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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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Hancoque
post Jun 15 2007, 01:13
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I've evaluated a DTS encoder and found out that the -3dB rear channel attenuation is in fact "hardcoded" into the stream. Therefore no additional attenuation of the rear channels should be necessary for a downmix.

I think it's sane to expect that the center channel is attenuated by -3dB. So the DTS downmixing formula should be as follows:
CODE
Lo = 1.0 * L + 0.7071 * C + 1.0 * Ls;
Ro = 1.0 * R + 0.7071 * C + 1.0 * Rs;

I derive the downmix factor of 1.0 for the rear channels from the fact that the rear speakers have the same distance to the listener as the front speakers and thus should be equally loud (ignoring the facing of the earlobes). But on the other hand the default value for AC-3 is -3dB. That's why I'm still not absolutely sure what to use.

This post has been edited by Hancoque: Jun 15 2007, 02:16
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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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