were mixed using Circle Surround (i.e. encoding of surround sound into 2 channel format).
Although I no longer work in that field, I believe I can answer at least what is SUPPOSED to happen with the rear channels on a properly encoded Surround Sound recording.
In a 5.1 channel setup it is slightly different than the formats that I worked in, however the concept is very similar. With true 5.1, one usually has discrete channels digitally encoded into some sort of format that can extract them at a later time back into their discrete channels.
I'm also not sure what you mean be 'decorrelated' but if you mean how does one derive rear channel information from 2 channel stereo information, I can help you with that.
In a properly encoded surround sound setup designed to encode 4 channels into 2, what is usually done is the Left and Right are combined into Sum and Difference components.
The center channel being the combined signal of the Left and Right channels and the read channels being the difference between the Left and Right channels.
The .1 part was usually just a hi pass filter placed on the center channel.
In all cases, the speakers were full range (Left, Center, Right, and Surround Left, Surround Right) however, the read speakers were usually 3db lower than the front 3.
As far as how to do the math within MAX for this, I don't know... but I know how to do it in the analog domain - I used to do it all the time with a standard console.
But the signals would be treated as follows:
Left (would be the regular Left channel)
Center (would be the COMBINED Left and Right channels)
Right (would be the regular Right Channel)
Rear Left and Rear Right (would be the DIFFERENCE between the Left and Right channels)
Sub (would be everything below approx 80hz from the Combined Left and Right channels)
I hope this helps.
Larry
Hi
thanks for the nice reply, yes your suggestion is a great help for me. let me clear a bit about my setup. I have two channel stereo input and i make it a a three channel stereo using the Trifield algorithm from Michale Gerzon.
For my rear channel, i take a stereo signal, sum up the right and the left together and generate a mono signal. So i want to have my rear channel different in phase from the front speakers. So that hearing effect is better. That's what i meant by decorrelation. I am using allpass filters for generating this random phase shift.
i guess now i am a bit clear about what i want to do. I am attaching a patch to clear my setup a bit maybe.