Many thanks Carl for creating DCP-o-matic and releasing it for free - what a great bit of software! Thanks also to everyone here for the feedback and help - I've spent the last few days reading through many of the threads.
My question is about the use of the LFE channel in surround audio mixes. I've made a DCP with 5.1 audio via un upmix from stereo that I implemented manually in Adobe Audition which provides a silent LFE channel. I've included the upmix parameters below for interest, it's a basic one we implemented at the BBC for use with digital TV broadcasts.
The upmix sounds fine in the cinema but I've noticed that when playing music the bass sounds quite weak and quieter than I'd expect from an equivalent stereo PA setup in the same room. When upmixing for TV we always kept the LFE channel silent (as you can see below) with the logic that it was only meant for special effects and it was correct to place a full-range signal on the other channels which would be routed to the subwoofer if needed by the playback equipment (for example, if the main speakers aren't themselves full range). This made sense since many consumer surround processors let you define whether the front and surround speakers are "small" or "large" for exactly this purpose. However, I'm wondering if this logic still holds in the cinema? Is the LFE track just for bangs and rhumbles or is it also meant to contain low-passed sound from other channels? Is the silent LFE channel the reason for the slightly weak bass in my DCP?
To put it another way, can I assume that a cinema sound processor knows to route sound to the subwoofer where it's needed, or does it just pass straight through via the fader when the input channels map exactly to the sound system (e.g. 5.1 in -> 5.1 out)? The processor in the cinema I'm using is a Dolby CP650.
And finally, if I provide stereo sound in my DCP, will it be easy for the operator in the cinema to configure the CP650 for an upmix and might this sound better than my basic manual one? Is that called a Prologic decode?
Any thoughts are very welcome and thanks again!
Code: Select all
Lf = (L * a1) + (R * a2) Rf = (R * a1) + (L * a2) C = b * (L + R) LFE = 0 Lsur = (L * c1) + (R * c2) Rsur = (R * c1) + (L * c2) where: b = 0.25 c1 = 0.25 c2 = -0.2 a1 = 1.0 - (cmix * b) - (smix * c1) a2 = -(cmix * b) - (smix * c2) where (these are the downmix coefficients): cmix = 0.707106781186548 smix = 0.707106781186548