Properly downmixing 5.1 to stereo
Reply #18 – 2008-05-02 15:28:04
Let me revive this thread. Looking for some embedded downmix coefficients isn't really the way to go for me as not all surround formats provide this information. AC-3 has it, but DTS doesn't (at least it's not available in the programs I know). DVD-Audio might provide it but only two of my seven discs actually have it. Then I thought again about the best general formula and came to this:L = Lf + C/2 + Ls R= Rf + C/2 + Rs I know that this looks quite different from what is most often used. But let me explain how I came to these values.I. Center Channel I was especially concerned about the center level as an attenuation of 3 dB seems to be the general rule of thumb instead of 6 dB. My goal was to achieve a phantom center channel that is equally loud than the original dedicated center channel. I imaged: What if I would split the speaker in half (halving the amplitude of each side, i.e. attenuating by 6 dB) and shove one half next to the left speaker and the other half next to the right one? In my theory this would not change the overall volume because both halved amplitudes would combine to the full amplitude when they both reach the listener (0.5 + 0.5 = 1). But I still wasn't sure if two speakers (each with half the amplitude) are really outputting the same energy as one speaker (with the full amplitude). So I conducted a test. I placed my two stereo speakers directly next to each other, placed a microphone in front of them, calibrated both sides so that the microphone picks up the same amplitude from left and right and then played back three test samples while measuring the amplitude the microphone registers. Test sample 1: A sound (sine wave of 500 Hz) of full amplitude on one side and the other side being silent (factor: 1.0). Test sample 2: The same sound distributed to both sides with an attenuation of 3 dB (factor: 0.707). Test sample 3: The same sound distributed to both sides with an attenuation of 6 dB (factor: 0.5). The conclusion is that my theory is correct in that sample 3 reproduces the same amplitude as sample 1. Sample 2 has a higher amplitude (by about 3 dB). So, I think that if one wants to downmix 5.1 to stereo and wants to keep the same volume for the center channel one has to use a center channel attenuation of 6 dB and not 3 dB.II. Surround Channels Now, why no attenuation of the surround channels? I think if you want to preserve the relative volume of the original channels you should not attenuate the surround channels. In a 5.1 setup all speakers (ignoring the LFE channel) are equidistant to the listener. So why should one channel that has the same distance to the listener like another channel be attenuated while the other one isn't? And I don't think that the human head attenuates signals coming from behind by 3 to 6 dB compared to signals coming from the front (correct me if I'm wrong). So, attenuating the surround channels cannot be called "most accurate reproduction" but can only be seen like "adjusting it to one's tastes". Or am I wrong?