Question (or Task)

What I want to achieve is something like the dolby decoding output. Do you have an idea what to do, I'd like to avoid pluging some external equipment?

-- ChristianMueller - 17 Mar 2004

Solution(s)

. Hi Christian

The Dolby decoder works in two halfs. the first half is just as you've got (I think), that is left goes to the left, the right goes to the right, the center is left and right mixed together and and surround is left minus the right or right minus the left. Either will do. Now (as you have found out) this on it's own has a problem and was no good for cinema .This was because the dialogue would bleed out of the left and right speaker and any one sitting near one of those speakers would hear an actors voice on one side yet see the actor speaking in front of them.

So Dolby added the second half called the steering circuit. What this did was assume that the signals coming through the system were created from just a single sound and that by messuring the levels of each of these four signals, it could work out where the sound should have been comming from and adjust the four levels to push the signal towards the correct speaker/speakers. i.e.if there was a level of one comming from the front , a level of half comming from the left and the right and a level of zero coming from the rear, this would equate to a signal being at the front only, so the left and right speakers were turned down compleatly and the front speaker was doubled in level. If however all the levels where the same then no speakers would be turned up or turned down. This does cause a problem in that if a solo dialogue voice was in the front and a new sound was intoduced into the left , then the voice would start to bleed into the left and right speakers. Also if that sound was a pumping sound like a train , then the voice itself would pump in and out of the left and right speakers. Well with dolby stereo (surround for home users) thats exactly what happens. Thats why so many sound engineers hated it so much, and why they loved Dolby Digital with it's descreate 5.1 channels.

Although Dolby sterio had this bleeding problem, it was ok for cinemas as the majority of the dialogue during the film was in the right place (the center speaker only).

This problem is why sound studio's had to have both the encoder and decoder in house and mix Dolby sterio while listening to the sound going through them. They couldn't just do a four channel mix and have it encoded afterwards as it could have sounded terrible.

So how do we make one ?

We could put the output signals of four level detectors into a formular and have the formula control four VCAs (Product modules). I haven't yet worked out the formula but it's based on doing the reverse of what you get out of an encoding matrix when you put in a single signal in. A low quality version could be made by using four stereo noise gates (expanders) where one which controls left and right level has a key input of the rear, another which also controls the left and right level has the center as the key input, another which controls the front and rear level and is keyed by the left and the last that also controls the front and rear and is keyed by the right. This won't be very even and could cause exccesive pumping but it might work ok in non extream cases.

If you are putting pre mixed music into it or any dolby sterio decoder, it will tend to keep the streering circuit open and flat and would be as if you hadn't used the steering bit at all.

One thing I have never tried but might work (with a correct steering circuit), is to split the sound up into many seperate bands and use a steering circuit to steer each band seperatly before remix the bands. This should minimize the interaction between different sounds in different places at the same time and could make old films in (dolby stereo only) sound better, and more like 5.1.

-- PeteJohnston - 22 Mar 2004