Dolby Pro Logic II pre-conditions

[Mackan]

Hi,

I wonder if a Dolby Pro Logic II downmix from a 5.1 audio source first requires the surround channels to be phase shifted 90 degrees?

For example, this pre-made phase shift is required for the more simple Dolby Pro Logic downmix.

[tebasuna51]

Sometimes.

If the 5.1 sound was recorded with at least 4 microphones around the audio source, it is not necesary.

If the 5.1 is from a AC3 Dolby compliant the surround channels are already shifted.

If the 5.1 was generated from a stereo with the appropriate soft the surround channels are already shifted.

Most the times you don't need make the shift. If you make the shift when is not necesary you can destroy the effect.

But there are axceptions.

[Mackan]

Thanks for the reply.

Hmm... the most typical case is that I want to make a DPLII track from a 5.1 DTS track. From my investigations, such DTS tracks are usually not phase shifted unless they are DTS-ES, which is more rare.

Though, this is still confusing to me. Let's say I have these scenarios, to make things more concrete:

1) 5.1 AC3, already phase shifted surround channels. Can apply standard DPLII downmix matrix here, no problem?

2) 5.1 AC3, no existing phase shift. Can apply standard DPLII downmix matrix here, no problem?

3) 5.1 DTS, no existing phase shift. Can apply standard DPLII downmix matrix here, no problem?

[tebasuna51]

If you are sure than not exist phase shift, no matter the kind of source, you must apply the phase shift.

You can use the Foobar plugin foo_dsp_downmix or eac3to with -downDpl -phaseShift

[Mackan]

If you are sure than not exist phase shift, no matter the kind of source, you must apply the phase shift.

You can use the Foobar plugin foo_dsp_downmix or eac3to with -downDpl -phaseShift

I see, thanks.

Sounds risky for many tools out there that convert audio. I mean, it's impossible for them to know if your source audio is phase shifted or not, yet they provide DPLII downmix.

Do you know of any tool that can be used to detect if the surround channels are phase shifted or not?

[tebasuna51]

Nope.
The best option is make the 2 downmix, with and without phase shift, and listen both.

[Mackan]

Nope.
The best option is make the 2 downmix, with and without phase shift, and listen both.

The problem is to interpet what you hear. For example, I can hear a clear difference between a normal stereo downmix (LoRo) and a Dolby Pro Logic (LtRt) one.

However, let's say there is an existing 90 degree phase shift, but I don't know it, so I apply another 90 degree phase shift. What does the end result sound like? Does a double phase shift cancel each other out, making it sound like normal stereo again, or does it just sound very wonky? Things like that...

[tebasuna51]

A double phase shift can cancel sound when you add Front channels with Surround channels.