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Hello everyone!

This post describes how to implement real-time on-DSP stereo-to-quad-channel conversion using DDRC-88A or DDRC-88D with the 88BM plugin.

The 4-channel output is generated by mixing direct and inverted Left and Right stereo components in certain proportions.
For example, this is the scheme I like to use:

Front Left = Left
Front Right = Right
Surround Left = 0.71 * (Left - 0.5 * Right)
Surround Right = 0.71 * (Right - 0.5 * Left)

Upmixing must be done before Dirac filters, i.e. on LFE Mgt and Routing tabs. This is not a problem when two instances of Left and Right signals are present on different inputs. For example, the Front Left/Right signals supplied to inputs 1/2 of DDRC-88 must also be present on inputs 7/8. Here are some ways to do that:
a) configure the source/transport device up the chain to duplicate L and R signals on two pairs of cables connected to the DDRC,
b) use an Y-cable to connect one output to multiple inputs of the DDRC (for example, an adapter like this will work with DDRC-88A or the S/PDIF version of DDRC-88D),
c)create unfiltered L and R signals on separate outputs of the DDRC and loop them back into a free pair of inputs (but note that this input will be affected by DDRC Master Volume and processing delay).

Once this part is done, only few simple steps remain.

In LFE Mgt tab, invert the extra instances of Front Left and Front Right by putting a custom biquad instead of high-pass filters:
b0=-1,
b1=0,
b2=0,
a1=0,
a2=0,

Mind that b0=-1 (negative one). The second biquad will be added automatically with coefficients of 1,0,0,0,0 (where b0=1 positive).

The result should be this:



Here, Front Left and Front Right signals are present on Ins 1-2 and 7-8, and the second pair is inverted. Frequency response does not indicate any change because inversion only changes the phase. Do not forget to press Process, disable Bypass mode, and apply the new biquad on both inputs.

In the Routing tab, mix Front Left with the inverted version of Front Right and vice versa, and send the result to Surround Left and Right:



The surround channels here are Dirac 5 and 6. Signals are mixed according to the matrix: attenuation by -3 dB corresponds to a multiplier of 0.71, and -9 dB to 0.71*0.5.

Done! The result will be processed by Dirac filters before being sent to speakers in the same manner as any quad-channel input.

Below are a few notes about this type of upmixing:

1. This scheme includes no delays or frequency-dependent gains and phase shifts, therefore the stereo image remains intact. The result will seem very close to the original unless you turn your head or move around a bit. Still, even at a fixed listening position, you may notice that reverberation tends to sound more natural after upmixing, and front speakers seem to be a bit wider or further than they really are (depending on mid/side balance in the material). I personally prefer this subtle kind of processing over Dolby ProLogic or other popular algorithms.

2. Depending on your configuration of filters, crossovers, delays, speakers, room and listening position, some attenuation may occur in the bass region after upmixing due to cancellation between front and rear. I’ve corrected for this by sending low-passed L and R signal copies to the LFE channel at -15 dB, but other solutions are possible (increasing subs volume, adding a high-pass, all-pass filter or delay to rear channels, etc.) or none may be required.

3. The matrix can be expanded to 5.0 or 5.1 or even 8 channels. Mixing for the extra channels can be done in the Routing tab, but additional high- and low-pass filters for Center and Sub have to be applied in the Outputs tab. You can try various filter settings, mixing coefficients, delays, etc. in search for the optimal setup.

That’s it. I hope you find this post useful and look forward to any feedback!

Woww... This is a cool idea! Definitely worth a sticky...
Isn't Dolby PL now out of its patent and can freely become implemented?

I wonder if it's a trick investigating for making a little button (miniLOGIC that would easily enable that upmixing inside the matrix.

Anyway, thanks for the great tip! We very much look forward to hearing more!

DevTeam

I'd definitely be interested in buying a Dolby Pro-Logic plugin for the Dirac Series (including 2x4 and SHD) if it was made available.

A 4.0 mix would be fine for me, with phase correction I find I don't need a centre channel, but having a SL and SR output would be a very nice option to have.

I generally find a lot of digital music is in some sort of surround sound format nowadays anyway. The type of music content I would like in surround is nearly always already in surround, the only other stuff is things like concerts which are meant to sound like they come from the front. I suppose it could be quite a cool addition if you needed more output though. Also I'm not very keen on the prologic format as it is quite lossy compared to newer formats such as dolbytrueHD/atmos, or DTS:X/Master audio.

I think it could be an awesome implementation for limited digital devices or any analogue formats though.