Dolby Pro-Logic

[joel2762]

A dvd labeled Dolby 5.1 Surround is a surround remix created by Dolby. But when it's labeled Dolby Pro Logic, does this simply activate the Pro-Logic decoders and lets them create the sound effects for other channels? That's what appears to me. Thanks in advance.

[mtrycraft]

A dvd labeled Dolby 5.1 Surround is a surround remix created by Dolby. But when it's labeled Dolby Pro Logic, does this simply activate the Pro-Logic decoders and lets them create the sound effects for other channels? That's what appears to me. Thanks in advance.

Pro logic is a surround matrixed encoding where the surround is mono, mixed electronically into the two front channels. When it is decoded, it has 4 channels with the rears in mono. Pro logic II splits the mono to give a stereo image in the rear.

[joel2762]

I understand now. Thanks. But for example, If a dvd says Mono 1.0, it is played through center channel. But if a dvd says Dolby Pro Logic, what do the decoders play back? A mix from Dolby or does it just activate the Pro Logic circuit and lets it create it's own mix. As you stated [B]When it is decoded, it has 4 channels with the rears in mono. Pro logic II splits the mono to give a stereo image in the rear. Does this mean that this format on dvd simply lets the Pro Logic decide the surround track, or did Dolby have to create a special one? Is this just a simple thing where instead of using time to create surround mixes for small tv episode dvd's, etc, they just let their technology do it instead? Thanks

[Sir Terrence the Terrible]

I understand now. Thanks. But for example, If a dvd says Mono 1.0, it is played through center channel. But if a dvd says Dolby Pro Logic, what do the decoders play back? A mix from Dolby or does it just activate the Pro Logic circuit and lets it create it's own mix. As you stated [B]When it is decoded, it has 4 channels with the rears in mono. Pro logic II splits the mono to give a stereo image in the rear. Does this mean that this format on dvd simply lets the Pro Logic decide the surround track, or did Dolby have to create a special one? Is this just a simple thing where instead of using time to create surround mixes for small tv episode dvd's, etc, they just let their technology do it instead? Thanks

First, a DVD should never read Dolby Prologic. Its should read Dolby Stereo or Stereo Surround. Dolby prologic is a decoding engine, not a audio format. And you are correct, Dolby prologic II creates the rear soundfield as the Dolby Stereo encoding does not contain a stereo surrounds, just mono. DPL II steers the mono surround with some information in the front channels to create a stereo surround in the rear. This process does not require a specially mixed soundtrack, but derives all of its decoding information directly from the stereo surround track.

[joel2762]

Yes you're right. The dvd actually reads Dolby Digital, and when played, my reciever indicates DOLBY 2/0 and the Pro Logic light is lit. I have a Sony STR-DE585 reciever, just wondering if anyone knows if this reciever has a seperate indicator for Pro Logic II or does it just show Pro Logic for both...And one more thing. So if a dvd is only labeled stereo..Not Dolby stereo, what will happen? Will the Pro Logic activate only when the Dolby signal is present or what?

[agtpunx40]

If a reciever has dolby pro logic II, it won't have the old version, dolby pro logic, so it won't have a separate light for each, obviously. You can turn on pro logic on any source you want, for example if I have the effect turned on on my dvd source on my yamaha reciever, and it doesn't pick up a 5.1 source, it automatically decodes it with pro logic, so if I'm listening to a cd through my dvd player, I turn it off (I don't like listening to music through the decoder although some people do). The difference between a regular stereo source and a dolby surround source, for example a cd, and a vhs tape or a tv show, is that the dolby surround source is specifically mixed to be played through the pro logic decoder. A TV show or vhs tape which is not specifically mixed to be listened to through the decoder may end up with strange things being played through the rear channel at one point or another. A properly mixed source will have the correct sound in the correct place throughout the movie or show.

[joel2762]

Thanks everyone for your help. I get it now. Curious about something else now. Does anyone know how the Pro Logic decoders work? Do they just decode the different sound frequencies or something? Maybe something like, the general voice frequency is directed to the center? Or something like that? Thanks..

[agtpunx40]

it has to do with phase differences, volume differences, and frequency. If you really want to get into it, http://www.dolby.com/tech/whtppr.html
it's pretty interesting stuff, especially considering it first came out in the early 80s.

[markw]

it's pretty interesting stuff, especially considering it first came out in the early 80s.

Actually, it owes it's beginnings to the original matrixed "quad" systems of the early 70's.

[kelsci]

Markw; DPL does owe something of its beginnings to quad sound of the 70's but if I am correct in being more specific, it was supposed to be similar to Sansui's QS system rather than Columbia's SQ system. For a time, Sansui bought out a QS device to decode Dolby Surround( and I think DPL) claiming their QS device did a better job then Dolby's decoders. I have never read any info whether this was proven.

[joel2762]

Thanks everyone for your help. Thanks agtpunx40 for the link. That's some amazing stuff! Great! Dolby has a webpage! I guess I'll be surfing that for the next few days :p One final question, when I switch between 2 channel mode and Normal surround mode on my reciever (just adding the rear speakers with no hall effects, etc) The front main speakers tend to lose a small amount of bass and get a small bit lower in volume. Is this due to amplifier or DPL processing? Thanks for all help!