# What is audio "phase" and is it important?

1. Originally Posted by 3dsnar
Do you want to add a new channel, creating 6.1 discrete multichannel signal,
or do you want to downmix the center surround channel to the surrounds L and R (of a 5.1 mix), resulting in (still) 5.1 signal, but containing audio from the center?
Yes, thats exactly what I want to do. A matrixed center surround in the Ls and Rs channels.

Being a DTS expert isnt necessary - this is not about DTS encoding, this is about matrixing audio signals. The DTS part only comes into play when you come to encode, which is simple anyway
2. Hmm, my question had two alternatives.
I assume that in fact you want to create 5.1 signal from 6.1 signal.
So, simply add the surround center to the surround left and surround right,
with changing the energy (of the centre channel) by half (i.e. amplitude should be 100*sqrt(0.5)% = 70.71%)
---
This may cause clipping (in the worst case), therefore before applying the downmix,
you may want to decrease the volume of all 6.1 channels by the following factor:
1/(1+0.7071) = 0.5858 = 58.58%
and than proceed with the downmixing (i.e. adding surround centre of 70.71% to the surround left and surround right.

Cheers, 3d.
3. with changing the energy (of the centre channel) by half (i.e. amplitude should be 100*sqrt(0.5)% = 70.71%)
What does all this mean?

Im getting more and more confused so again will ask:

* To make a 6.1 matrixed mix (having already mixed L, C, R, LS, RS and LFE), do I simply mix 50% of the centre surround into LS and RS? And do I need to do any processing (e.g. invert the channel, phase correction?)
4. To make it as simple as possible
1) create 5.1 signal from the 6.1 signal as I described in the previous post
2) If you want to create a DPLII stereo downmix, take the resulting 5.1 signal
and proceed with the standard dolby prologic downmix (already discussed in this thread)

--
or please describe in details what is the source and what do you want to obtain as result.
Maybe I will be able to help, because I am also confused
and do not really know what exactly you want to do
5. Originally Posted by 3dsnar
1) create 5.1 signal from the 6.1 signal as I described in the previous post... or please describe in details what is the source and what do you want to obtain as result
I wish to know how to matrix a centre surround track into a 5.1 mix, to make a 6.1 track that will play via a 6.1 decoder.

Is this simply a case of creating the centre track and splitting it 50/50 between LS and RS? If so, what do I need to do? Is there anything I need to do do this additional centre surround track before I downmix it into the LS and RS channels?

Thats as simple as I can make it!

You asked me earlier in this very thread. "Do you want to downmix the center surround channel to the surrounds L and R (of a 5.1 mix), resulting in (still) 5.1 signal, but containing audio from the center?" Thats exactly what I want to do, BUT I'd like this centre channel to be directed to the centre surround speaker on 6.1 home cinema systems, i.e. a matrixed 6.1 mix.
6. Gav,
it seems that you want to have combination of 5.1 and matrix encoding.

But I will try to explain this in steps:
Let's assume that you have 7 tracks (of the 6.1 signal) with the following names:
FL, FR, FC, LFE, SL, SC, SR

1) In the first step modify gain of each track by 58.58%.
Lets call the new channels with modified gain: FL2, FR2, FC2, LFE2, SL2, SC2, SR2

2) Next: modify gain of the surround centre channel (SC2) by 70.71%. Let's call the modified centre surround CS3

3) Mix the surround left and the prepared surround centre:
SL2 mix with CS3. Let's call the produced signal SL4

4) Mix the surround right and the prepared surround centre:
SR2 mix with CS3. Let's call the produced signal SR4
-----------------------------------------------------------------------------
Now your 5.1 mix has all channels prepared. It should be created from:
FL2, FR2, FC2, LFE2, SL4, SR4

I cannot be more specific than the above detailed explanation.
So I hope this is enough for you. If not, maybe someone else
could help, because it seems that I am not skilful enough to share
any knowlede
Cheers, 3d
7. OK, that is easy enough to follow, sure. But will a 6.1 system extract the matrixed Centre Surround channel and route it to the CS speaker?
8. Has anyone ever noticed that in Star Trek, technical problems are always overcome by adjusting the phase variance !..It dont matter wether its the warp coil or the coffee machine..adjust the phase variance and hey presto..it works !
9. Normally in 'Voyager'
10. Originally Posted by GavSalkeld
OK, that is easy enough to follow, sure. But will a 6.1 system extract the matrixed Centre Surround channel and route it to the CS speaker?
Actually... I have no idea
11. Originally Posted by 3dsnar
Actually... I have no idea
Oh dear, lol! Not the answer I was hoping for!
12. Originally Posted by GavSalkeld
OK, that is easy enough to follow, sure. But will a 6.1 system extract the matrixed Centre Surround channel and route it to the CS speaker?
That is pretty much what they're "supposed" to do. Whether they actually do that, or something slightly different with different coefficients, is up to the chip maker/manufacturer.

Scott
13. Would applying any sort of processing to the centre surround help? Phase alteration or something?
14. Probablly not. I.e.
by mixing to both surround left and surround right the same signal (the surround centre channel signal), you are in fact applying the correct phase information (common phase components from both channels should be extracted as centre during decoding process - this is how it works in DPLII decoding, for extracting the front centre).
Cheers, 3d.
15. So I dont have to do anything to the centre surround?
16. Exactly. If the hardware that you are using has the ability to extract the surround centre,
than the way to create downmix that I've describe should be fine.
If it does not have such ability, than you cannot do much about it...
17. Well I will certainly have a try. Thanks for all your help, its greatly appreciated.
18. Originally Posted by GavSalkeld
Im wanting to create a stereo wave with matrixed surround information in it, for encoding to Pro Logic II. Someone on this board once said I have to do this to the surround channels:

LS -->(50%L + PhaseInverted 50%R) w/ +90º PhaseShift (if you know how to do that, otherwise ignore the phaseshift)
RS -->(50%R + PhaseInverted 50%L) w/ -90º PhaseShift (same as above)

LS -->100%L, but down by -3dB
LS --> 100%R, Down by -3dB, AND Phase Invert
Same for RS

What is "phase" in the world of audio, and why does Phase Invertion and Phase Shifting have to be used? What does this process do?

Thank you as always.
Sound travels thru air, its arrival time is depended on distant, thus have different delays from Left speaker and right speaker to the right ear, same effect on the left ear.

The different in delay manifests as phase shift.

When the sound hits the back wall and reflects back, it get a 180 phase reverse. due to reflection.

The phase delay should be proportional to the length of the room.

This discussion is phase only, that omitted the fact that the range ( aka spectrum ) of sound get reflected back. These factors include the material( example concrete vs wood) of the back wall, and the secondary reflection ( via roof, side wall, etc.).

If the simulated surround is played at a high level, then I assumed it is pretty much up to the person to tune the amount, the phase, and the spectrum.. to his/her enjoyment/interpretation of the music.
19. Originally Posted by SingSing
Sound travels thru air, its arrival time is depended on distant, thus have different delays from Left speaker and right speaker to the right ear, same effect on the left ear.

The different in delay manifests as phase shift.

When the sound hits the back wall and reflects back, it get a 180 phase reverse. due to reflection.

The phase delay should be proportional to the length of the room.

This discussion is phase only, that omitted the fact that the range ( aka spectrum ) of sound get reflected back. These factors include the material( example concrete vs wood) of the back wall, and the secondary reflection ( via roof, side wall, etc.).

If the simulated surround is played at a high level, then I assumed it is pretty much up to the person to tune the amount, the phase, and the spectrum.. to his/her enjoyment/interpretation of the music.
Time-delay, whether interaural, room-based or whatever, is a slightly different matter from what's been spoken here before. Notice that a reflection delay of 2.272 milliseconds of a concert A440 note would return EXACTLY out of phase from the original and completely cancel it out. But wait--it would completely cancel out only the fundamental!
The difference is that there are so many harmonics to that note and each harmonic would have a differing amount of phase delay. The note wouldn't be completely cancelled out, only "colored a different shade". Similar to some EQ adjustments. So the mixture shouldn't really be called phase-related, it's much more delay-related.
This only is really a factor when, with the Hall and Haas effects, there is a closeness in delay which somewhat fuses the sounds and changes the character. The phase matrixing encode/decode stages are fairly separate in interaction from the playback environment. Remember that the electronic/digital phase adjustment in matrixing does NOT, for most intents and purposes (theoretically), introduce a linear time delay. The math is combining coincident signals.
In, for example, a polarity change, ALL frequencies are equally changed 180°, which would not be the kind of interaction you'd see in a reflection or a speaker or ear delay.

Scott
20. Everything is nice
However phase inverting in DPLII has nothing to do with
room acoustics simulation. It is applied only to enable
DPLII stereo stream decoding to 5.1
This topic has been exploited here:
Cheers, 3d
21. Cornucopia,
and others interested in DPLII specs,
please take a look at this poll.
This may be interesting for you.