Dolby Pro Logic II encoding test (90° vs 180° phase shift)

[Rockaria]

Originally Posted by raquete

...
DPL II channels are full range(less LFE of course)

He threw up another meaningful words again, I was thinking about that too though, and finally had some time to messup with some richer harmony.

One of my receivers I am testing is not directing the mixed LFE to the Subwoofer. The center speaker is set to 'large'(for more rich bass) so setting it to small/medium might help to direct it to the subwoofer.
The fact that if I mix the LFE to Lf or Rf, it goes to the subwoofer, gave me a hint to mix the LFE to both Lf and Rf in invert & 90phase shifted way.

There can be several ways to achieve the LFE to be redirected to the SUBWoofer by :
. equaly positioning the LFE to the fronts(like center) : no redirection(in my setup), but when changed the center to 'small', it redirected
. panning the LFE to some unbalanced positions to the front(Lf-Rf) : some portion will go to the SUB, will also affect the unbalanced normal stereo playback
. phaseshift(Lf -90), invert(Rf) of the LFE : complicated, might have some side effects depending on the existing fronts LFE portion(cancellation)
.other combinations of these (normal, shift, invert) gave me less SUB Woofer volumes.
. mixing the LFE to the rears before the rear shift & invert process

Some changes from the latest model :
Lt = (L, LFE<, -3dB(C), (-1.25dB(Ls), -6.02dB(Rs))<*)
Rt = (R, LFE*, -3dB(C), (-6.02dB(Ls), -1.25dB(Rs))<)

changing it to the linear gain ratio format would look like :
Lt = (1L, 1LFE<, 0.7071C, (0.866Ls, 0.5Rs)<*)
Rt = (1R, 1LFE*, 0.7071C, (0.5Ls, 0.866Rs)<)

. resulting max linear ratio = 4.07 == 12.20dB(10log(4.07^2)), max(safe) pre-gaining -12.20dB
. this way gave richer LFE directed to the subwoofer
. might affect the existing LFE portions of the fronts(cancellation)
. it seems giving better rear seperations surprisingly(?)
. the PhantomCenter(of the LFE) might stay in the center though
. also it has very similar volume in the stereo mode playback as before(not too loud).

PreMixing the LFE to the Rears :
Ls = (Ls, LFE), Rs = (Rs, LFE)

dbFormat ratio :
Lt = (L, -3dB(C), (-1.25dB(Ls), -6.02dB(Rs))<*)
Rt = (R, -3dB(C), (-6.02dB(Ls), -1.25dB(Rs))<)

Sound Power ratio :
Lt = (1L, 0.7071C, (0.866Ls, 0.5Rs)<*)
Rt = (1R, 0.7071C, (0.5Ls, 0.866Rs)<)
. no negative effect, the safe-pregaining would be similar to above.

<conclusion>
. if the receiver supports the speaker size setting(or center-LFE-SUB redirecting) or portable use, pre-mix it to the center
. if the receiver supports rear-LFE-SUB redirect, pre-mix it to the rears.
. otherwise, test the front invert-shift-mix

<history>
[Jun 29] initial version tested with some 6ch music. : good results
[Jun 29] added a method premixing to the rears, conclusions.

[bleo]

I made a 6-channel test sample in Cool Edit that contained the following:
0-3 sec, silence
3-6 sec, L 300Hz square wave
6-9 sec, C 400Hz square wave
9-12 sec, R 500Hz square wave
12-15 sec, Ls 600Hz square wave
15-18 sec, Rs 700Hz square wave

I then downmixed it to DPL2 in Cool Edit using the matrix:
Lt = L + 0.7071 C + 0.866 Ls + 0.5 Rs
Rt = R + 0.7071 C - 0.5 Ls - 0.866 Rs

I then upmixed it back to 6 channels using WinDVD DPL2 decoder in Movie mode.

Results:
- The square waves were intact in the output indicating that no phase shift occurred.
- There was a 5msec delay of the entire output, and an additional 15msec delay of Ls' & Rs'.
- After accounting for this delay, the phases of all output channels were identical to the inputs, and there was no inverting either.
- The amplitude of the square waves fluctuated a bit at the channel switchover points (3, 6, 9, 12, 15 secs). This was probably an artifact of the steering logic.

Discussion:
This matrix produced output that was closest to the original input. The phases were the same and the relative channel volumes were the same with an overall reduction of -3dB. However this is NOT a DPL2 spec. compliant downmix. A discussion on 90° phase shifts will follow...

[bleo]

Let's say we forget about DPL2 and DD5.1 for the moment. Then we take a 5-channel soundtrack and phase shift both rears by 90°. What is the result to the listener?... In most cases, nothing! Especially for movie soundtracks, this is because there is usually no phase correlation required between the fronts and the rears. Furthermore, it would be difficult to keep such a phase correlation intact because the listener does not necessarily sit in the exact center of all 5 channels, and the rears are usually time delayed as well. (This difficulty may have been why quadraphonic sound did not become popular).

Now for DPL2, the rears MUST be 90° phase shifted to DEcorrelate them from the fronts in the downmix. Otherwise, if C and S are identical, they will cancel each other out on either Lt or Rt of the downmix. Once they cancel out in the downmix, you cannot get them back--this is the disadvantage of matrix encoding compared to discrete multi-channels.

So you 90° phase shift the input rears to make sure they are not cancelled out in the DPL2 downmix. After you decode back to 5 channels, the output rears are still 90° phase shifted. What is the consequence to the listener? Again, nothing! The input & output rears do not need to have any phase correlation with the input & output fronts.

Now for DD5.1, why do the Dolby specs say to 90° phase shift the rears? Firstly because it has no consequence for a listener using 5.1 channels--the rears do not need to have any phase correlation with the fronts. Secondly (and more importantly for our discussion), when downmixing DD5.1 to DPL 1 or 2, PRE- 90° phase shifting the rears allows the DD5.1 DEcoder to downmix using a very simple arithmetic matrix:

Lt = L + 0.7071 C - 0.866 Ls - 0.5 Rs
Rt = R + 0.7071 C + 0.5 Ls + 0.866 Rs

or

Lt = L + 0.7071 C + 0.866 Ls + 0.5 Rs
Rt = R + 0.7071 C - 0.5 Ls - 0.866 Rs

(signs to be determined by further experiments)

The final DPL2 decoded rears will of course be 90° phase shifted compared to the original sountrack before DD5.1 encoding, but have the same phase as the DD5.1 output. What is the consequence to the listener? You guessed it--nothing!

So when downmixing a DD5.1 soundtrack to DPL2, use one of the above matrices. DO NOT 90° phase shift the rears--it should have already been done when encoding to DD5.1.

When SHOULD you apply 90° phase shifts to the rears when downmixing to DPL2? These are mostly unique cases:
- If your multichannel material was never encoded to DD5.1 (e.g. you created it yourself).
- If you want to experiment...

[raquete]

Now for DD5.1, why do the Dolby specs say to 90° phase shift the rears? Firstly because it has no consequence for a listener using 5.1 channels--the rears do not need to have any phase correlation with the fronts.

sorry,i don't agree and it's nothing have to do with DD5.1,follow me:


think that you listen the sound and not decoding
q1
what happen if the surround left and right have the same phase as front left and right?
a1- cancellation!

q2
what if the sL and sR have +90 degrees?
a2- cancellation!

q3
what if the sL and sR have -90 degrees?
a3- cancelation!

q4
what happen if sL have +90 and sR -90 degrees?
a4-
the sL is 90 degrees from the L channel.
the sR is 90 degrees from the R channel.
the sL and sR are now 180 degrees
no more cancellations and:
as show the picture,the surrounds are placed round 90 degrees from the frontal channels this is why they are turned 90 degrees and one of them with +90 and other with -90 to don't have cancellation because one speaker in front other sounding with the same phase will give.... cancellation.
this is what i understood(with experience) why they used 180 degrees between surrounds and at the same time 90 degrees from the fronts!

[3dsnar]

Raquette,
we are talking about 90 deg phase shift of the waveform
(or shifting by PI/2 in radians). It has not much to do with angles between speakers

[raquete]

@ 3dsnar

It has not much to do with angles between speakers

but have.
maybe i was unclear,let me try to explain better:

has no consequence for a listener using 5.1 channels--the rears do not need to have any phase correlation with the fronts.

if the surround channels have the same phase(no matter if phase 0,+90 or -90) and one speaker is in front of the other,you get cancellation.
as the surrounds are placed in the left and right sides,they turn his phases in (+ and -)90 degrees in the audio too.this result in differents phases between the surrounds and no cancellations will happen.
this not means that the sound with turned phase sounds better
then,if it not sounds better,we have to questions:
1-why they turn the phases?
because one speaker in front other playing the sound with the same phase give cancellation
2- and why -90 and +90?
because the surrounds are nearly (or perfectly) 90 degrees from the central channel(and can't have the same phase)

(of course,poor english here)

later i(will if needed)post another picture showing how and why the surrounds have to stay round 90 degrees from the frontals and 180 degrees(between sL/sR)

[Rockaria]

Originally Posted by bleo

I made a 6-channel test sample in Cool Edit that contained the following:
0-3 sec, silence
3-6 sec, L 300Hz square wave
6-9 sec, C 400Hz square wave
9-12 sec, R 500Hz square wave
12-15 sec, Ls 600Hz square wave
15-18 sec, Rs 700Hz square wave

I then downmixed it to DPL2 in Cool Edit using the matrix:
Lt = L + 0.7071 C + 0.866 Ls + 0.5 Rs
Rt = R + 0.7071 C - 0.5 Ls - 0.866 Rs

I then upmixed it back to 6 channels using WinDVD DPL2 decoder in Movie mode.

Results:
- The square waves were intact in the output indicating that no phase shift occurred.
- There was a 5msec delay of the entire output, and an additional 15msec delay of Ls' & Rs'.
- After accounting for this delay, the phases of all output channels were identical to the inputs, and there was no inverting either.
- The amplitude of the square waves fluctuated a bit at the channel switchover points (3, 6, 9, 12, 15 secs). This was probably an artifact of the steering logic.

Discussion:
This matrix produced output that was closest to the original input. The phases were the same and the relative channel volumes were the same with an overall reduction of -3dB. However this is NOT a DPL2 spec. compliant downmix. A discussion on 90° phase shifts will follow...

Well, whatever is the purpose of this test, there always are another eyes trying to see what it means actually..

I try to figure out conditions not described clearly :
. purpose : to prove if the DPL II decoders work correctly without the 90 deg shift process with non-pre-shifted clips
. conditions : very similar to tebasuna's powerdvd test before where he recommended the matrix3 to be used
- Windvd should represent the standard of the DPL II decoders
- the input streams should represent the normal combinations

The conclusion implied in the test result : although nothing specified explicitly
. windvd or powerdvd are de facto standard : whether complient to the DPL II spec or simply Dolby..
. the 90 phase shift is not necessary, matrix3 is the rule
. not regaining the -3dB on rears is correct

The conclusion I could derive from the result : although the previous test with 90 deg shift looked more reliable
. the decoder performs invert on one channel, does not perform 'reverse phase shift' as expected
. not sure if it is complient or not with the two test results : does not meet the conditions to conclude
. the -3dB rear output attenuation says not correct volume regaining by either wrong input or logic
. for the invert-only versions(and these s/w dpl II decoders), the matrix3 is the way to go!

/believing is personal freedom

[tebasuna51]

Originally Posted by bleo

I made a 6-channel test sample in Cool Edit that contained the following:
0-3 sec, silence
3-6 sec, L 300Hz square wave
6-9 sec, C 400Hz square wave
9-12 sec, R 500Hz square wave
12-15 sec, Ls 600Hz square wave
15-18 sec, Rs 700Hz square wave

I then downmixed it to DPL2 in Cool Edit using the matrix:
Lt = L + 0.7071 C + 0.866 Ls + 0.5 Rs
Rt = R + 0.7071 C - 0.5 Ls - 0.866 Rs

I then upmixed it back to 6 channels using WinDVD DPL2 decoder in Movie mode.

Same test using PowerDVD6 DPL2 decoder in Movie mode. Exact results, only:

Originally Posted by bleo

- There was a 5msec delay of the entire output, and an additional 15msec delay of Ls' & Rs'.

I get a 15msec total delay of Ls' & Rs' (5 global + 10 additional)

[Rockaria]

@raqueue, there are three possible cases that can happen by mixing the two identical signals before & after the DAC process.
Suppose you are playing the identical stereo streams in stereo mode(DPL II will aggressively decode them in surrounds) :
combinations : (0,0),(0, -180),(0, 90), (0,-90), (90,90), (90,-90), (-90,-90)....
a. mixing the streams in one channel before the DAC :
- the stream will completely cancel each other : (90,-90), (0, -180)...
- increased volumes : (0,0), (90,90), (-90,-90) > (0,+-90)...

b. phantom center speaker effect : (0,0), (90, 90), (-90,-90) > (0,+-90)..
c. difussion effect : (0, -180), (90,-90) > (0,+-90)...

@ bleo, at least we seem to agree the final DPL II stream must contain +-90 cross stacked mixes for the better seperation quality.
Some problems I am noticing is that the conditions to be proved are confused :

a. 90 shifts are nessary for normal 5.1 streams for better seperations : clearly explained in the Dolby's doc, doesn't need to prove anything else.

b. 90 shifts are not necessary at all except for some experiments : need to prove lots of things before claiming
- the DPL II encoding is mostly used only for DD5.1 movie clips
- all the DD5.1 movie clips rears are already 90 deg phase shifted
- there are actually some certified equipments for normal users which can do the invert-only downmix
- the 90 deg shifts has no negative effects at all for movie clips in the correlations between the channels maybe
- the MOVIE clips are always different from the MUSIC in the correlations between the channels maybe
- all the non-pre-90 shifted clips are just for hobby

Not an easy job .. impossible. Why not just let the methods be chosen by those who know what they are doing?

[3dsnar]

Originally Posted by raquete

sorry,i don't agree and it's nothing have to do with DD5.1,follow me:


think that you listen the sound and not decoding
q1
what happen if the surround left and right have the same phase as front left and right?
a1- cancellation!

q2
what if the sL and sR have +90 degrees?
a2- cancellation!

q3
what if the sL and sR have -90 degrees?
a3- cancelation!

q4
what happen if sL have +90 and sR -90 degrees?
a4-
the sL is 90 degrees from the L channel.
the sR is 90 degrees from the R channel.
the sL and sR are now 180 degrees
no more cancellations and:
as show the picture,the surrounds are placed round 90 degrees from the frontal channels this is why they are turned 90 degrees and one of them with +90 and other with -90 to don't have cancellation because one speaker in front other sounding with the same phase will give.... cancellation.
this is what i understood(with experience) why they used 180 degrees between surrounds and at the same time 90 degrees from the fronts!

Ofcourse the speakers position are of importance. But this is a completely different issue. Here we are talking about the problems related to phase properties for downmixing and decoding (before the decoded sound gets outside the decoder).
So it is not that I disagree / agree with you
I just wanted to clear things out.

[raquete]

@ 3dsnar

So it is not that I disagree / agree with you

of course,i (we all) know that.

I just wanted to clear things out.

no need because you are always very clear in your posts.
remember,you are one "creator",i'm one single "listener".



@ Rockaria

c. difussion effect : (0, -180), (90,-90) > (0,+-90)...

difussion effect takes to listening effects when the audio is up or downmixed.(this is why i posted about speakers positions and respectives audio phases too)

all i can say about your (complete) last post is:
perfect (clever explanations,word by word)

edit: typos

[Rockaria]

The DIFFUSION effect looks like another expression(transformed energy) of the cancellation.
Like throwing another stone in the pond, the area between them will have some counter-wave effect making the power weaker.
If you change the phase from 0 - +-90 - +-180 between the two speakers, you will FEEL the center sound becoming weaker(scattered).

Not the entire cancellation(like digitally when mixing) but some type of transformed(to different type of energy : speaker) interference..
So I agree by including the digital phase altering mixing process(& delay,,,), the speaker diffusion/phantom center effects would make a full consideration of this environment.

[raquete]

ok. (one more)

one single test for "cancellations" :

1-create(generate tones) 100Hz -30 seconds stereo wave-same phase in L/R.
listen in one stereo amplifier(or in pc with good sound)where the speakers are in front of you(of course,left and right speakers)
you will listen one very clean sound.

2- now place one speaker in front the other.(face to face)
L --> <-- R
see(or better,listen) how much cancellation happen.

3- most important: i will that you understand what i mean

[Rockaria]

I see the sound becoming almost muted with extremely close faced speakers & identical stereo playback with a ch invert phased.

The speaker's transformed energy consists of several factors :
- direction : it has 360 degree but more directed to the front of the speaker
- carrier : air(thiner density of the matriels than water or circuit in digital/analog formats), it will be delayed, weakened by the distance.
- set up : 5.1(3 fronts and 2 rears facing each other but with some enough distance), angle & distance(&delay),,, fators are crucial
- room(ambience) : interferences, reflections(matriels), decay, echos...

Thanks for providing the excellent extreme example.

[edit]
The same DIFFUSION or cancellation effect will also happen when changing the one speaker polarity(swapping the red & black wires).

[Rockaria]

Originally Posted by Rockaria

. for the invert-only versions(and these s/w dpl II decoders), the matrix3 is the way to go!

http://forum.doom9.org/showpost.php?...&postcount=134
Based on the Dolby's and surecode's docs, the invert only versions should use the tebasuna's matrix3(in avs or similar scripts) :
- the DPL II is expected to have the out of phase signal in Rs : the matrix1 will have (-Ls, +Rs) encoding time
- the decoder movie mode inverts the Rs(as explained in the surecode doc) : the matrix1 will have (-Ls, -Rs) both inverted
- so the matrix3 will have (+Ls, -Rs) encoding time and (+Ls, +Rs) decoding time.
- but these models do not fully seperate the channels(unbalanced), although will never be agreed or admitted..

Because most 90 phase deg shift routines perform -90 deg shifts effectively, invert mixing the Ls(coeffs) will have (+Ls, -Rs) encoding time and (+Ls, +Rs) decoding time.

I have tried to find a plugin or ways to implement/utilize the 90 deg phase shift in avisynth with no success yet. Developing the plugin also does not have the proper environment(esp. time) for me. But as I found the softEncode surround 90 deg phase shift option performs correctly for the DD downmix encoding/play, I conclude it's the ac3 encoder's role to do the optional 90deg shift + downmix + (DD2.0 / WAV encode/STDOUT).

http://forum.doom9.org/showpost.php?...3&postcount=15
http://forum.doom9.org/showpost.php?...6&postcount=50
And the complete background of the DPL II downmixing process in DD encoding(rear 90deg phase shifts option) and decoding(auto downmix mode selection based on annex->dmixmod meta info) : http://forum.doom9.org/showpost.php?...0&postcount=98

I think the softEncode's batch mode will still be useful for the DPL II encoding automation if we don't care about the temporary WAV AC3 files.

As for the Dynamic DPL II encoding for the ds filter use(FFDShow...), there are some conditions I can think of for now :
. the freq-delay method must be applied to the all-pass(granular frequency bands) and aligned
. the buffer must be large enough to sync(align) the delays between the freq-bands as well as other channels.
. the accuracy of the phase shifts degree does not affect the quality that much
...
This technology is already fading even without being utilized of the full potentials and advantages.. that's too bad..

/.
[edit] added a reference in the same thread