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Dynamic Range Compression Note  

post #1 of 11
Thread Starter 
I was listening to 'Reign of Fire' yesterday at fairly high levels (-15Db on 3802 with P-25 Power-amp) and used this opportunity to test out the DRC setting on my AMP. I have three settings, Low - Med - High. I chose Low. I think that very heavy settings (High or 100%) should only really be used when listening late at night with a baby asleep upstairs!

Much to my surprise, I found the setting was rather useful. I had some pre-conception that it may destroy 'the purist' feel of Home Cinema but I was wrong. Voices were indeed highly intelligible and the bass (although present) was less overwhelming; almost better blended into the soundtrack. You could definitely tell that there was a difference in sound but it was still rather involving and did not destroy the soundtrack in anyway.

Dolby Digital soundtracks are usually not mixed appropriately (or specifically) for the home market. Which is why on most action DVD's you will find that you can have the volume set quite high to obtain intelligible dialogue but, as a side effect, this comes with massively overwhelming sound effects. This isn't really a 'bad' mixing issue rather more a 'straight from print' version of the sound track. If any of you have been to see the new 'James Bond' or 'Lord of the Rings' in a Dolby Digital equipped Cinema, you will notice that the dialogue is of a normal and comfortable level and that the rest of the soundtrack is hugely explosive; James Bond actually had me covering my ears at one point!

Therefore, its no wonder that most people have to crank up the volume to hear the dialogue on the DVD version but 'ride the remote' when big special effects scenes appear. For this reason alone, Dolby Digital incorporated dynamic range compression on all of its tracks allowing you to watch films at high levels without reaching for the remote. From what I have read, its quite a sophisticated and clever piece of technology which, on certain films, I will be using.

As for the purists out there, well, there's no argument really. Yes, it is being processed and the dynamic range of soundtracks has been altered to 'up' higher frequencies and 'down' lower frequencies, Therefore its not as pure. However, for listening at loud levels on a majority of consumer equipment that struggle to reach reference levels and maintain quality, DRC is the answer.

Dolby would class the use of DRC as 'typical' and expect it to be used in the consumer market for listening to DVD's. It done a fabulous job last night on 'Reign of Fire' and would recommend auditioning a loud-score film (at loud volume levels) with this on and then off. As said before, I found that the soundtrack level was clear and comfortable and I did not wrestle with the remote once when SFX-heavy scenes appeared.
post #2 of 11
The dynamic range compression feature that is part of the Dolby Digital standard is a very cool feature. It goes way beyond a conventional "after-the-fact" compressor and actually relies on compression instructions inserted in the bitstream when it is encoded. These instructions basically tell the decoder when, and how much, to scale the incoming signals. Playing with the no compression setting is basically telling the decoder to ignore the instructions. Very sophisticated.

In reading between the lines of various Dolby white papers describing the compression system, I draw two clear inferences:

a) Dolby views the compression feature as a MAJOR feature of Dolby Digital and one of the key reasons that Dolby Digital is a format that can be used with any kind of system: from a high-end home theater set-up to playing movies through tiny built-in TV speakers.

b) I get the sense that Dolby expects the compression feature (to one degree or another) to be used with the majority of systems and those playing sountracks with NO compression to be in the minority. For example the standard light compression setting (what Dolby refers to as "line" mode) is specifically designed to result in dynamic range comparable to a good quality VHS Hi-Fi recording.

I'm not absolutely positive, but I believe that all of the compression options built into most receivers are variations of this fairly mild "line" mode. If so, then the maximum compression available on a receiver would only be about a 10 dB reduction of peak levels. If you were playing at Dolby reference levels, this would make the loudest possible peaks in any one channel 95 dB instead of 105 dB -- still some 20 dB louder than average dialog levels.

Dolby Digital also features a much more aggression compression mode which they refer to as "RF mode". This cuts the dynamic range down to that of a standard TV broadcast and is designed for use with cable set-top boxes feeding the signal to a TV using RF modulators (Channel 3 antenna connections). The total dynamic range in RF mode appears to be about 35 db or less than half of a non-compressed Dolby Digital signal. For anyone with a Motorola digital cable box, this is the default "heavy" compression setting.

One note: Dolby Digital compression only works on Dolby Digital signals and is usually accessed somewhere in the surround sound setup menus. Some receivers have a different button (labelled "midnight" on Pioneer receivers). This appears to be a different after the fact compression system (still done in the digital domain) that works with any kind of source material.
post #3 of 11
Thread Starter 
I got to say I agree with hwc entirely here. There are plenty of 'purists' that I come across that believe that this technology degrades or destroys the sound field to some degree. I disagree and after watching a few films the other night, I am pretty much always going to leave my Denon set to 'Low' DRC.

In short, DRC enables me watch movies at a higher level (more cinematic) without wrestling with the remote. As said before, dialogue is so much more intelligible as well at higher volumes so this just enhances the whole experience.....

Brilliant. Just brilliant. Expensive, also, but Brilliant. :D
post #4 of 11
The decision to use the compression feature depends on a lot of factors including the capability of your system, the size of your room, whether your speaker levels are properly calibrated, and what is "loud" too you.

I don't use compression with movie soundtracks. I find that 6 to 8 dB below Dolby reference levels provides natural dialog level, my system can play at that level without strain, and the loudest passages don't send me diving for the remote, despite being quite loud.

However, there are many system configurations where I would say that you MUST use the compression feature (for example, any home-theater-in-a-box grade system or any speaker system with 3.5 inch woofers). Also, many people probably don't like soundtracks as loud as I do.

Dolby's compression is a great feature. I reckon that more people should use it.
post #5 of 11
Say NO to compression! Get your neighbors duck in fear with their ears covered! :D
post #6 of 11
Is there any standard provided by Dolby as to how much compression is applied? HWC mentioned 10dB in the max setting, but what about those processors that just have on or off?
Does Dolby say specifically that dynamic range compression ON should be X dB, and then it is up to the manufacturer to provide additional settings just as long as one of them is X dB?
post #7 of 11
Originally posted by Dinn
Is there any standard provided by Dolby as to how much compression is applied? HWC mentioned 10dB in the max setting, but what about those processors that just have on or off?
Does Dolby say specifically that dynamic range compression ON should be X dB, and then it is up to the manufacturer to provide additional settings just as long as one of them is X dB?
For certification of a line level decoder product such as a pre/pro or a receiver, Dolby REQUIRES two dynamic range control options:

The MAX DYNAMIC RANGE setting is full dynamic range with no compression.

The STANDARD DYNAMIC RANGE setting responds to the dynamic range control "instructions" encoded in the bitstream and reduces dynamic range to approximately the level of of a VHS Hi-Fi recorder.

It is not possible to put an exact number on it because it's a very complex system. It leaves sounds near the level of dialog alone. It uses a limter at the very extremes, cutting ultra high level signals and boosting ultra low level signals. In between, the instructions inserted in the bitstream call for variable compression ratios. Not only do you have these five "bands", but the compresssion instructions are different depending on type of recording. There are two flavors of instructions for film content, two flavors of instructions for music content, plus a set of instructions for speech content. The engineer selects which to use when encoding the Dolby Digital signal. These five sets of instructions vary considerably. So all you can really say about STANDARD DYNAMIC RANGE is that it behaves according to the instructions encoded on that particular disc.

So there are your two settings on a receiver that offers MIN/MAX or OFF/ON settings for dynamic range control.

Dolby also allows decoder manufacturers to offer variable settings -- i.e. the MAX DYNAMIC RANGE setting, the STANDARD DYNAMIC RANGE setting, and any setting they like in between. These middle settings are achieved by using the STANDARD DYNAMIC RANGE instructions, but scaling them. A scaling factor of 1.0 would give you Standard, a scaling factor of 0.0 would give you MAX, a scaling factor of 0.5 would give you something halfway in between. So receivers that offer middle options are just allowing the selection of one or more "scaling factors" in between the two extremes.

The examples Dolby gives show a maximum reduction of loud level sounds of 10dB and a max boost of low level sounds of 15 db. However, those are just "representative" numbers to give you an idea. They don't appear to really correspond to any of actual sets of compression instructions. I think that its fair to say that using the STANDARD DYNAMIC RANGE setting (compression on) in most receivers will give you an appropriate amount of compression for that type of recording that will result in an overall dynamic range roughly comparable to a VHS Hi-Fi tape, which is still a pretty high quality, dynamic signal. Using compression on a Dolby Digital receiver is never going to produce something that sounds noticeably compressed. It's basically just going to ease back on how loud the explosions and shattering glass get relative to the dialog and do it without any bad side effects.
post #8 of 11
I've used it. On my processor, the options for DRC are 0 - 50 - 100.

I think it's effective, and non-degrading except to the extent, that limiting dynamic range is counter-intuitive to those of us old enough to remember the old days when all we had were compressed recordings. We used dynamic range expansion from dbx and Phase Linear to expand 50-60 db dynamic range of tapes and albums.

All in all, DRC is effective, non-intrusive, but I won't use it unless it's late night, wife upstairs sleeping etc. Reaching for the clicker is an old habit to break. But I don't want to take the chance of missing a few dbs of impact just so my wife sleeps better...

Boom! "What?"

post #9 of 11
I use it too on my Meridian processor, which has Low, Med, Top and Max. Each varying the amount of compression at top-up and bottom-down part of the output signal. Usually use it late at night, so as not to wake everybody else up.
post #10 of 11
I have experienced quite a bit with this for the last few days. I always watch late at night with my young daughter sleeping in the room next door, so I thought it would be useful for me. But so far I think that the compression is to small to have any real effect. To be honest, I don't hear much difference. But I have decided to use it anyway.

And I know that the compression in my processor corresponds with Dolbys scaling factor 1.0, I have checked it out.
post #11 of 11
It's easier to tell if it's working by toggling through the settings when the movie scene is already loud. You can hear it quiet down. I believe it only works for DD5.1, but I'm not positive.

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AVS › AVS Forum › Audio › Audio theory, Setup and Chat › Dynamic Range Compression Note