|
FAQ:
rec.audio.* Recording 2/99 (part 7 of 13)
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 and rewritable CD recorders and
discs are available, and costs are dropping. As of Dec 1997, recorders
have shown up for <$300 and blank disks are advertised as low as $2.00
each in bulk. 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. For
more on CD-R read this excellent document:| http://www.fadden.com/cdrfaq/
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.
Carver made a remote with timer which
could be programmed to start recording at a specific time, if you have a
recorder with remote control capability.
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
You can also 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.
You can also use a computer's hard drive
to record audio. Cybercorder 2000 shareware ($19.95 to register)
schedules recordings on the computer sound card Line-In jack. http://skyhawktech.com
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. Progress in HiFi has
modern VHS HiFi equipment on par with the best analog cassette recorders
and close to that of the digital formats. VHS HiFi suffers generational
loss and noise, but because of the high quality of the AFM (HiFi) track,
these generational losses are minimal and not as severe as those of
audio cassettes.
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. |
