mat@hou5e.UUCP (06/06/83)
I am replying to a couple of things that appeared in an article recently. I am proud owner of one of Carver's magic boxes, and I would like to balance my opinions against these below. SONIC HOLOGRAPHY*: ... . I have found the holography effect to be convincing on but a small number of records, primarily small groups. This is of course subjective. I find that it improves the imaging on most stereo recordings, detracts from it on none that I have heard, and makes a tremendous improvement on a few. The latter include both Pink Floyd's Dark Side of the Moon and the Telarc Carmina Burana (Orff). Placement of speakers is critical for sonic holography. Very. On the other hand, I am now able to check the speaker and chair position by walking across the room with a familiar recording playing. Proper (for holography) speaker placement may by itself improve the stereo imaging sufficiently to make the exercise worthwhile. Best placement for holography is usually closer together than for regular stereo so this is not always true. If your speakers are very well placed as it is, the hologram will probably still require some adjustment. Sometimes recordings sound muddy with the holography switched in. ... I haven't found this, but it is subjective, as well as depending on speakers, the room, etc. Unfortunately, proper speaker placement (for best imaging) may render the room useless for other activities. Sometimes. Usually there are compromises which can be made on both sides. It just requires a little work. AUTOCORRELATION NOISE REDUCTION: ... To my ears, autocorrelation does NOT reduce hiss without affecting instrumental timbers as the advertising suggests. Of course, my ears sre not as well trained as some, but my system seems to work pretty well. In fact I have only heard it ``fall down'' once. This was an old piano recording played on WNCN and the autocorrelator apparently had to suddenly open every band to let the signal through, dumping all of the disk noise through. Notice that this was cleaning up not only the FM noise but also the disk surface noise from a 20-odd year old disk. In any case, if someone were to offer to buy my Carver preamp for what I paid, with interest, I would turn him down. I still have a little work to do to finish find-tuning the room for the hologram, but it works very well anyhow. *SONIC HOLOGRAPHY is a trademark for a process implemented by a set of patented devices marketed by the Carver Corporation. This audio process injects into each channel of a stereo signal a signal derived from the opposite channel, delayed, inverted, and shaped so that the added signal just cancels the signal that the ear hears from opposite speaker. The result is claimed to be an enhancement of the stereo illusion, providing depth, height, and apparent sound sources not only between the speakers, but to the sides of the speaker pair as well. Mark Terribile Duke of deNet
caf@cdi.UUCP (06/11/83)
The recent articles about equalizers has tempted me to post my 2 cents' worth about various audio gadgets. EQUALIZER: Get (borrow?) a unit with a spectrum display and microphone. Use the microphone and the noise source to equalize your system by moving the speakers, not by equalizing the audio. If you can't get the response reasonably flat by moving the speakers around, you should have spent the money on better speakers or a different room. The usefulness of an equalizer is "directly" proportional to the variability of your audio sources. Most (not all!) movies and concerts on videodisc are especially amenable to improvement, if only because some engineer has munged the sound with HIS equalizer. Recent, HIGH QUALITY "mainstream" audio recordings rarely need correction with an equalizer. Your equalizer should be located where you can hear the speakers fairly well. For greatest enjoyment, the RTA display should be visible from the listening position. Read the article in June 83 High Fidelity. SONIC HOLOGRAPHY: In many of Carver's products, the invention lies as much in the sophisticated name as in the actual circuit itself. I have found the holography effect to be convincing on but a small number of records, primarily small groups. Placement of speakers is critical for sonic holography. Proper (for holography) speaker placement may by itself improve the stereo imaging sufficiently to make the exercise worthwhile. Sometimes recordings sound muddy with the holography switched in. Be sure to read the article on holography in the May 83 Audio. Unfortunately, proper speaker placement (for best imaging) may render the room useless for other activities. AUTOCORRELATION NOISE REDUCTION: The word correlation had me confused about how this circuit works; I had been thinking of multipliers in signal detection. Actually, autocorrelation is merely an evolution of the 1950's Scott Dynaural noise filter. To my ears, autocorrelation does NOT reduce hiss without affecting instrumental timbers as the advertising suggests. I find the noise reduction that results from a slight (1.15 or 1.2 to 1 linear expansion) to be somewhat more musical in most cases. Sometimes, (with particularly noisy sources) autocorrelation in auto threshold mode is useful in conjunction with expansion to reduce swishing. The low frequency correlation filtering is ofen effective in cleaning up record rumble without undue side effects. On the Carver preamp, the "correlation threshold" can be set so that only the low frequency filtering is effective. LINEAR EXPANSION: On many program sources, a linear expansion of from 1.1 to 1.3 to 1 will simultaneuosly reduce background noise and add/restore emotional "punch" to the program. Sometimes I can use expansion and equalization to make a 10 year old record sound almost as good as a CD. (Well, good enough to put off buying the CD until prices come down ...) I use a DBX 128 which was on sale at $200 off (discontinued model?) an am quite happy with it. DBX RECORDS: To my ears, DBX records are almost as musical as CD's on most material. The main problem is limited selection. CX NOISE REDUCTION: CX encoding was CBS's "answer" to DBX. CX sacrifices some of the awesome noise reduction potental and relative freedom from side effects that characterize DBX for a measure of compatibility with undecoded playback. There are two distinctly different CX systems. The original used compansion mainly controlled by frequencies above 500 Hz., and this version is used on Laser videodiscs. For records (and, I think, CED discs) this cutoff frequency was changed to 100 Hz, resulting in stronger compansion at the expense of audible swishing accompanying low frequency sounds. Although this cutoff frequency is even lower with DBX, swishing is usually inaudible because the greater compansion keeps signal levels well above record hiss. To date, there has been bloody little classical music (the only kind I really care about) on CX'ed records. Fortunately, it is easy to modify early CX disc decoders for use with old laser viseodisc players. A recent Billboard article casts doubt on the future of CX records. STEREO DOLBY VHS: At the two hour speed, the JVC 2650 gives passable frequency response and s/n ratio for much material. Just don't try to record a harpsichord recital. The dynamic range can be improved by using DBX tape encoding in place of the Dolby B that is built in to the 2650. Chrome tapes seem to sound better with DBX because of less modulation noise (asperity). Most tapes have unberable drouputs except for T-160's, but this may be a problem with my particular VCR. Since the 2650 has no manual record level controls, I use an old preamp to set the recording level. Before spending money for a stereo VCR, I suggest you listen carefully to the sound quality of simulcasts available in your area. Beta HI-FI or the VHS equivalent (if it ever appears) would be an improvement because of the improved sound at the > 2 hour speeds (assuming you have something to record that justifies the better sound). GROUND LOOPS: Ground loops are more troublesome in combined audio-visual setups. Amplifiers with SCR power supplies (such as the Carver M400) exacerbate the situation. Look for video gear with pseudo differential video inputs (shield not connected directly to chassis) for help. LASER VIDEODISC PLAYER: The sound quality of Laserdiscs has been improving to the point that the best (with possibly some help from DBX and/or an equalizer) sound nearly as good as any record, almost as good as DBX or CD. Unfortunately, the mechanical noise from the player can be a nusiance. The original Pioneer player is not so great at driving long, high capacity audio cables. PROJECTION TV: The large screen of a projection TV will affect room acoustics to some degree, especially stereo imaging or holography effect. The best location (sonically) for the speakers may be a few feet behind and to the sides of the screen. DOLBY STEREO: The background channel in Dolby Stereo encoded movies may be recovered with a speaker or pair of speakers driven by the sifference signal between the left and right channels. I have not heard a true Dolby Stereo decoder yet; I would appreciate comments about how this compares with the Hafler type connection. SYNERGISM: Although a LaserDisc feeding a Kloss projection TV is no visual match for 70 millimeter film, a fine home audio system can easily provide sound better than that heard in almost all theatres. The home setup is effective for programs with a strong emphasis on sound, such as operas and movies with great music and/or sound effects. Chuck Forsberg cdi!caf -- Chuck Forsberg, Chief Engr, Computer Development Inc. 6700 S. W. 105th, Beaverton OR 97005 (503) 646-1599 cdi!caf