Sound Recording FAQ

Sound Recording FAQ

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!

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 2 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).

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 December 1992, DCC is very new, DCC equipment is very expensive, and the ultimate future of DCC is not assured.

What about writable compact discs? What is the status today?

Recordable CD's are available, but are very expensive. Blank discs sell for approximately $35 each, and the recorders start at around $6,000. These units are mostly used by recording studios and other audio professionals.

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 tothe 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.

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.

You CAN hear PASC, but it is very difficult, since it 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.

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.


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.

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.

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.

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. 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.

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.

Where can I get new pinch rollers or drive belts?

    Projector-Recorder Belt Company
    Whitewater WI USA

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

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.

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.

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.

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.
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