|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.