SonicBirth

ayskura · Post by **ayskura** » Thu Sep 11, 2008 8:48 pm

Hi

I would like to realize a Dolby Pro Logic Encoder but I don't really know how to make it out of this maths....

Please anybody can help translating this to sonicbirth?

especially i don't know how to realize the 90 degree phase shift...

I know it's about a transform of hilbert but how can i apply it to the expression?

http://en.wikipedia.org/wiki/Dolby_Pro_Logic
Thanks!

zmix · Post by **zmix** » Fri Sep 19, 2008 1:50 am

A cosine is 90˚ out of phase with a sine function...

ayskura · Post by **ayskura** » Sat Sep 20, 2008 3:27 pm

Ok but how do you apply it then to an audio signal?

dyne · Post by **dyne** » Fri Mar 06, 2009 2:43 pm

ayskura wrote:Hi

I would like to realize a Dolby Pro Logic Encoder but I don't really know how to make it out of this maths....

Please anybody can help translating this to sonicbirth?

especially i don't know how to realize the 90 degree phase shift...

I know it's about a transform of hilbert but how can i apply it to the expression?

http://en.wikipedia.org/wiki/Dolby_Pro_Logic
Thanks!

90° phase shift means=
Surround cannel is mixed to L + R output but at the left side with 180° phase shift.
Dont forget to filter it: Hi-pass around 500 and Low-pass around 8khz. (variable at best)
Without Filtering you will "muddy" your mix.
-3db PAD is also important.

Also you can put a short variable delay (up to 100ms) for the matrix surround channel

If you you get your Plugin to work let me hear it.

ayskura · Post by **ayskura** » Sat Mar 28, 2009 3:27 pm

Thank you dyne.

This is what i was looking for.

But are you sure that the +/- 90° degree phase shift is the same as keeping the file at normal phase and invert one channel?

I know that the differences is always 180° (out of phase) but if you apply a phase shift to an audio file. So far i found that in a Hilbert Transformation you have a square wave becoming a kind of saw wave (although quite strange one).

The same thing happen if I phase shift of 90° with the option from the new iZotope Ozone Version 4 : everything sound quite weird comparing with the original due to this 90° phase shift...

I decided anyway to implement the way you said, dyne.

Here come a different problem: I don't seem to be able to address a specific thing from Sonicbirth:
I can't set a 4 channels in > 2 channels out to work with Logic Pro (v8)...

How can i make it seen by Logic?

Also do you have idea how to implement the opposite (Pro Logic Decoder) to test it?

Thank you !!

I hope to be able to make it!!

tatsf · Post by **tatsf** » Sun Mar 29, 2009 7:02 am

Hi!
Have you checked "Sidechain" option?
It's in window's Settings.

tatsf · Post by **tatsf** » Sun Mar 29, 2009 11:11 am

Hi!
I think decoding is easier than encoding.
The easier way is:
L=Lt
R=Rt
C=(Lt+Rt)/sqrt(2)
S=(Rt-Lt)/sqrt(2)

And there is another way for better separation.
The decoding is the same.
You keep watching the level of each channel,and if one is the biggest,you decrease the neighbour channels.
For example,when L is the biggest,as it contains C and S,you decrease the C and S level.

Are these right? I'm not sure.

tatsf · Post by **tatsf** » Sun Mar 29, 2009 4:03 pm

Hi!
I tried the phase-shifting.
It is:
FowardFFT>>complexToPolar
phas=(phas+pi+pi/2)mod(2*pi)-pi
polarToComplex>>inverseFFT

It seems to work well.
Is it right?

tatsf · Post by **tatsf** » Sun Mar 29, 2009 4:25 pm

Sorry,I didn't count the latency FFT causes.
It depends FFT's block size.
You'd better take it as small as you can.

dyne · Post by **dyne** » Thu Apr 02, 2009 12:14 pm

tatsf wrote:Sorry,I didn't count the latency FFT causes.
It depends FFT's block size.
You'd better take it as small as you can.

when you want to convert an PCM stream (fixed bitdeph)
you only have to invert the binerys 0=1 and 1=0.

I don't know if this is working at floating point.
Cubase or Logic working internally at floating Point.

Test it like that:

1. you neet 2 mono channels with the same signal (would be 6db louder)
2. a phase shift plugin (mono)
3. load the plugin in channel 1 (leave channel 2)
4. level both the same
5. if the audio disappears the phase shift work

tell me how it works

tatsf · Post by **tatsf** » Thu Apr 02, 2009 12:42 pm

Sorry,but I mean 90º(PI/2) phase-shifting,not inverting.
(And it's on surround line in your figure.)

Any other better idea for 90°phase-shifting?

dyne · Post by **dyne** » Thu Apr 02, 2009 2:11 pm

tatsf wrote:Sorry,but I mean 90º(PI/2) phase-shifting,not inverting.
(And it's on surround line in your figure.)

Any other better idea for 90°phase-shifting?

I have searched again in my documentations of the SAE and its actually how i wrote it.

0° to the left and 180° to the right

tatsf · Post by **tatsf** » Thu Apr 09, 2009 4:52 am

I found the document about Dolby surround(or pro logic).

http://www.labdv.com/guide/audio/dolby_surround-en.html

It says:
"The surround input S is attenuated by 3dB, bandpass filtered from 100Hz to 7kHz, and passed through a Dolby B-type encoder modified to produced 5dB of noise reduction rather than the normal 10dB. The result is 90° phase shifted and added to feed the Rt signal and subtracted to feed Lt signal."

If this is correct, some 90ºphase-shifting is needed,isn't it?

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