Meridian Unplugged - Powered by MrTechGuy.co.uk Forums Discussion Meridian - Systems Cinema / Discrete / 10dB - definitive answers required!

I really want to understand the Dolby Digital LFE channel from both a recording and a playback perspective. I've been doing some reading of past threads and I'm as confused as ever.

The confusion is, of course, surrounding the +/- 10dB issue. I've skimmed through a couple of Dolby white papers and I'm still in the dark. So...

1) What level should the LFE channel for movies be recorded at vs. the other channels?

2) What level should the LFE channel for music be recorded at vs. the other channels?

3) What level is the LFE channel calibrated at in the playback system vs. the other channels? Edit: It appears that the LFE channel is calibrated for +10dB response vs. the other channels at the mixing desk.

4) On playback, does the DD decoder alter the LFE channel level by 10dB?

5) The Dolby spec. apparently requires +10dB to show as 0dB. Does the "Cinema" DSP preset alter LFE levels or just what is displayed?

6) The "Discrete" DSP preset sets the LFE channel to -10dB. Does this correspond to any actual change in the LFE levels?

7) Did DTS follow Dolby's lead in an identical, or similar way?

8) What happens to the LFE level in the DTS decoder?

Please: No speculation in this thread. Only reply if you actually know the answers.

Thanks in advance,
Joel

Ok, here's what I think is going on... (subject to improvement over time!)

At the mixing desk:

1a) The sub is calibrated for a flat response with the other speakers given the same input level.

2a) For movies, the LFE channel is calibrated for a +10dB response vs. the other speakers.

3a) For movies, the soundtrack will actually be recorded at -10dB vs. the other channels for the same SPL.

4a) in their "5.1-Channel Music Production Guidelines", Dolby says:

As discussed in Section 2.2, bass content can easily overload a recording system. This is the historical reason for the creation of the LFE channel and the practice of recording to tape 10 dB below reference level and boosting the playback 10 dB to achieve a flat response. In this way, plenty of bass can be included in the recording without causing tape saturation or distortion.

Because lower frequencies require more energy than higher frequencies to be perceived as the same level of loudness, recording energy-sapping deep bass on a separate track at a lower level to allow for a wider dynamic range is still good practice. For these reasons, plus the added bonus of legacy cinematic recording playback compatibility (both in the studio and the home), the +10 dB practice is still recommended.

This implies that the LFE channel for music should also recorded at -10dB.

At home:

1b) The sub is calibrated for a flat response with the other speakers giving the same input level.

2b) For movies, the LFE channel must actually be played back at +10dB to sound the same as when it was mixed. As this corresponds to a net 0dB gain (recorded at -10dB, played back at +10dB), it is correct for the "Cinema" DSP preset to display "0dB".

3b) Given (4a), for music the LFE channel must also be played back at +10dB. Assuming that multi-channel "music" soundtracks are always decoded in the DVDA/SACD player, this means that the DVD player must add 10dB to the LFE channel on decoding.

Cinema
The Cinema preset is recommended for listening to multichannel
movie soundtracks when no further processing is required.

Discrete
The Discrete preset is similar to Cinema, except that it uses the
Music speaker layout and sets the LFE level to -10dB.

In summary, I reckon that "Cinema" boosts the input LFE signal by 10dB (displaying the LFE level as 0dB), and "Discrete" does *nothing* to the LFE signal (displaying the LFE level as 0dB). What this means is that "Discrete" should not be used with a DD/DTS soundtrack decoded in the processor without the risk of losing 10dB of LFE. In other words, "Discrete" should only be used with multi-channel PCM, MHR or analogue inputs.

Do music tracks have an LFE channel on the recorded media?

In the case of my 40+ DVD-A music collection, almost all of 'em have an active LFE channel.

Happy Trails!
Vince@Freewheelcycle.com

the 10dB LFE offset stems from the original 5.1 channel 70mm magnetic soundtracks. The LFE channel was placed on the two tracks for the surround channels. The surround channels were then high pass filtered at 250 Hz. To provide additional headroom, the LFE channels were recorded -10dB below reference to a) avoid distortion and, b) to avoid partially erasing the HF of the surround channels it shared through mag head-gap saturation. The LFE channel on playback would be passed through a low pass filter and then boosted by 10dB to restore it's original level. This would give one a max SPL of 105dBC for each of the 5 channels and 115 dBC for the LFE channel. To avoid calibration errors and confusion, the practice was continued in the cinemas for Dolby Digital, DTS, and SDDS sound tracks as they came into use.

Calibration of each channel in the home (using the internal test signals) is to the same SPL, assuming a 2 octave band limited signal (REF=-30dBfs); 20-80 Hz for the LFE and 500-2 KHz for all other channels. The +10 dB adjustment is applied by the surround decoder for Dolby Digital sources. DTS may actually use something different. When a Discrete digital input on a Meridian controller is used, the controller provides no +10 dB offset as this is anticipated to have been applied by an upstream surround decoder. In the case of a Meridian source, the decoding and offset (if required) would be done by the DVD player.

Dolby Digital (and one hopes DTS) have flags in the originating bit-stream which would identify film material from other sources such as music. This flag tells the decoder whether or not to apply the +10 dB offset or not.

As for music tracks, it's up to the producer to use the LFE track on Dolby Digital or not. Most will simply use 5 full range channels, as we rarely have the need for 115 dBC SPL in music.

Did that help?

I myself have found most DVD-A's need LFE to be set to -10 for it to sound correct. However, I have at least one that I need to keep it at zero. So I think sound engineers were a little confused at what level to set it too and some early DVD-A's have LFE set wrong or too low requiring a setting of zero. Problem with that is they threw away some bass resolution doing it.

That's what I figured: the "Discrete" preset does nothing at all to the signal (LFE is at -10dB vs. the "Cinema" preset).

I'd hope so.

I'm not sure this is possible with the latest Meridian processors, because with the 861v4 preset model (also used by the G9x and 568 v2.5), decoding is separated from the preset. Yet it is the "Cinema" preset that appears to apply the +10dB boost. What I'm getting at is that, with any DD/DTS music with the LFE channel at -10dB that is decoded by the processor, the "Cinema" preset must be used rather than "Discrete".

Of course, this is where the Meridian DVDA player + SmartLink comes into play, because the SmartLink metadata will tell the processor what type of disc the (decoded) PCM came from (Music6 or Movie6), so that the processor can apply the correct default preset.

Good thread. Should make a Wiki out of the conclusions...

Mark

Cheers Mark, that's the plan. I think I know what is going on now. The only thing to confirm (with Meridian) is which discs and/or multi-channel encoded formats are considered "Music" by SmartLink and which are considered "Movie".

For instance, are all DVDA discs "Music"? Or are DD/DTS 5.1 considered to be "Movie" in all cases? What about DD/DTS 5.1 on a DVDA disc? etc. Similar considerations for 2.0 DD/DTS tracks on DVDA and DTS CDs (though that's a slight tangent from this particular thread).

[quote=VirusKiller
the SmartLink metadata will tell the processor what type of disc the (decoded) PCM came from (Music6 or Movie6), so that the processor can apply the correct default preset.[/quote]

This begs the question - should we change the default preset for DVD-A from Discrete to Cinema?

A second question, which might be related or not to this, and which I have wanted to ask for some time is this. On most DVD-A disks that I have, the 800 player detects that are MLP tracks on the disc and the 861 processor defaults automatically to Discrete playback.

On some DVD-As, however, the 800 detects only 'Multi' tracks, no MLP, and then the selection of Discrete is NOT automatic, though I still select it manually.
Do not ALL DVD-As use MLP, and if so is Discrete NOT the most appropriate mode to select?

I guess I'm in the minority here in that I have a 7.0 channel system with no sub. I have been wondering what happens to the bass from the LFE channel in my instance.

No, because your 800 should apply a +10dB gain to the LFE channel on decoding. You don't want the 861 to apply another +10dB gain which is what Cinema would do. We have to assume that the output from a Meridian DVD-A player (digital or analogue) has the correct LFE level. That is why Discrete should be used as it does nothing to the input levels (speaker time-alignment and MRC still applied of course).

Correct. Not all DVD-As use MLP. Your observations would suggest that Meridian players only treat MLP as "Music" from DVD-A.

I should be re-routed to the other speakers. Meridian said on Hiker's day that "we will not allow you to lose bass".