rzdz@fluke.UUCP (Rick Chinn) (01/11/84)
re: hafler circuit I tried the Hafler circuit years ago when it first hit audio (sort of like quad did). I seem to remember that it more or less did what it claimed. The big minus (seemed to me) was that any ambience was reproduced in MONO. Then I built a primitive quad decoder, using the matrix coefficients promulgated by Electro-Voice. No level-shifting or any funny business like that, just matrixing. I was unable to find any records encoded with this process, so I tried a standard stereo recording. What a surprise! With levels adjusted properly, so you could just *barely* hear the rear speakers, the effect was quite pleasing. It's hard to describe it in subjective terms. What I percieved was the ability to "fill" the room, without having to resort to excessive volume (although I still like to do that). If you turned off the rear speakers, the room suddenly felt "emptier". The usual reaction was: "Whatever you just did, Undo! it." The EV decoder used fixed matrix coefficients. It wasn't anything super- complicated, just cross matrixing of out of phase signals. Later, I built another version of the same thing into my system's remote control. This time I made the matrix coefficients variable. Now I can adjust to one degree or another, the amount of ambience present in the rear channels. For most pop recordings (multitrack stuff is what I listen to, mostly), the ambience is any reverberation that was added, and any cross mixed delays. Center channel information drops out. On recordings made with "classical" technique (spaced omnis, xy, ortf, blumlein, mid-side), the effect is still present, and pleasing. You could describe what I did mathematically as: LeftFront = Left RightFront= Right LeftRear = Left - k * Right RightRear = Right - k * Left where 0 > k < 1 I've been living with 4 channel ambience recovery now for about 9 years. I feel really strongly that it does make a worthy difference. I've tried delaying the rear channels, but haven't had time to really closely evaluate the results. Rick Chinn John Fluke Mfg. Co MS 232E PO Box C9090 Everett WA 98043 {uw-beaver,decvax!microsof,ucbvax!lbl-csam,allegra,ssc-vax}!fluke!rzdz (206) 356-5232