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Path: senator-bedfellow.mit.edu!faqserv From: neidorff@unixyztrode.com Newsgroups: rec.audio.tech,rec.audio.opinion,rec.audio.misc,rec.audio.marketplace,rec.answers,news.answers Subject: FAQ: rec.audio.* Recording 5/97 (part 7 of 13) Supersedes: <AudioFAQ/part7_872763743@rtfm.mit.edu> Followup-To: poster Date: 28 Sep 1997 09:06:23 GMT Organization: Unitrode Corp. Lines: 681 Approved: news-answers-request@mit.edu Expires: 11 Nov 1997 08:58:45 GMT Message-ID: <AudioFAQ/part7_875437125@rtfm.mit.edu> References: <AudioFAQ/part6_875437125@rtfm.mit.edu> Reply-To: neidorff@unixyztrode.com NNTP-Posting-Host: penguin-lust.mit.edu Summary: Answers to common questions about audio equipment, selecting, buying, set-up, tuning, use, repair, developments, and philosophy. X-Last-Updated: 1997/07/31 Originator: faqserv@penguin-lust.MIT.EDU Xref: senator-bedfellow.mit.edu rec.audio.tech:57579 rec.audio.opinion:130206 rec.audio.misc:39675 rec.audio.marketplace:129390 rec.answers:34317 news.answers:113252 Archive-name: AudioFAQ/part7 Last-modified: 1997/7/31 Version: 2.12 14.0 Recording There are more different recording systems available today than ever before. Digital and analog are both available to the consumer. With the advent of consumer digital recorders, used pro analog recorders are becoming available for surprisingly low prices. Now may be the time for you to buy a microphone and recorder and make your first! 14.1 What is DAT? What is its status today? DAT (Digital Audio Tape) is currently the standard professional digital format for 2-track digital recording. DAT had a short-lived consumer presence, but never "made it". As digital recorders have no tolerance for clipping, using a DAT recorder takes a slightly different knack. The results can be worth it, however, as DAT format offers the same resolution and dynamic range as CDs. DATs record for up to 3 hours on a tape, and can run at three different sampling rates: 32 kHz, 44.1 kHz (for CD), and 48 kHz (the DAT standard). Longplay mode cuts frequency response to 14kHz but adds even more recording time. 14.2 What is DCC? What is its status today? DCC is Philips' attempt to modernize the regular cassette. DCC decks can play analog cassettes, and can record new Digital Compact Cassettes. They use stationary heads (DATs use rotary heads as do VCR's), and although they are digital, they use lossy compression to fit all the data on the cassette. Although DCC sound quality is far better than the 1960 standard cassette, the DCC does not have the sound quality present in DAT or CD. DCC may be a good choice for consumers who want to assemble mix tapes for cars or walkmans, but is not suitable for any professional applications. As of October 1996, DCC is quite affordable in price. Some DCC home recorers are under $200. However, blank DCC tapes are still hard to find and fairly expensive ($10 each for 90 minute lengths). Also, DCC manufacturers are dropping DCC from their lines, indicating that it is either on the way out or never made it in. Although the ability to play analog cassettes is a strong advantage of DCC, many people have had trouble with oxide particles falling off analog cassettes and clogging the gap of the DCC head. This may be due to the extremely low quality of some analog cassette tapes and may be due to the very tiny gap of DCC heads. Caution: NEVER demagnetize DCC heads. This will permanently damage the heads. As of May 1997, Philips has announced plans to discontinue DCC. 14.3 What about writable compact discs? What is the status today? Recordable CDs are available, and costs are dropping. As of May 1997, recorders have shown up for <$400 and blank disks are advertised as low as $5.50 each in bulk. Almost all recordable CDs can only be recorded once. Also most CD recorders do not allow start-and-stop but require that the whole disk be written at the same time. This means that you really need a computer with a large hard disk to master a CD. Many people report destroying many disks before getting their machine working correctly, but once people learn the software and hardware steps, archival CDs can be made inexpensively and routinely. 14.4 What are Dolby B, C, and S, HX Pro, and DBX? Are they compatible? Dolby B, C, S, and DBX are techniques for increasing the signal/noise ratio of recordings. All work in similar ways: they compress the dynamic range of the sound during recording, then expand it back upon playback. As much as we would like it to be otherwise, you only get correct reproduction if you use Dolby B to play back a Dolby B tape. Same for Dolby C, Dolby S, and DBX. Dolby HX Pro is the exception. Dolby B works mostly with higher frequencies; it increases their levels during recording and decreases their levels, and the levels of high-frequency noise such as tape hiss, during playback. Dolby B tapes can be played back without Dolby B processing, but high frequencies are over-emphasized and the sound will be excessively bright. This can be compensated for to some extent by turning down the treble control. Audio novices often remark that commercially recorded tapes recorded using Dolby B sound dull when played back with Dolby B; this is because they are accustomed to the boosted high frequencies they hear when playing these tapes without Dolby. Dolby C achieves greater noise reduction (about 8-10 db) than Dolby B by working with a greater range of frequencies and altering relative levels more; this means that playing Dolby C tapes back with no Dolby processing or with Dolby B, leads to very bad frequency response and a sound that most people find unpleasent. Dolby C may also be more sensitive to variations among decks in exact frequency response, alignment, etc. Some people find that tapes recorded using Dolby C sound best only when played back on the deck on which they were recorded. Dolby S works with an even broader range of frequencies than Dolby C, and achieves slightly greater noise reduction. Its has three advantages over Dolby C: (1) many people find that tapes recorded and played back using Dolby S sound closer to the original than tapes done using Dolby C; (2) tapes recorded using Dolby S don't sound awful if played back on Dolby B decks, and (3) Dolby S seems to be less sensitive to variations among decks. DBX is similar to Dolby B, C, and S, but uses the same compression on all frequencies, high and low. However, DBX is mostly used in the professional market. Very little home DBX equipment is available, and some of that home equipment is no better than comparable Dolby B home systems. All DBX systems are compatible with all other DBX systems, but incompatible with Dolby. A DBX tape will sound terrible without DBX processing during playback. All compression/expansion systems suffer two problems. One is due to the fact that compressors can't compress a loud signal before they have heard a bit of it, so that little bit of loud signal will get through uncompressed. Likewise, quiet passages will not be expanded until after they are detected. These delays give rise to an audible problem often called "breathing". The other problem inherent in all compression/expansion systems is that if there are any frequency response errors in the tape recorder, they will be made worse by the compression/expansion. For example, if there is a 2dB dip in frequency response at 1kHz in the tape recorder, this will be accentuated to a 4dB dip if the compressor is using a 2:1 ratio. So compression/expansion trades noise for frequency response error. For that reason and the previously mentioned breathing, some people prefer to use their recorder without any noise reduction at all. They prefer a bit of noise to the other errors. Dolby HX Pro is not noise reduction and does not use compression or expansion. HX Pro is a technique developed by Dolby Labs to increase tape headroom by decreasing the bias when recording signals with a large high frequency component. This allows better transient response, particularly on less expensive tapes, and requires no processing when the tape is played back. Dolby HX tapes can be played back on any system with no decrease in quality. Dolby Corporation has developed other techniques and other acronyms for products related to surround sound. The phrase "contains Dolby" isn't as meaningful today as it used to be. 14.5 What is the best cassette deck under $400? 14.6 What is PASC? Can I hear the effects? PASC (Perceptual Audio Sub-band Coding) is a data-compression algorithm. It increases the length of recording that can be stored in a given number of data bits by eliminating sounds that the developers' research claims can not be perceived by human listeners. Its most important component is the omission of quiet sounds that occur at the same time and near the frequency of louder sounds. It provides up to a 4x increase in the length of recordings a given digital medium can hold; this is essential to allow full-length digital recordings on DCC (and on MD, which uses a different compression technique). It is not necessary to translate CD data to analog before compressing it using PASC, nor the reverse. It is very difficult to hear any degradation from PASC, but it is possible, depending on the source and listener. The effect is not a distinctive noise (like a hiss) nor a consistent diminution (like a notch in a speaker's response), but a broad, uncorrelated dropout in a changing collection of sounds that are masked by sounds that you can hear very easily. Since it is lossy, repeated PASC recording will cause progressive loss, and this signal damage may become easily noticeable. This is a side effect that recording companies hope will have the effect of discouraging piracy via DCC. DCC recorders do have digital inputs so can make one perfect copy of a master, but copy protection prevents digital duplication of a copy. For more information on audio compression, consult these articles (courtesy of Jonas Palm): R. Veldhuis, M. Breeuwer, R. van der Waal, "Subband Coding of Digital Audio Signals Without Loss of Quality," IEEE ICASSP, 1989, pp. 2009-2012. J. Johnston, "Perceptual Transform Coding of Wideband Stereo Signals," IEEE ICASSP, 1989, pp. 1993-1996. G. Davidson, L. Fielder, M. Antill, "High-Quality Audio Transform Coding at 128 kbits/s," IEEE ICASSP, 1990, pp. 1117-1120. J. Princen, A. Bradley, "Analysis/Synthesis Filter Bank Design Based on Time Domain Aliasing Cancellation," IEEE Trans ASSP, Oct. 1986, v. 34 n. 5, pp. 2161-2164. P. Duhamel, Y. Mahieux, J. Petit, "A Fast Algorithm for the Implementation of Filter Banks Based On 'Time Domain Aliasing Cancellation,'" IEEE ICASSP, 1991, pp. 2209-2212. J. Johnson, "Transform Coding of Audio Signals Using Perceptual Noise Criteria," Journ. Acoustical Society of America, Feb. 1988, pp. 314-323. 2nd Draft-Proposed Standard on Information Technology Coding of Moving Pictures and Associated Audio, document ISO/IEC JTC1/SC2/WG11 MPEG 90/001, Sept. 1990. G.Thiele, G. Stoll and M. Link "Low bit-rate coding of high-quality audio signals. An introduction to the MASCAM system." EBU Review No. 230 14.7 What is SCMS? Can I hear the effects? SCMS (Serial Copy Management System) is a copy-protection system intended to stop rampant piracy of commercial recordings to digital tape. SCMS allows the home taper to copy from a CD to a digital tape, but prevents anyone from digitally copying that new digital tape. You CANNOT hear SCMS. 14.8 How can I bypass SCMS? There are professional devices used by engineers to manipulate the digital bitstream, but they cost several hundred dollars and are not cost effective for consumers. If you need to make perfect digital copies of digital copies, buy a professional digital recorder. Pro models do not have SCMS, are more durable than consumer recorders, and may have better quality electronics than consumer models. 14.9 What's this about a tax on DAT? Every digital audio tape recorder and every blank digital tape sold in the USA is priced to include a "premium" or "tax". This tax is collected by the US Copyright Office and distributed to the recording artists and record companies that own the copyrights to commercial music. These fees are supposed to repay them for lost royalties. Many believe that this "tax" is illegal, because it represents an assumption that the buyer will use the recorder and tape to violate a copyright, and not to record their own works. A founding principle of the USA legal system is that everyone is assumed innocent until proven guilty. If you believe that this law is unjust, write your elected representatives. 14.10 Is it legal to copy an LP, CD, or pre-recorded tape? In the US today, it may be legal to copy LP's, CD's, etc. for your own private use (such as to copy a CD to play on your walkman). UK law specifically prohibits this, but it is almost never enforced. It is definitely not legal in the US, UK, or almost anywhere else, to copy these sources for commercial purposes, or to give the copies to others. It is as of yet unclear whether you own the rights to sell or give away a copy of a recording if you made the copy on media which was sold with an included digital audio tax. 14.11 How do I clean and demagnetize tape heads? First, a caution: DAT recorder tape heads are VERY fragile. Before cleaning the heads on a DAT recorder, get specific recommendations from a very knowledgeable source that is intimately familiar with DAT head cleaning. In the internet, a good source is the DAT-Heads-Digest FAQ. For more information on DAT-Heads-Digest, see section 20.2, below. To clean tape heads, use pure isopropyl alcohol and lint-free swabs. Wipe the metal parts of the transport with alcohol (DON'T wipe the rollers!) and allow them to dry. Throw the swab away after use. Be exceedingly careful when cleaning the heads on a DAT. DAT heads are notoriously easy to misalign by incorrect cleaning. Practical tape head demagnetizers are available for under $10. Try to find one with a plastic coated tip. If you can't find one which is plastic coated. you can slip a drinking straw or plastic tube over the tip for the same effect. This plastic will prevent the demagnetizer from scratching the head. Before plugging in the demagnetizer, remove all tapes from your working area and unplug the recorder. Hold the demagnetizer away from the recorder as you plug it in. Slowly bring the tip of the demagnetizer up to the tape head and slide it back and forth across each tape head for five one-second strokes. Then pull it away from the head slowly and go on to the next. After demagnetizing the heads, use the tip on each metal tape guide with a similar five strokes. Last, slowly pull the demagnetizer far away from the recorder and unplug it. Recording engineers use a demagnetizer before each recording session. 14.12 How do I adjust a tape recorder for best results? Adjusting a tape machine for best results usually requires special equipment and test tapes. Unless you know what you're doing, leave it for a pro. If you are serious about doing it, buy the service manual for your particular tape recorder. It will list a detailed procedure, as well as describe the correct test tape and tools. As for setting of record levels, it is best to experiment with different levels on different tape brands. Different formulation will reach saturation for different levels. Generally speaking, the transients on a Chrome tape should peak at about +6 dB above 0, though some formulations can take significantly hotter signals. 14.13 Where can I get new pinch rollers or drive belts? Projector-Recorder Belt Company Whitewater WI USA 800-558-9572 14.14 What is a good rubber (pinch) roller cleaner? Teac RC-1 available from J&R Music World 59-50 Queens-Midtown Expressway Maspeth NY 11378-9896 USA 800-221-8180 or 718-417-3737 Tascam Rubber Cleaner RC-2 available from: Tape Warehouse Chamblee GA 1-404-458-1679 14.15 How can I program a recorder to tape a radio broadcast? Radio Shack and Panasonic make a clock/radio/cassette that can be set to record at a specific time. Radio Shack also sells 120 minute cassettes, which can be used for 60 minutes per side. The recorders are not high quality, and the long tapes are fragile, but it works. You can buy "appliance timers" at hardware stores that will start and stop an appliance at a specific time. Radio Shack sells fancier versions of the same thing for more money. Gadget freaks love "X-10" control systems. These can be configured to do the same thing. All require a recorder that can be left in RECORD mode. Such recorders are identified by a "TIMER" switch on the front panel. Many cassette decks have a TIMER switch for use with timers. This can be set to start a recorder at a particular time. As the recorder will be started from a remote control rather than by the power line voltage, no timer switch is required. Radio Shack has a very similar product available for $99.95, may be less on sale. Damark also sells a learning remote with a built in on/off timer and sleep timer. It can learn 17 commands per device on a total of 5 devices and should be great for taping a few shows. They also sell a 8 device remote with timer. As Damark sells close-outs, these items may not be available in the future. Damark 7101 Winnetka Avenue North PO Box 29900 Minneapolis MN 55429-0900 USA 800-729-9000 or 612-531-0066 There is a similar learning remote available from MCM Electronics. They call it a Lonestar Learning Universal Remote, item 80-450, $24.95 on sale. MCM Electronics (Speakers, A/V Repair Parts, Etc) (+) (C) 650 Congress Park Dr Centerville Ohio 45459-4072 USA 513-434-0031 or 800-543-4330 Another timer-remote is the Fox 800 (approx. US $80 retail). The Fox 800 was spotted at Damark 800-729-9000 for $30 in March of 1995. There is also a Carver remote with timer. For the true nerd, there's the programmable remote sold as a Scientific Calculator, the HP-48. Audio remote control software for this fine adding machine exists. For more information, consult the HP-48 FAQ. The HP-48 FAQ contains pointers to a few remote control programs. The FAQ is archived at site rtfm.mit.edu in /pub/usenet-by-group/comp.sys.hp48 Use a VCR for audio-only recording. Hook the audio in to the output of a radio, tuner, or receiver. You may also have to connect some video signal to the VCR so that the sync circuits work correctly. 14.16 Will CrO2 or Metal tapes damage a deck made for normal tape? No. They will work fine. They are no more abrasive than common tape and may actually be less abrasive than very cheap tapes. Recorders which are designed for CrO2 or Metal tape have different bias settings and equalization settings to take best advantage of the greater headroom and to give flat response with these different types of tape. However, they use similar if not identical heads as less expensive tape recorders. Almost all tapes are in some way lubricated, and these lubricants minimize wear and squeaking. 14.17 Why do my old tapes squeak in my car cassette deck? One problem that will cause this is "binder ooze". The binder is the glue which holds the oxide particles to the backing. With time, this binder can ooze forward and actually get past the oxide particles, so that there is sticky stuff on the surface of the tape. When this sticky stuff goes past the heads, it can cause a slight stick, which will sound like a squeak. You won't feel it with your fingers, but it is there. If you have a prized tape with this problem, consider baking the tape in a home oven at a very low temperature, like 150F. This might cure the problem by drying out the binder. 14.18 Is VHS Hi-Fi sound perfect? Is Beta Hi-Fi sound perfect? The HiFi recording format is subject to two different problems: Head-switching noise and compression errors. To get perfect reproduction, the FM subcarrier waveform being played back by one audio head must perfectly match the waveform from the other head at the point of head switching if a glitch is to be avoided. If you record and then play the tape on the same VCR under exactly the same conditions, you have a reasonable chance of this working. But if the tape stretches just a bit, or you play it on another VCR whose heads are not in exactly the same position, or the tracking is off, the waveforms will no longer match exactly, and you will get a glitch in the recovered waveform every time the heads switch. This sounds like a 60 Hz buzz in the audio, which is often audible through headphones even if not through speakers. The same glitch will occur in the video waveform too, but since head switching always happens during vertical retrace, you won't see it. Some VCRs have azimuth correctors or Dynamic Track Following which minimize these problems (Philips V2000 and some VHS). The wonderful signal to noise ratio of VHS HiFi is achieved through the use of compression before recording and expansion after playback. The actual signal to noise ratio of the tape itself is about 35 dB and a 2.5:1 compressor is used to "squeeze" things to fit. Like all companders, this produces audible errors at certain places on certain signals, such as noise "tails" immediately after the end of particularly loud passages. Worse, compressors often have problems simply getting levels right. That is, if you record a series of tones, starting at -90 dB and working up in 1 dB increments to 0 dB, and then play them back, you will almost invariably have level errors. The trend from soft to loud will be there but the steps won't be accurate. Two or three of your tones might come out at essentially the same level, then the next one takes a big jump to catch up or even overshoot. For music, the result will be that the relative levels of some instruments, passages, etc. will not be accurate. This doesn't matter as much for movies, which tend to have steady volume level. Also, movie enjoyment is rarely hurt by these level errors. VHS and Beta HiFi is fine for reproduction of movie and tv soundtracks. They are also perfectly fine for non-critical audio applications. But VHS and Beta HiFi are not serious competitors to DAT, CD, open-reel analog tape, or even a high quality cassette deck. 14.19 How do HiFi VCRs compare to cassette recorders? DAT recorders? VHS HiFi and Beta HiFi are analog recording formats which use modulation techniques to record a video signal and a stereo audio signal on a videocassette. The audio capabilities typically surpass that of the "linear" audio tracks found on all video recorders, thus the "HiFi" designation. "HiFi" is essential for getting good sound quality on your video recordings and out of pre-recorded videos. HiFi is also touted as an excellent audio recorder for audio-only (no picture) applications. On paper, the specifications are typically superior to analog cassette but inferior to DAT. In reality, the quality of HiFi video recorders is better than low quality cassette recorders but not as good as high quality cassette recorders when they are used with noise reduction systems. In no case can a HiFi video recorder compare to DAT. It suffers from generational loss and audible noise. Many people use VHS HiFi for recording radio broadcasts, since VCRs often have built-in timers and can record for up to 9 hours. If you use a HiFi video recorder to record from an audio-only source, beware that some decks will not function properly without a video signal for synchronization. If you are interested in very good quality sound, use a deck with manual level control. 14.20 What is the difference between VHS HiFi and Beta HiFi? To record the video and HiFi sound signals onto the same tape area, VHS HiFi uses "depth multiplexing", while Beta HiFi uses "frequency multiplexing". That is, the FM signal for Beta HiFi occupies a different frequency band than do the Beta format's luminance and chroma signals, and is simply mixed with those signals and laid down on the tape by the video heads. In VHS the luminance and chroma signals were too close together in frequency for this to work. VHS HiFi uses a separate pair of heads on the spinning head drum to record the HiFi carrier. These heads' gaps are shaped so that the HiFi carrier is actually recorded at a different depth in the tape than the luminance and chroma signals. 14.21 Is there any good reason to buy a HiFi VCR for common TV shows? If you do not own a stereo TV, the purchase of a HiFi VCR will give you the capability to listen to stereo TV broadcasts to your system. 14.22 What is the best cassette tape? One simple answer to this question is that the best tape is the tape which was used to align your tape recorder. A second simple answer is that more expensive tapes are frequently better in terms of quality of the backing, durability of the oxide, accuracy of the shell and guides, and life. Background: When you make a tape recorder, you build electronic circuits which have specific, non-flat frequency response. These circuits correct for the non-flat response of the tape heads, the recording process, and the tape. These circuits can be adjusted after the recorder is made, but adjustment is tricky, and may or may not be successful with every tape made. The designer of the tape recorder picked one tape as their standard when they did the design, and built that recorder to work well with that particular tape. It may work better with a different tape, but it won't necessarily sound the best with what one person calls the best sounding tape. From a review of frequently given answers to this question, it is obvious that almost every brand of tape has its advocates. Many brands also have their detractors. Maxell and TDK tend to have a strong following, but that is in part because they own a large share of the US tape distribution market. 14.23 What is the best Reel-to-Reel tape? See 14.22. Just as cassette tape recorders are set up specifically for one type of tape, reel-to-reel tape recorders are equalized and biased so that they are best with one specific brand and model of tape. Just as more expensive cassette tapes will last longer and have less noise than cheaper ones, you can expect fewer dropouts, better quality control, and lower noise from more expensive reel-to-reel tapes. The major brands in reel-to-reel tape include Ampex, Scotch (3M), AGFA/BASF, and Maxell. 14.24 What is Type I, Type II, Type III, and Type IV cassette tape? These are IEC (International Electrotechnical Committee) standards. They provide broad standards for all tapes, and end the need to align a deck for an individual tape. Type 1 is for normal "iron oxide" tapes (Fe2O3), Type 2 is for high-bias "chromium oxide" tapes (CrO2), Type 3 (obsolete) is for FeCr (ferric chrome), and Type 4 is for Fe (Metal). Type 2 tapes tend to be more expensive than type 1, and type 4 tapes are the most expensive. This is because type 2 tapes tend to have less noise and flatter high frequency response than type 1, and type 4 tapes tend to have even flatter highs and even less noise. Some Type 1 tapes are more expensive than other Type 2 tapes, and may be worth the extra price. More expensive tapes come in better shells, have better lubrication, fewer dropouts, smoother frequency response, and better uniformity from tape to tape. Even though the types imply a particular tape formulations, the type really refers to the tape performance. For example, some iron oxide tapes have an unusual oxide formulation with very small grains that conforms to the type 2 standard better than the type 1 standard. These tapes will be labeled type 2, but may not have any chrome in them. Most modern cassette recorders sense the tape type by the holes in the back of the housing and adjust bias and equalization to compensate for the differences. A few top cassette recorders (the Revox and several Nakamichis) automatically align to a particular tape by recording test tones and then setting their own equalization. In practice, each brand and model tape is slightly different. For the very best recordings, adjust your recorder for the tape you use most, or buy the tape which works best in your recorder. Manufacturers adjust each recorder for a specific tape at the factory. So the best tape might be the one referenced in the recorder owner's manual. In a recording studio, it is common to align the bias and equalization for the specific tape used, and stick with that tape. 14.25 Why do I have hum when I connect cable to my VCR (or TV), which is connected to my audio system? What you are experiencing is probably a "ground loop", caused by multiple connections from your equipment chassis ground to building ground. Since disconnecting the cable or building antenna from the VCR eliminates the hum, the cure is simple. The following info talks about "the cable" but works the same with a coax from a master antenna system. Go to Radio Shack and buy one each of: 15-1253, "300-ohm TV-VCR Matching Transformer" This looks like a little box with two screw terminals and a push-on male F (coax) connector. 15-1140, "75-ohm coax/300-ohm twin lead indoor/outdoor matching transformer" This is a longish box or tube, with a female F connector on one end and a bit of twin-lead coming from the other. The twin-lead ends in a pair of what are called "spade lugs" (shaped like U's). Note: each of these part numbers may have a "B" or other letter at the end. These indicate slightly different details of functionally equivalent parts. Don't worry about it. Connect the two spade lugs on the -1140 to the two screw terminals on the -1253. Make sure they don't touch each other; this shouldn't be difficult to get right. This gives you the "isolator", with a female coax connector on one end and a male coax connector on the other. Just insert the isolator "in line" in the incoming cable lead. ie treat it as you would a (very short) extension cord. You can do this right at the back of the VCR (or whatever the cable is hooked to). Only one of the two units called out here (15-1140) actually provides isolation. Two of the 15-1253 units back to back will NOT work. Two of the 15-1140 units back to back will work fine but will be less convenient. If you can't find these specific parts, and want to know if the substitutes you've found will work, test them with an ohmmeter, measuring from either the pin or shield of the coax side to either wire of the twin-lead side. If it's not an autoranging unit, set the meter to its highest resistance range. You want to see no connection (ie: infinite resistance, an open circuit) between them. As with the parts described above, only one of the coax/twinlead adapters needs to pass the test. This trick runs the signal through a PAIR of baluns. This is more than is absolutely required to solve this problem, and may weaken the signal slightly. This should not be a problem on most cable systems. But, some audio stores are beginning to carry a unit made expressly for this purpose. It contains a single 75 ohm to 75 ohm isolation transformer. This should introduce less signal loss. It will also be better shielded than the two baluns (see next paragraph). Under $10 would be an appropriate price. The back-to-back baluns may allow "ingress". That is, if you are near to a TV transmitter, the short length of twinlead may pick up broadcast TV signals and mix them with the cable, causing interference. If you can find a prepackaged 75 ohm isolation transformer as described in the preceding paragraph, it should be better in this regard. Mondial is selling a unit dubbed the "Magic-1"; this does the same job but with three capacitors instead of transformers. It is said to cause less than 1 dB of signal loss. On the other hand, it costs about $90. Yet another solution is to attack the problem at the line-level audio connection between the VCR and the rest of your stereo. Radio Shack's stereo ground isolators (270-054) are made for this purpose. These go in the line-level AUDIO connections between the VCR (or TV) and the rest of your sound system. If both the line in and line out jacks on the VCR are connected to the sound system, you'll need two of these isolators. They are audio frequency transformers and may add some distortion and frequency response error. COPYRIGHT NOTICE The information contained here is collectively copyrighted by the authors. The right to reproduce this is hereby given, provided it is copied intact, with the text of sections 1 through 8, inclusive. However, the authors explicitly prohibit selling this document, any of its parts, or any document which contains parts of this document. Note: My e-mail address in this FAQ is "Spam-Blocked". Remove "xyz" to use. -- Bob Neidorff; Unitrode Corporation | Internet: neidorff@unixyztrode.com 7 Continental Blvd. | Voice : (US) 603-429-8541 Merrimack, NH 03054-0399 USA | FAX : (US) 603-429-8564 Note: Unitrode Corporation has openings for Analog and Mixed Signal Design Engineers in Merrimack, New Hampshire. If interested, please send resume in confidence to address above.