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12dB difference from two licensed Dolby down-mixer

smilenight

Novice Member
I am very confused, because two Dolby 5.1 to stereo down-mixer, both manufactured under license from Dolby Laboratories, give very noticeable sound level difference.


By my crude measurement, the difference is about 12dB sound level difference. How can I know which one is correct, or whether both are incorrect?


Movie passage played: 0:54:46 – 0:54:52, English Dolby 5.1, Chapter 13 of Sherlock Holmes, Warner Bros., C 2009. Right channel was measured.


Disk playback: Sony BDP-S6500, Audio DRC off


Sony BDP-S6500's down-mixer (digital audio output set to PCM on Sony BDP-S6500 and DA conversion done by MuxLab 500088): -18.57dBu
(Referenced to 0 dBFS PCM: -12.35dB)

MuxLab 500088’s down-mixer (digital audio output set to Auto on Sony BDP-S6500, and Dolby down-mix and DA conversion done by MuxLab 500088): -6.06dBu
(Referenced to 0 dBFS PCM: +0.16dB)

0dBFS PCM source: -6.22dBu


Level measurement was done by the following equipment and software.


Mackie Onyx Blackjack USB audio interface.


Line-in gain knob on the USB audio interface was set so that the 0dBFS 200Hz sinusoidal tone track on Pierre Verany CD played on Sony BDP-S6500 (same results no matter whether digital audio output was set to PCM or Auto) and DA conversion done by MuxLab50088, gives the reading -6.22dBu on TrueRTA.


The software TrueRTA on a Toshiba laptop with Windows 10 was used. The overall level was read from dBu peak display.


The output voltage level from 0dBFS 200Hz sinusoidal tone track through MuxLab 50088 is about 1.86 Vrms measured by a multi-meter.



======================


I made another measurement.


Movie passage played: 1:12:57 – 1:13:07, English Dolby 5.1, Cinderella,Disney, C 2015. Left channel was measured.


Sony BDP-S6500's down-mixer (digital audio output set to PCM on Sony BDP-S6500 and DA conversion done by MuxLab 500088): -11.20dBu
(Referenced to 0 dBFS PCM: -5.13dB)


MuxLab 500088’s down-mixer (digital audio output set to Auto on Sony BDP-S6500, and Dolby down-mix and DA conversion done by MuxLab 500088): -5.71dBu
(Referenced to 0 dBFS PCM: +0.36dB)


0dBFS PCM source: -6.07dBu



Pierre Verany CD (PCM) 1kHz +0dBFS track: -6.09dBu
Pierre Verany CD (PCM) 1kHz +3dBFS track: -5.76dBu
Pierre Verany CD (PCM) 1kHz +6dBFS track: -5.57dBu
 
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smilenight

Novice Member
MuxLab 500088 produced louder volume when it does 5.1 to 2.0. I returned it anyway, because it made audible high frequency mechanically generated noise from something inside the enclosure.

Now I tried Wyrestorm Digital to Analogue Audio Converter with Dolby Downmixing (EXP-CON-DAC-D). The behavior of this one is opposite to MuxLab 500088.

When it does 5.1 to 2.0, it is too quiet, about 10dB lower than when it does 2.0 to 2.0.
The 10dB figure is by my measurement with RadioShack sound level meter, and the audio test tracks in Wow World of Wonder DVD (by Disney).

Compared to Wyrestorm, the Sony blu-ray player exhibited consistent volume level no matter whether it did 5.1 to 2.0 or 2.0 to 2.0.

What DAC with Dolby Downmix is reliable in its performance?

I wish all TVs have down mix feature, but many (such as LG) do not offer that feature.
 

smilenight

Novice Member
I did another test with another Blu-ray player: LG BP550. With auto DRC off, of course.


The Dolby downmixer in LG BP550 behaves like the one in Sony BDP-S6500, except that LG BP550 does not offer choice between Surround (I guess it is Lt/Rt from the test tracks in Disney Wow World of Wonder DVD audio set up track.) and Stereo ((I guess it is Lo/Ro).


If I choose output to be PCM on LG BP550 (I use the downmixer in LG BP550 for 5.1 to 2.0 conversion), the sound level of Cinderella (Disney) seems consistent no matter whether I choose the 5.1 audio track or 2.0 audio track on the disk.


If I choose output to be Auto on LG BP550 (I use the downmixer in Wyrestorm Digital to Analogue Audio Converter with Dolby Downmixing EXP-CON-DAC-D (Serial number: WSA10002099) for 5.1 to 2.0 conversion), the sound level of Cinderella (Disney) is very consistent depending on whether I choose the 5.1 audio track or 2.0 audio track on the disk: much quieter when Wyrestorm EXP-CON-DAC-D does 5.1 to 2.0 than when Wyrestorm EXP-CON-DAC-D does 2.0 to 2.0.




Gefen GTV-DD-2-AA Digital Audio Decoder showed similar loudness level inconsistency as Wyrestorm EXP-CON-DAC-D. MuxLab 500088 showed opposite kind of loudness level inconsistency: it sounds louder (to the point it seems to clip on loud passages) when it does 5.1 to 2.0 than when it does 2.0 to 2.0.


These three, Gefen GTV-DD-2-AA, Wyrestorm EXP-CON-DAC-D, and MuxLab 500088 are practically all external Dolby downmixing DACs (without additional features such as amplification to drive loudspeaker, etc.) licensed by Dolby Laboratories in the US consumer market. All three are doing incorrect Dolby downmixing? How could they be licensed by Dolby while doing incorrect Dolby downmixing?


The three DVD or blue-ray players I tested (two Sonys and one LG, all of them seems to be licensed by Dolby Laboratories) behave similar to each other, and seem to be correct by common sense: no volume change shock when the source changes from 5.1 to 2.0 or from 2.0 to 5.1.
 
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