From roberts@pharm.som.sunysb.edu Tue Feb 27 16:08:54 1996 27 Feb 96 19:08:06 +0500 From: "Roy Roberts" Organization: Pharmacology, SUNY Stony Brook Date: Tue, 27 Feb 1996 19:07:11 EST Subject: (Fwd) Re: surround sound Priority: normal -------- Forwarded Message Follows ------- Date: Sun, 25 Feb 1996 23:52:43 +1030 From: "The Dark force of dance" Subject: Re: surround sound Y-ellow all. I've posted this to the list because it'll save me writing it again for another year maybe. :-) This is in the interests of "Information want's to be free". A philosophy that Dolby Labs obviously don't S*u*b*s*c*r*i*b*e to. At 12:34 PM 2/24/96 EST, SEAN@ecology.coa.edu wrote: > > > Hello Batz! You wrote the following and I wonder if you have any > specific suggestions as to what sort of hardware I would need to mix > for surround sound. Thanks. > > sean Y-ellow Sean. It kind of depends on what you are doing and what your level of understanding is. If you were doing film sound and needed to place live sound of actors and sound effects in specific locations within the surround field, then you'll definatly need a Prologic encoder. You can't actually buy these. You can only rent them from Dolby Labs. In the same way that you can't buy a steddycam. However if you're doing music for surround it's handy to be aware of a couple of things. Surround works in principal by creating a difference signal between the left and right channels. That is to say that the product will be an audio signal that contains only those sounds which are different in each speaker. Sounds which are the same will be cancelled out. So for example, bass sounds which are generally mono and in the centre will not come out of the rear speakers. However a sound that may only be in the left channel will come out of the rear. Basic surround will produce only one rear channel however prologic uses some tricky analogue logic to produce two or more rear speakers. The basic processing chain is as follows. Differencing amp, 7Khz lowpass filter, Dolby B decoder, 10ms analogue delay line. In a simple surround system the two rear speakers are wired out of phase to give a pseudo stereo effect. Often the Delay line doubles as a stereo chorus unit for added effect. Prologic has two chains with an envelope follower on each. This controls 4 VCAs and improves channel separation whilst being able to give a reasonably good stereo image front and rear. A simple Surround system makes no compensation for the extra over all SPL created by one or more extra channels. Prologic does and therefore doesn't screw with your MIX too much. In essence, If you were to take a mono sound but invert it out of phase in one channel, it would come out of the rear speakers and be somewhat cancelled in the front. A slight delay or comb filter will give you a little more control over it's positioning. Phase shifting networks will give better control but the disadvantage is that such networks are frequency dependant. It is worthy of note that Dolby Prologic Surround was never intended for musical applications. It was only intended for Movie soundtracks. For placing special effects in a quasi 3D space etc. Music can often sound cluttered through surround so if you wish to master for it, you have to be able to hear it. You also have to do an A/B check between surround an Stereo. Often you may get a good mix in surround but loose definition in stereo or worse, some sounds will be way to loud in stereo. The trick is to make sure it sounds OK in stereo but mess with it in surround. Also make sure you haven't gone overboard and that nothing cancels out in MONO. Unless that is your intention. There is a Newzealand band called the HEADLESS CHICKENS. One of their songs has sound in it that can only be heard on a Surround system. It's pretty wikkeed actually and quite a shock when you first hear it. However on our previous album CYBERBLADE, we processed the hi-hats into what we thought was surround and indeed the over all effect was that you were surrounded by the drum kit. However when played back in mono, the hats disappeared altogether. What is worse is that if you are mastering for vinyl, be very careful. CYBERBLADE could not be cut onto vinyl because it's antiphase components would require the cutting head to move in two opposing directions at once. One last point to note. Dolby prologic isn't patented. They just keep it top secret. Suggesting that it has been done before. My investigations into Prologic show it to be somewhat similar to the old SQ4 or CD4 quadrophonic decoders of the 70s. With a few added embellishments to make it work on hi-fi video machines rather than specially pressed vinyl. It is not discrete, it does not actually have 4 channels and is somewhat of a fudge. As for equipment. All you need is a Surround Amp and an extra set of speakers. For the truly keen, one of the Australian Electronic magazines had a project for a Prologic decoder. They sell the kit through one of the electronic toyshop chains here which uses a licensed Prologic chip. They do not tell you what's actually inside this chip. I'm told that the same Magazine also had a Surround encoder project but I've not seen it nor been able to verify it's authenticity. Hope this helps Be absolutely Icebox _ __ _ | "_ \ | | batzman@dove.mtx.net.au | |_)/ __ _| |_ ____ Batz Goodfortune | _ \ / _` | __|___ | All Electric Kitchen | |_) | (_| | |_ / / _-_|\ |_,__/ \__,_|\__|/ / / \ / ,__ \_.-*_/ |_____| South Australia Alive to Synthesize