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Properly downmixing 5.1 to stereo
Hancoque
post Jun 9 2007, 01:24
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I'm currently working on a plugin to downmix 5.1 to stereo. First, I thought about how an ideal speaker setup looks like. I came up with this image (omitting the subwoofer):



What I conclude from this image is that the stereo separation of the rear channels is stronger than the stereo separation of the front channels. This means I have to mix the front channels differently into the stereo channels than the rear channels.

There are two extreme points that a speaker can have. It can be located at 0° (like the center channel). In that case the channel should go equally to the left and right channel. Or it can be located at ±90°, which means that 100% of the channel goes either to the left or to the right.

The front channels are positioned 30° from the 0° point, so the calculation would be as follows:
Front: 30° / 90° * 50 + 50 = 67%
So 67% of the channel goes to the same side, while the rest (33%) goes to the other side.

The calculation for the rear channels is similar:
Rear: 70° / 90° * 50 + 50 = 89%
So 89% of the channel goes to the same side, while the rest (11%) goes to the other side.

But then I noticed that this would be suitable for headphones but not for speakers. So I decided to set 70° as the maximum and not 90°:
Front: 30° / 70° * 50 + 50 = 71% (other side: 29%)
Rear: 70° / 70° * 50 + 50 = 100% (other side: 0%)

This way I have the widest possible stereo separation while maintaining the separation ratio between front and rear. But I still feel that it's just a compromise and not an ideal solution.

Then something else came to my mind. I noticed that most applications don't mix the center channel 50%/50% into the stereo channels but 71%/71% (-3.01dB = square root of 2, divided by 2). So, aren't two speakers with half the amplitude as loud as one speaker? If I should indeed use 71% instead of 50% I wonder how I have to apply this to the other channels.

This post has been edited by Hancoque: Jun 9 2007, 01:26
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SebastianG
post Jun 15 2007, 19:53
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From: Göttingen (DE)
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This is "only" the white paper, btw.
...looks quite comprehensive. But I'm positive I got the real spec for free.
IIRC it was from ETSI.org / ETSI, free standards download page.

Cheers!
SG

This post has been edited by SebastianG: Jun 15 2007, 20:00
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Posts in this topic
- Hancoque   Properly downmixing 5.1 to stereo   Jun 9 2007, 01:24
- - SebastianG   QUOTE (Hancoque @ Jun 9 2007, 02:24) But ...   Jun 11 2007, 12:24
|- - Hancoque   QUOTE (SebastianG @ Jun 11 2007, 13:24) I...   Jun 11 2007, 17:24
- - Woodinville   It seems to me that there is no direct formulaic w...   Jun 11 2007, 21:15
- - robaer   How is the downmix done on a standalone dvd player...   Jun 11 2007, 21:39
|- - Woodinville   QUOTE (robaer @ Jun 11 2007, 13:39) How i...   Jun 11 2007, 23:15
|- - robaer   QUOTE (Woodinville @ Jun 11 2007, 23:15) ...   Jun 11 2007, 23:57
|- - mcbear   QUOTE (robaer @ Jun 12 2007, 00:57) QUOTE...   Jun 12 2007, 08:45
- - Hancoque   According to the AC-3 specification on atsc.org th...   Jun 12 2007, 14:22
|- - mcbear   QUOTE (Hancoque @ Jun 12 2007, 15:22) Acc...   Jun 13 2007, 15:24
|- - SebastianG   The decoder should do the downmixing since it has ...   Jun 13 2007, 16:04
- - Hancoque   So, clev and slev are only used for downmixing? In...   Jun 14 2007, 01:03
|- - mcbear   QUOTE (Hancoque @ Jun 14 2007, 02:03) So,...   Jun 14 2007, 08:53
|- - SebastianG   QUOTE (mcbear @ Jun 14 2007, 09:53) DTS i...   Jun 15 2007, 18:23
- - Hancoque   I've evaluated a DTS encoder and found out tha...   Jun 15 2007, 01:13
- - Hancoque   There's a publicly available technical documen...   Jun 15 2007, 19:24
|- - mcbear   QUOTE (Hancoque @ Jun 15 2007, 20:24) The...   Jun 19 2007, 11:26
- - SebastianG   This is "only" the white paper, btw. ......   Jun 15 2007, 19:53
- - Hancoque   Let me revive this thread. Looking for some embedd...   May 2 2008, 15:28


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