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DIY ProLogic II
DIY ProLogic II
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Old 26th May 2017, 12:06 AM   #1
Patrick Bateman is offline Patrick Bateman  United States
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Default DIY ProLogic II

I am not using a receiver for my home theater. My source is a personal computer and it's output goes to a four channel amp. (Not a receiver.)

If any of you are in a similar boat, and you're interested in doing some Dolby ProLogic II type stuff, here's how.

OK, first off, what is ProLogic II?

The short answer is that ProLogic II is a decoding scheme that takes a stereo signal and turns it into six channels. There were some later iterations that can turn it into eight channels and there's really nothing stopping you from turning it into a hundred channels. The only limit is your amp channels and your sound card channels.

Click the image to open in full size.
Stereophile did an excellent interview with Jim Fosgate (of Rockford Fosgate) about his ProLogic II decoder and it's history : https://www.stereophile.com/interviews/1204fosgate/

Click the image to open in full size.

ProLogic II uses a matrix to do the decoding. The pic above is that matrix. Dolby has published their spec and this is it. Let me translate the pic into English:

1) We take the left channel of the source and feed it to the left channel of the output
2) We take the right channel of the source and feed it to the right channel of the output
3) We take .707% of the left channel and .707% of the right channel, combine them, and feed them to the center channel of the output. (Note that this means the center channel is louder than both the left and the right channels.)
4) We take .87% of the left channel, invert it, and combine it with .49% of the right channel, and feed that to the left surround channel
5) We take .87% of the right channel, invert it, and combine it with .49% of the left channel, and feed that to the right surround channel
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Old 26th May 2017, 12:08 AM   #2
Patrick Bateman is offline Patrick Bateman  United States
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The easiest way to do ProLogic II is to do it with the three front speakers.
No processing is required. Just combine the left and the right channel, feed them to a dedicated amplifier, and adjust the center speaker so that it receives 41.4% more power than the left and the right speakers.
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Old 26th May 2017, 12:15 AM   #3
Patrick Bateman is offline Patrick Bateman  United States
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The next easiest way is with a MiniDSP.
You need the "2x4 ADV" plugin.

This plugin is intended for people that want to do sub-satellite setups. IE, you can combine a stereo input to produce a mono output, for a sub.

But there's nothing stopping you from using this plugin for Dolby ProLogic II.

Just go to the 'routing' tab of the plugin, and create a mono output. This mono output will correspond to your center channel. Then use the 'delay/gain/rms' tab to adjust the output of the center so that the output is 41% higher than the left and the right.

To produce the rear channels you'll need another miniDSP because four outputs isn't sufficient.

Note that the plugin doesn't offer a way to invert the inputs, so I think you'd literally have to make a cable to create the surround with MiniDSP.

IE, to make the left surround channel, we need .87% of the left channel and .49% of the right channel combined, but the left portion is inverted. Since MiniDSP can't invert an input, you would have to make an RCA cable that inverts it. (Just cut the cable in half, swap the positive and negative, and put it back together.) Obviously you'll want to mark this cable because it could really make troubleshooting confusing
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Old 26th May 2017, 12:20 AM   #4
Patrick Bateman is offline Patrick Bateman  United States
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Another nice thing about MiniDSP is that we can still use all of it's features along with the matrix. For instance, in my home the main speakers are close to three meters away from the couch, but the surrounds are a fraction of a meter away. With MiniDSP you can implement attenuation and delay to 'fix' the speaker locations. This makes all the difference in the world; IMHO the main reason that a lot of surround sound setups sound awful is because you MUST compensate for loudspeaker location and amplitude. Even if you're perfectly equidistant from all five speakers, you'll likely find that the room itself is changing the frequency response, which can 'drag' the image towards the louder speakers.
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Old 26th May 2017, 12:22 AM   #5
Patrick Bateman is offline Patrick Bateman  United States
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https://sourceforge.net/p/equalizera...n%20reference/

If you're running a home PC, APO can do nearly everything that MiniDSP can do.

This is the main reason that I wrote this post; I wanted to ditch my MiniDSP. I love the thing, but it sure is a mess of cables that doesn't look too good in my living room.

So you can implement all the filters using APO, and if you're using a HTPC you can have a setup that looks really clean.

In particular, we need the copy function:

opy (since version 0.9)
Syntax:
Copy: <Target channel>=<Factor>*<Source channel>+...
Copy: <Target channel>=<Source channel>+...
Copy: <Target channel>=<Constant value>+...
Description:
Replaces the audio on the target channel by the sum of the given source channels with optional factors. To add instead of replace the audio on the target channel, the target channel itself can also be a source channel. The factor can also be specified in dB by appending dB. Multiple channel assignments can be specified on a single line by separating them with spaces, therefore a single assignment must not contain spaces. Instead of channel and factor, a constant value can be specified. To avoid ambiguity with numerical channel indices, the constant value must contain a decimal point. For more information about channel identifiers, see the Channel command.
Example:
# Adds the audio on channel R multiplied by 0.5 to channel L
Copy: L=L+0.5*R
# Replaces the audio on channel L by the audio on channel R
# plus the audio on channel C attenuated by 6 dB
Copy: L=R+-6dB*C
# Replaces the audio on the first channel by the audio previously on channel R
# Also sets the audio on channel R to the constant value 0.5
Copy: 1=R R=0.5
# Attention: Sets the audio on channel L to the audio on the second channel
# (not to constant value 2, because no decimal point is present)
Copy: L=2
# Real world example: Replaces the audio on the subwoofer channel with
# the audio on the left channel while muting all other channels of a 5.1 speaker system
# (useful for measuring the subwoofer response in REW)
Copy: SUB=L L=0.0 R=0.0 C=0.0 RL=0.0 RR=0.0
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Old 26th May 2017, 12:23 AM   #6
Patrick Bateman is offline Patrick Bateman  United States
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Click the image to open in full size.

There's a GUI for APO which makes the program way more user friendly. Get it here:

https://sourceforge.net/projects/pea...apo-extension/
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Old 26th May 2017, 12:32 AM   #7
billshurv is offline billshurv  United Kingdom
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DIY ProLogic II
It's worth an experiment. I have 3 JBL control-1s waiting to be centre and surround just no surround decoder. But in my case the driver is that I want to boost dialog (centre) as for late night films when kids in bed the effects are too loud if you turn it up so you can hear the speech. I've got channels to burn on my 2x10 so why not...
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Old 26th May 2017, 12:53 AM   #8
Patrick Bateman is offline Patrick Bateman  United States
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Quote:
Originally Posted by billshurv View Post
It's worth an experiment. I have 3 JBL control-1s waiting to be centre and surround just no surround decoder. But in my case the driver is that I want to boost dialog (centre) as for late night films when kids in bed the effects are too loud if you turn it up so you can hear the speech. I've got channels to burn on my 2x10 so why not...
At low frequency our perception of location is based on phase. Due to that, the presence of a center channel can narrow the stage. IE, if you have two speakers, the stage is defined by those two speakers. Add in a third and the location gets 'pulled' toward the center because the center features sounds that exist in both the left and in the right.

The simplest way to fix this is with a simple high pass; you just let the left and the right speakers cover the low frequencies, and high pass the center.

Click the image to open in full size.
If you think about it, this is also one of the reasons that Opsodis works so nicely. Removing the low frequencies from the speaker in the center allows you to make a larger soundstage.
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Old 26th May 2017, 12:57 AM   #9
billshurv is offline billshurv  United Kingdom
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I'll have to read up on opsodis. I need to high pass the mains as well a little as they struggle with the extreme LF on blu-rays. I have a sub, just not integrated. Some research for the long weekend
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Old 26th May 2017, 02:25 AM   #10
eriksquires is offline eriksquires  United States
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Nice info, Bill, but I don't see an important component here in the thread, which is the channel steering.

Just quickly, I worked for one of Dolby's competitors and was able to look at the reverse engineered schematics, as well as evaluated our own version. So I know at least the pro end as well as anyone left around. If you know what a CP-50 was, you'll know what I'm talking about. What follows here specifically excludes mag-tape movies, and Dolby Digital or DTS.

You got the encoding right (as far as I can tell) but since Dolby Stereo tried to encode 4 (L,C,R,S) or 5 channels (split surround) into two analog tracks that were actually on the film, the decoding was never this straight forward. When VHS came out of course it was first stereo then Hi-Fi stereo, and they took the tracks as-is from the film versions.

While we can mathematically make sense of adding L and R to get a center, removing the center from the L and R signals is quite tricky in an analog realm.

Dolby Stereo (as the pro version was called) steered channels aggressively. The first home version was called Dolby Surround and if memory serves, did not steer at all. The first home version to incorporate the channel steering was called Dolby Pro-Logic, and AFAIK, was basically Dolby Stereo for home, and on a chip.

What I mean by "channel steering" is that there were a number of VCAs (Voltage Controlled Amplifiers) which would, depending on a number of factors, raise and lower the gain of each channel. The logic seemed to want to choose between immersion, where all channels are active, and effects, when events would take place predominantly L, C, or R for instance. So, an explosion on the right would cause the L,C and S channels to drop.

As I recall (and we're talking decades back) there was also a number of phase inversion happening to try to isolate the center from L and R. A decoding matrix involved around 8 VCA channels and 16 op amps which were used for mixing various combinations of signals together, and altering the mix dynamically. So VCA's were used to control not just the output of each channel, but also the mixing used to try to isolate each channel.

Dolby Prologic II of course was an enhancement which added L/R surround steering (my company actually did this first!) and later on, consumer gear also got a music mode too. It is my belief (without data) that DPL II also had a nicer overall steering mechanism, which was not as aggressive. ( BTW, if you have Neo6, the music mode is really great with a center channel! ).

The idea in theaters was to give as many people as good of an experience as possible. If you were up front, channel steering was essential. I imagine it was quite maddening for recording engineers, as doing a delicate pan from L to R was probably a giant PITA, as you could not avoid the steering mechanism.

We are very fortunate to be in an era of mostly discrete channels now.


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Last edited by eriksquires; 26th May 2017 at 02:54 AM.
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