Simplified REW Setup and Use (USB Mic & HDMI Connection) Including Measurement Techniques and How To Interpret Graphs - Page 519 - AVS Forum | Home Theater Discussions And Reviews
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post #15541 of 15555 Old 07-31-2015, 04:56 PM
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Originally Posted by AustinJerry View Post
The LFE track is encoded at 10dB lower than the other tracks. The AVR automatically adds the 10dB back, so there is no action required on your part, nor should you be concerned.
So the reason why REW ends up +10db for the sub LFE channel is because they aren't factoring in -10db to compensate for your AVRs +10db, which is assuming an input of -10db. Wow.

What started all this mess in the first place, I wonder?
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post #15542 of 15555 Old 07-31-2015, 05:18 PM
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Originally Posted by Soulburner View Post
So the reason why REW ends up +10db for the sub LFE channel is because they aren't factoring in -10db to compensate for your AVRs +10db, which is assuming an input of -10db. Wow.
No, the reason it ends up +10dB is because you don't understand how the LFE channel works and therefore don't adjust your system to get the same SPL as the others when measuring.

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What started all this mess in the first place, I wonder?
Someone decided the LFE channel should be capable of 115dB whereas other channels were capable of 105dB. That means the same digital signal will yield different playback levels depending on the channel.
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post #15543 of 15555 Old 07-31-2015, 06:29 PM
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No, the reason it ends up +10dB is because you don't understand how the LFE channel works and therefore don't adjust your system to get the same SPL as the others when measuring.
I don't think you understood the question. In REW, it is commonly reported in this thread that HDMI 4 gives +10db over the other channels. Jerry told us this is because the AVR makers add +10db to the LFE channel.

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Originally Posted by Stereodude View Post
Someone decided the LFE channel should be capable of 115dB whereas other channels were capable of 105dB. That means the same digital signal will yield different playback levels depending on the channel.
But if the AVR is adding 10db to LFE, and the LFE track really is, as Jerry tells us, encoded 10db lower by the movie studios, they'd cancel out. Hence the confusion.
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post #15544 of 15555 Old 07-31-2015, 08:35 PM
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Originally Posted by Soulburner View Post
In REW, it is commonly reported in this thread that HDMI 4 gives +10db over the other channels. Jerry told us this is because the AVR makers add +10db to the LFE channel.
Yes, that's correct.


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But if the AVR is adding 10db to LFE, and the LFE track really is, as Jerry tells us, encoded 10db lower by the movie studios, they'd cancel out. Hence the confusion.
If the studio wants something in the LFE channel to play at the same SPL as the other channels them they would reduce the signal by 10dB compared to the other channels. However, they're certainly not going to do that for all LFE content. Ultimately, they mix for what they want during playback and encode the signal accordingly. It seems like you're over thinking this 10dB offset that gives the LFE channel an additional 10dB of output capability.
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post #15545 of 15555 Old Yesterday, 12:52 AM
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Originally Posted by Soulburner View Post
But if the AVR is adding 10db to LFE, and the LFE track really is, as Jerry tells us, encoded 10db lower by the movie studios, they'd cancel out. Hence the confusion.
The confusion seems to be that you think the REW signal is encoded differently according to which channel you use and respects some domain specific (film) encoding standards (hence is designed for use with AVRs only).

This isn't the case, it is just a signal generator that sends a certain scale signal and leaves any downstream processing to do whatever it wants. You therefore have to understand your signal chain if you want to understand the reported output.
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post #15546 of 15555 Old Yesterday, 03:59 AM
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Originally Posted by Stereodude View Post
If the studio wants something in the LFE channel to play at the same SPL as the other channels them they would reduce the signal by 10dB compared to the other channels. However, they're certainly not going to do that for all LFE content. Ultimately, they mix for what they want during playback and encode the signal accordingly.
That makes more sense.

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The confusion seems to be that you think the REW signal is encoded differently according to which channel you use
Not quite. From what I was reading it sounded like this: all signal levels from REW are the same, but AVR is boosting LFE 10db. However I saw what Jerry said and it got me caught up on the idea that ALL movies will have their LFE -10db, and since REW isn't a movie with that offset you end up with an extra 10db over the other channels. While the math works out in the end, the reasons for it I didn't understand correctly.
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post #15547 of 15555 Old Yesterday, 04:13 AM
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Originally Posted by Soulburner View Post
all signal levels from REW are the same, but AVR is boosting LFE 10db
It doesn't boost LFE 10dB (if it was done on the DSP we would get heavy clipping on the subwoofer). It is better think about it this way:
1) AVR *calibrates* subwoofer output so it has 10dB more loudness headroom than satellites (the voltage at the pre-outs could still be the same or even lower than satellites - it depends on the gain set at the subwoofer, so it is not 'electrical' or 'digital' boost, it is setting the gain structure of the system so the maximum loudness of the channel is 10dB higher).
2) Pass LFE as is to the subwoofer output channel (that is calibrated to sound 10dB louder for the same input signal level).
3) Attenuates redirected bass by 10dB so it is the same loudness at both sides of the crossover.
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post #15548 of 15555 Old Yesterday, 03:32 PM
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post #15550 of 15555 Old Yesterday, 06:21 PM
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post #15552 of 15555 Old Today, 07:17 AM
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Originally Posted by Soulburner View Post
Not quite. From what I was reading it sounded like this: all signal levels from REW are the same, but AVR is boosting LFE 10db. However I saw what Jerry said and it got me caught up on the idea that ALL movies will have their LFE -10db, and since REW isn't a movie with that offset you end up with an extra 10db over the other channels. While the math works out in the end, the reasons for it I didn't understand correctly.
The problem is that you are confusing the LFE channel and the subwoofer output. They are 2 different things. The subwoofer output contains BOTH the LFE channel and the re-directed bass from any channels that have Bass Management applied, (i.e., set to "Small.") However, the LFE channel 10dB boost in ONLY applied to the LFE channel, and it is applied BEFORE the LFE channel is combined with the re-directed bass.

REW doesn't output anything on the LFE channel. Therefore, the signal from REW never "sees" the LFE boost. If you're setup with the left and right channels being output to the receiver, and the speakers are set to "Small" in the receiver, the subwoofer output only contains the re-directed bass from the left and right channels. There is no LFE channel... and no 10 dB LFE channel boost.

Having said that, if you calibrate with REW using the re-directed bass from the L/R channels, you'll be properly calibrated and your receiver will take care of adding the LFE channel boost. If you want to measure the LFE channel, you need to get a test disc with test tones on the LFE channel. The only one I know of is the Goldline 5.1 Audio Toolkit:
http://www.gold-line.com/51atdvd.htm
See attachment, Chapter 16.

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Originally Posted by craig john View Post
The problem is that you are confusing the LFE channel and the subwoofer output. They are 2 different things. The subwoofer output contains BOTH the LFE channel and the re-directed bass from any channels that have Bass Management applied, (i.e., set to "Small.") However, the LFE channel 10dB boost in ONLY applied to the LFE channel, and it is applied BEFORE the LFE channel is combined with the re-directed bass.

REW doesn't output anything on the LFE channel. Therefore, the signal from REW never "sees" the LFE boost. If you're setup with the left and right channels being output to the receiver, and the speakers are set to "Small" in the receiver, the subwoofer output only contains the re-directed bass from the left and right channels. There is no LFE channel... and no 10 dB LFE channel boost.

Having said that, if you calibrate with REW using the re-directed bass from the L/R channels, you'll be properly calibrated and your receiver will take care of adding the LFE channel boost. If you want to measure the LFE channel, you need to get a test disc with test tones on the LFE channel. The only one I know of is the Goldline 5.1 Audio Toolkit:
http://www.gold-line.com/51atdvd.htm
See attachment, Chapter 16.

Craig
AFAIK, Craig, the output from REW HDMI4 is indeed the LFE channel. If your REW setup supports a full 7.1 HDMI interface, you could easily test this out. Send the low frequency test tone to Left+Right, turn the Left and Right amps off, and measure the signal coming from the sub channel only. Then output the same test tone to HDMI4, measure the signal, and see if it is 10dB higher. Unfortunately, my laptop only supports 2.0 HDMI, so I cannot run the test myself.
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Quote:
Originally Posted by AustinJerry View Post
AFAIK, Craig, the output from REW HDMI4 is indeed the LFE channel. If your REW setup supports a full 7.1 HDMI interface, you could easily test this out. Send the low frequency test tone to Left+Right, turn the Left and Right amps off, and measure the signal coming from the sub channel only. Then output the same test tone to HDMI4, measure the signal, and see if it is 10dB higher. Unfortunately, my laptop only supports 2.0 HDMI, so I cannot run the test myself.
Mine has 7.1 support and the LFE channel is 10 dB higher.

One thing I'm worried about is losing full 7.1 HDMI support when upgrading to Windows 10.

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post #15555 of 15555 Old Today, 08:57 AM
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Originally Posted by AustinJerry View Post
AFAIK, Craig, the output from REW HDMI4 is indeed the LFE channel. If your REW setup supports a full 7.1 HDMI interface, you could easily test this out. Send the low frequency test tone to Left+Right, turn the Left and Right amps off, and measure the signal coming from the sub channel only. Then output the same test tone to HDMI4, measure the signal, and see if it is 10dB higher. Unfortunately, my laptop only supports 2.0 HDMI, so I cannot run the test myself.
Thanks for the clarification Jerry. I'll check this the next time I set up REW for measurements. I've only ever used REW in 2-channel mode to measure the L, C, R and subs.

Craig

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