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**rbinck** · 03-13-2005, 10:18 AM

Dolby ProLogic II is a surround system that uses only two (stereo) sound channels to develop the four channels: Left, Center, Right and Surround. Most also have a subwoofer output also.

The way the ProLogic decoder works is (with a bit of technical poetic licence here) the Center channel is developed from the Left and Right channel where the signals that are equal in both the Left and the Right channel are removed from the Left and the Right channels and fed to the center channel. This has the effect of putting the dialog into the center of the screen.

Simularly, the surround channel is developed from the Left and the Right stereo channels and the issue is the encoding and decoding needs to be very precise for good results. The technology goes back many years and is not much different than the old 4 channel records.

Sonic problems include harmonics in the original recordings that trigger surround information where it should not be in the surround channel and ends up with some popping and a weird wave like almost echo effect in the surround information. Usually due to a poor recording with not enough effort in the sound mixing and encoding. One of the worse offenders, ironically, was the VHS tape Top Gun which was used extensively to demonstrate the surround effect back when ProLogic was state of the art. Some of the surround was very good when the planes were flying, but in other areas of the movie it was just plain (no pun) annoying.

Now we move to the 5.1 system where there is five full and independent channels for Left, Center, Right, Left Rear and Right Rear. Each channel is an independant track and none of these channels are derived from each other. There are 6.1 systems where there is a Center Surround that currently is derived from the Left Rear and the Right Rear in much the same way the Center channel was derived from the Left and Right channels in the ProLogic systems. Simularly, the 7.1 systems will develop a Left Side and Right Side channels.

In addition in all of the #.1 systems there is a LFE (Low Frequency Effects) channel to drive a subwoofer. By separating the subwoofer channel some of the bandwidth of the LFE channel can be used to extend the dynamic range (the lowest to loudest volume level possible) of the LFE audio. This is possibly the largest sonic difference people notice when moving from ProLogic to a #.1 system, the increased dynamic range of the audio.

Other subtile effects can be noticed, but they generally are noticed by people who have heard a lot of surround material. What usually happens when people get surround systems of any type, is they want all of the speakers to make sound. They want this whether there is something to be at that location or not. Their perception is that the room should be filled with sound. This is the reason for the simulated surround settings. There are different settings that different systems employ, but the most popular is the music hall simulation where the surround channels will output a delayed decaying signal that would simulate the reflections that would exist in a large hall.

Eventually if I can get them to sit and watch a well mixed 5.1 movie like Saving Private Ryan (good blowing up stuff) or believe it or not Midnight In The Garden Of Good And Evil, they soon get the idea of bullets traveling from the front to the back on one side or the other as opposed to traveling from front to back from one side to the center rear that is the ProLogic tendency. In the case of Midnight.... different birds chirping in the Left Rear and Right Rear channels giving you the sense that you are actually sitting in the garden. I must admit though, the sound was the only thing I enjoyed about that movie. Midnight..... that is.

If you are new to surround, I would say you should give your ProLogic some time until you get very accoustomed to the surround effects. Notice how the dialog is drawn to your screen and the surround effects actually sound better when the levels are set by the test tone as opposed to being jacked up all of the time.

When you move to 5.1, I would say go for a high level system. Imo, the Home Theaters In A Box systems just don't cut the mustard. A good 5.1 Home Theater receiver with at least 100w per channel, and a great subwoofer that will not slap on high output, is required at a minimum. Some of the HTIBs sound pretty good musically, but then go to hell when something blows up. You would be better off staying with your ProLogic system than moving to a medocre 5.1 system, imo.

03-13-2005, 10:18 AM	#2
rbinck Administrator Join Date: Apr 2004 Location: Katy, Texas Posts: 12,983	Dolby ProLogic II is a surround system that uses only two (stereo) sound channels to develop the four channels: Left, Center, Right and Surround. Most also have a subwoofer output also. The way the ProLogic decoder works is (with a bit of technical poetic licence here) the Center channel is developed from the Left and Right channel where the signals that are equal in both the Left and the Right channel are removed from the Left and the Right channels and fed to the center channel. This has the effect of putting the dialog into the center of the screen. Simularly, the surround channel is developed from the Left and the Right stereo channels and the issue is the encoding and decoding needs to be very precise for good results. The technology goes back many years and is not much different than the old 4 channel records. Sonic problems include harmonics in the original recordings that trigger surround information where it should not be in the surround channel and ends up with some popping and a weird wave like almost echo effect in the surround information. Usually due to a poor recording with not enough effort in the sound mixing and encoding. One of the worse offenders, ironically, was the VHS tape Top Gun which was used extensively to demonstrate the surround effect back when ProLogic was state of the art. Some of the surround was very good when the planes were flying, but in other areas of the movie it was just plain (no pun) annoying. Now we move to the 5.1 system where there is five full and independent channels for Left, Center, Right, Left Rear and Right Rear. Each channel is an independant track and none of these channels are derived from each other. There are 6.1 systems where there is a Center Surround that currently is derived from the Left Rear and the Right Rear in much the same way the Center channel was derived from the Left and Right channels in the ProLogic systems. Simularly, the 7.1 systems will develop a Left Side and Right Side channels. In addition in all of the #.1 systems there is a LFE (Low Frequency Effects) channel to drive a subwoofer. By separating the subwoofer channel some of the bandwidth of the LFE channel can be used to extend the dynamic range (the lowest to loudest volume level possible) of the LFE audio. This is possibly the largest sonic difference people notice when moving from ProLogic to a #.1 system, the increased dynamic range of the audio. Other subtile effects can be noticed, but they generally are noticed by people who have heard a lot of surround material. What usually happens when people get surround systems of any type, is they want all of the speakers to make sound. They want this whether there is something to be at that location or not. Their perception is that the room should be filled with sound. This is the reason for the simulated surround settings. There are different settings that different systems employ, but the most popular is the music hall simulation where the surround channels will output a delayed decaying signal that would simulate the reflections that would exist in a large hall. Eventually if I can get them to sit and watch a well mixed 5.1 movie like Saving Private Ryan (good blowing up stuff) or believe it or not Midnight In The Garden Of Good And Evil, they soon get the idea of bullets traveling from the front to the back on one side or the other as opposed to traveling from front to back from one side to the center rear that is the ProLogic tendency. In the case of Midnight.... different birds chirping in the Left Rear and Right Rear channels giving you the sense that you are actually sitting in the garden. I must admit though, the sound was the only thing I enjoyed about that movie. Midnight..... that is. If you are new to surround, I would say you should give your ProLogic some time until you get very accoustomed to the surround effects. Notice how the dialog is drawn to your screen and the surround effects actually sound better when the levels are set by the test tone as opposed to being jacked up all of the time. When you move to 5.1, I would say go for a high level system. Imo, the Home Theaters In A Box systems just don't cut the mustard. A good 5.1 Home Theater receiver with at least 100w per channel, and a great subwoofer that will not slap on high output, is required at a minimum. Some of the HTIBs sound pretty good musically, but then go to hell when something blows up. You would be better off staying with your ProLogic system than moving to a medocre 5.1 system, imo.