Azimuth loss	high frequency losses caused by head misalignment

bias	An electrical signal or relatively high frequency (nominally 100kHz) applied to magnetic tape along with the audio signal during the recording process. The bias current 'moves' the audio signal into the most linear portion of the magnetic transfer curve aiming to get the best frequency response, lowest distortion and maximum signal-to-noise ratio.

Bias noise	The noise from a tape which was recorded with bias only. In audio reproduction this is called hiss.

Capstan	A revolving shaft on a flangeless pulley that drives the tape by squeezing it against a pinch roller which controls the rate the tape passes over the heads of the tape recorder or deck

Coercivity	The demagnetizing (coercive) force required to reduce the induction of a piece of tape from saturation to zero, expressed as oersted.

Distortion	Extraneous signals not present in the original, but the term may also broadly include noise, hum and peaks.

Dolby	A name of noise-reduction systems available as a special circuit on some stereo cassette tape decks. Use of this circuit when the tape is recorded and during play back can reduce hiss level by between 10dB to 20dB.

Dropout:	The momentary loss or reduction in level of a recorded signal. This is caused by an impression in the magnetic coating on the tape at the point where that signal is recorded, or by variations in tape movements as it passes across the head.

Dubbing	Copying of already recorded material. in tape recording, playing a tape a tape or disc on one machine while recording it on another. The copy is called a dub.

Dynamic Range	The ratio expressed in dB between the softest and loudest sounds that can be reproduced without undesirable distortion.

Equalization	Used to achieve an overall uniform frequency response. Employed in both the recording and replay circuits - works similar to the tone control on audio systems

Erasure	The process of removing signals from the tape by applying and external magnetic field.

Frequency Response	The measure of a system or tape's ability to reproduce a range of frequencies in a linear fashion. Ideally, the 'flatter' or linear the better.

Harmonic Distortion	The production of spurious frequencies not present in the original sound, that are multiples of the original sound frequency. For example, a 100 Hertz tone in the original may produce spurious tones of 200Hz, 300Hz and so on. The result is an audible blurring or or loss of clarity in the reproduced sound.

Head Alignment	Adjustment of the recording or playback head so that it is at right angles to the longitudinal axis of the tape - also called Azimuth alignment.

Leader	A section of plain, non-magnetic tape, usually plastic affixed to the beginning of a length of recording tape. Attached to the end it is called a trailer.

Linearity	Frequency linearity referring to the straightness of a frequency response curve.

Metal Particle	A microscopic fragment of metal alloy (usually consisting mainly of iron) designed for high-fidelity magnetic recording.

Oxide (Gamma Ferric Oxide)	Minute ferric (iron) oxide particles that are coated on to the recording tape. Usually black or brown in colour.

Particles	The very small magnetic pieces that constitute the magnetic formulation used on the tape. Generally, the smaller and more uniform the particles, the better the tapes signal-to-noise ratio and bias characteristics.

Peak	The maximum level of a sound or electrical signal

Print-through	The transfer of magnetization of the recorded sound from one layer of tape to immediately adjacent layers. Print-through usually is precipitated by overloading during recording. The audible effect of print-through is echo.

Reference Tape	A tape whose characteristics are known under specified operating conditions and which is used as a comparison standard for tape evaluation.

Remanence	The induction which remains when the magnetizing field is reduced to zero from a level producing saturation. magnetic recording tapes having high remanence have a correspondingly high output.

Saturation	The point at which a tape is fully magnetized, any additional signal beyond this point causes distortion.

Sensitivity	The input signal level required by the tape to produce a stated output. The lower the necessary input, the higher the sensitivity. Tape sensitivity is expressed in dB as the difference of output levels between the test tape and the reference tape. For testing, signals are recorded at a recording level of -20dB.

Signal-to-Noise Ratio (S/N)	The ratio between the desired signal and the noise level. The bigger the difference, the less objectionable the noise.

Squareness Ratio	A measure of tape efficiency, relative to recording.

Q4.1 -	What does it mean to "align" a tape machine?

There are a number of standard adjustments on any analogue tape machine, which can roughly be broken up into mechanical and electronic adjustments. The mechanical adjustments include the head position (height, skew, and azimuth), and sometimes tape speed. Incorrect head height will result in poor S/N and leakage between channels, because the tracks on the head do not match up exactly with those on the tape. Incorrect tape skew will result in level differences between channels and uneven head wear, because there is more pressure on the top of the head than the bottom (or vice versa). Incorrect azimuth will result in loss of high frequency response and strange skewing of the stereo image. Tape speed error will result in tonal shifts, although on many machines with capstan speed controlled by crystal or line frequency, it is not adjustable.

Electronic adjustments include level and bias adjustments for each channel. Some machines may have bias frequency adjustments, equalization adjustments for playback and record emphasis, pre-distortion adjustments, and a varied bevy of adjustments for noise reduction systems.

Alignment is relatively simple, and the same general method applies from the smallest cassette deck to the largest multitrack machine. First, put a test tape on the machine. Use a real reference tape, from the manufacturer, from MRL, or a similarly legitimate lab. DO NOT EVER use a homebrew test tape that was recorded on a "known good" machine. You will regret it someday. Spend the money and get a real test tape (and not one of the flaky ones from RCA).

1.	Speed adjustment (if necessary). Play back a 1 KHz reference tone and, using a frequency counter, adjust the tape speed for proper frequency output. There are strobe tapes available for this as well, but with cheap frequency counters available, this method is much easier.

2.	Head height and skew adjustments. Better see your machine's manual on this one, because I have seen a variety of ways of doing this.

3.	Azimuth adjustment. I find the easiest way to do this is to take the left and right outputs and connect them to the X and Y inputs of an oscilloscope, and play back a 1 KHz reference tone, while adjusting the azimuth until a perfectly-diagonal line appears.

You can do this by ear if you are desperate, but I strongly recommend the lissajous method, which is faster and more accurate. On multitrack decks, use the two tracks as close as possible to the edge of the tape. Now you have the playback head azimuth set... put a 1 KHz source into the record input, with a blank tape on the machine, and adjust the azimuth of the record head for the proper diagonal line.

4.	Playback eq adjustment (if necessary). This is a case of playing back various test tones at different frequencies, and adjusting the response curve of the deck to produce a flat output. You can also do this by playing back white noise and using a third-octave spectrum analyzer of great accuracy to adjust for flat response. Again, this is one to check your deck's manual for, because the actual adjustments vary from one machine to another, and you will want to use the test tape once again.

5.	Record eq adjustment (if necessary). How this is done (and whether you want to do it after biasing the tape) depends a lot on your deck.

6.	Bias adjustment. There are a lot of ways to do this. My favorite method is to use a white noise source, and adjust the bias until the source and tape output sound identical. Some people prefer to use a signal generator and set so that the levels of recorded tones at 1 KHz and 20 KHz are identical. I find I can get within .5 dB by ear, though your mileage may differ. [Ed. note: Many tapes have recommended overbias settings, and many decks will also provide a chart that correlates the amount of overbias against available tape formulations. -Gabe]

7.	Record level adjustment. I use a distortion analyzer, and set the level so that at +3 dB, I get 3% distortion on the output. Some folks who are using very hot tape set the machines so that a certain magnetic flux is produced at the heads given a certain input, but I find setting for a given distortion point does well for me. If you don't have a distortion analyzer, use a 1 KHz tone source and set so that you have the onset of audible distortion at +3 dB, and you will be extremely close.

[Ed. note: The traditional way to do this is to align the repro side of the machine using a calibration tape, and then to put the machine into record. Monitoring off the repro head, the operator then aligns the record electronics until the output is flat. -Gabe]

At this point, you will be pretty much set. Whether you want to do this all on a regular basis is a good question. You should definitely go through the complete procedure if you ever change brands of tape. Checking the mechanical parameters on a regular basis is a good idea with some decks (like the Ampex 350), which tend to drift. Clean your heads every time you put a new reel on, and demagnetize regularly. [Scott]

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Q4.2 -	What is bias? What is overbias?

With just the audio signal applied to a tape, the frequency response is very poor. High frequency response is much better than low frequency, and the low frequency distortion is very high. In 1906, the Poulson Telegraphone managed to record an intelligible voice on a magnetic medium, but it was not until the 1930s when this problem was solved by German engineers.

To compensate for the tape characteristic, a very high frequency signal is applied to the tape in addition to the audio. This is typically in the 100 KHz range, far above the audio range. With the bias adjusted properly, the frequency response should be flat across the audible range. With too low bias, bass distortion will be the first audible sign, but with too much bias, the high frequency response will drop off.

Incidentally, digital recording equipment takes advantage of the very nonlinearity that is a problem with analogue methods. It records a square wave on the tape, driving the tape into saturation at all times, and extracts the signal from the waveform edges. As a result, no bias is required. (For a good example of the various digital recording methods, check out NASA SP 5038, _Magnetic Tape Recording_.) [Scott]

[Ed. note: For those looking for an understanding of why we need bias in the first place, here is one way to think about it. Tape consists of lots of small magnetic particles called domains. These domains are exposed to a magnetic field from the record head and oscillate in polarity as the AC signal voltage changes. Domains, being physical objects, have inertia. Every time the analog signal crosses from positive to negative and back again, the voltage passes the zero point for an instant. At this moment, the domain is at rest, and like any other physical object, there is a short period of inertia before it gets moving again. The result is the bizarre high-frequency performance characteristic that Scott described. The high frequency of a bias signal simply ensures that the domains are always kept in motion, negating the effect of inertia at audio frequencies. -Gabe]

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Q4.3 -	What is the difference between Dolby A, B, C, S, and SR? How do each of these systems work?

The Dolby A, B, C, SR, and S noise reduction (NR) systems are non-linear level-dependent companders (compressors/expanders). They offer various amounts of noise reduction, as shown in the table below.

Dolby HF NR LF NR Number Of Active Target

System Effect Effect Frequency Bands Market Year

------ ------ ------ ---------------------------- --------- ----

A 10 dB 10 dB 4 fixed Pro audio 1967

B 10 dB -- 1 sliding (HF) Domestic 1970

C 20 dB -- 1 sliding (HF) Domestic 1981

SR 24 dB 10 dB 1 sliding (HF), 1 fixed (LF) Pro audio 1986

S 24 dB 10 dB 1 sliding (HF), 1 fixed (LF) Domestic 1990

------ ------ ----- ---------------------------- --------- ----

The band-splitting system used with Dolby A NR is a relatively costly technique, although it can deal with noise at all frequencies. The single sliding band techniques used in Dolby B and C systems are less costly, making them more suitable for consumer tape recording applications where the dominant noise contribution occurs at high frequencies.

The typical on-record frequency response curves for the Dolby B NR system look something like those depicted below. The curves for Dolby C, SR, and S are similar, but the actual response levels and behaviour at high frequencies are modified to extract better performance form these more advanced systems.

|

0dB -|----------------------------------------------------

|

|

-10dB -|

| /-------------------------------

| /

-20dB -|------------------/

|

|

-30dB -| /-----------------------------

| /

| /

-40dB -|----------------/

|______________________________________________________

| | |

20Hz 1kHz 20kHz

The above picture attempts to show that the encoding process provides selective boost to high frequency signals (decoding is the exact reciprocal), and the curves correspond to the results achieved when no musical signal is applied. The amount of boost during the compansion depends on the signal level and its spectral content. For a tone at -40dB at 3 kHz, the boost applied to signals with frequencies above this would probably be the full 10dB allowed by the system. If the same tone were at a level of -20dB, then the boost would be less, maybe about 5dB. If the tone was at 0dB, then no boost would be supplied, as tape saturation would be increased (beyond it's normal amount).

The single band of compansion utilized with Dolby B NR reaches sufficiently low in frequency to provide useful noise reduction when no signal is present. Its width changes dynamically in response to the spectral content of music signals. As an example, when used with a solo drum note the companding system will slide up in frequency so that the low frequency content of the drum will be passed through at its full level. On replay, the playback of the bass drum is allowed to pass through without modification to its level, while the expander lowers the volume at high frequencies above those of the bass drum, thus providing a reduction in tape hiss where there is no musical signal. If a guitar is now added to the music signal, the companding band slides further up in frequency allowing the bass drum and guitar signals through without any compansion, while still producing a worthwhile noise reduction effect at frequencies above those of the guitar.

The Dolby B NR system is designed to start taking effect from 300Hz, and its action increases until it reaches a maximum of 10dB upwards of 4kHz. Dolby C improves on this by taking effect from 100Hz and providing about 15dB of NR at 400Hz, increasing to a maximum of 20dB in the critical hiss region from 2kHz to 10kHz. Dolby C also includes spectral skewing networks which introduce a rolloff above 10kHz prior to the compander when in encoding mode. This helps to reduce compander errors caused by unpredictable cassette response above 10kHz, and an inverse boost is added after the expander to compensate. Although this reduces the noise reduction effect above 10kHz, the ear's sensitivity to noise in that region is diminished, and the improved encode/decode tracking provides important improvements in overall system performance. An anti-saturation shelving network, beginning at about 2kHz, also acts on the high frequencies but it only affects the high-level signals that would cause tape saturation. A complementary network is provided in the decode chain to provide overall flat response.

When the tape is played back, the inverse of the above process takes place. For an accurate decoding to occur, it is necessary that playback takes place with no offsets in levels between record and replay. i.e. If a 400 Hz tone is recorded at 0dB (or -20dB), then it must play back at 0dB (or -20dB). This will help ensure correct Dolby "tracking".

Just think about it: if a -40dB tone at 8kHz was recorded with Dolby B on, then it would actually have a level of -30dB on tape. The same tone, if it were at a -20dB level, would have a level of about -15dB on tape. If the sensitivity of the tape was such that anything recorded at 0dB actually went on tape as -10dB, then you can see that the Dolby encoded tones would actually be at a lower level, and the system would have no way of determining this. It assumes 0dB in = 0dB out. Hence the signal would be decoded with the incorrect amount of de-boost.

The Dolby SR and S NR systems provide slightly more NR than Dolby C at high frequencies, 24dB vs 20dB, but they also achieve a 10dB NR effect at low frequencies below 200Hz as well. This is obtained using a two-band approach, the low-frequencies being handled by a fixed-band processor, while the high frequencies are tackled by a sliding band processor. This reduces the potential for problems such as "noise pumping", caused by high-level low frequency transient signals (bass

notes from drums, double basses, organs), raising the sound level in a cyclic fashion. Dolby SR and S also contain the spectral skewing and anti-saturation circuits for high-level high-frequency signals that are implemented with Dolby C. The performance of the sliding band is improved over that obtained with Dolby B and C NR systems by reducing the degree of sliding that occurs in the presence of high-frequency signals. This increases the noise reduction effect available at frequencies below those occurring in the music signal.

An additional benefit of the Dolby S NR system for consumers is that the manufacturers of cassette decks who are licensed to use the system must adhere to a range of strict performance standards. These include an extended high frequency response, tighter overall response tolerances, a new standard ensuring head height accuracy, increased overload margin in the electronics, lower wow and flutter, and a head azimuth standard. These benefit users by enhancing the performance of cassette recorders as well as helping to ensure that tapes recorded on one deck will play back accurately on any other. [Witold Waldman - witold@aed.dsto.gov.au]

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Q4.4 -	What is Dolby HX-Pro?

HX-Pro is a scheme to reduce the level of the bias signal when high frequency information is present in the recorded signal. Sufficient high frequency information will act to bias the tape itself, and by reducing the AC bias signal somewhat, additional signal can be applied without saturating the tape. This is a single-ended system; it requires no decoding on playback, because it merely permits more signal to be recorded on the tape. In theory it is an excellent idea, and some implementations have lived up to the promise of the method, although some other implementations have produced unpleasant artifacts. [Scott]

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Q4.6 -	How much better are external microphone preamplifiers than those found in my portable recorder?

Going by the rule that "external is better than internal," the external preamps are likely to sound better. Besides the issue of electrical shielding and interaction, it is simply the case that a designer who is spending *all* his time on a project designing only a preamp is likely to do a better job of it than a tape machine design team that has to worry how they're going to fit the preamp into the box and still have enough room for the rest of the tape machine. [Gabe]

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Q4.7	What is an MRL? Where do I get one?

An MRL is a reference alignment tape from Magnetic Reference Laboratory. These tapes, available in every conceivable tape speed, tape width, equalization, and field strength, contain alignment tones useful in calibrating the electronics of analog tape machines.

These tapes can be ordered from many pro audio dealers. If not, you can contact MRL directly at:

Magnetic Reference Laboratory, Inc.

229 Polaris Avenue, Suite 4

Mountain View, CA 94043

Tel: (415) 965-8187

Fax: (415) 965-8548

Tape playback

The first is that when you play a tape is that the voltage rise 6 dB per octave that you have to compensate then you have to compensate for the losses in the gap, and after that you have the play back equalize curve that differ with the tape ( its not possible to have as high level in high frequency as in the low freq.)

Bias Optimization in Audio Recording

Recall that the magnetization curve for the original magnetic material (Mr) is linearized by including a large ac bias current.

There is an optimal value of bias that delivers the best linearity for a given dynamic range. Unfortunately, since the field decreases with distance from the recording head, the bias field is really only optimal for one section of the recording media. Adjusting the bias down for optimal short wavelength performance will result in distorted long wavelength signals. However, adjusting the bias up for optimal long wavelength performance will generate losses in the short wavelength signals. Thus, the bias is typically adjusted a bit on the high side in order to minimize long wavelength distortion at the cost of short wavelength losses.

Equalization

Equalization is a process by which electronics compensates for frequency dependent losses in the recorder or reader. Equalization can occur in the recorder (pre-equalization) or in the reader (post-equalization). Pre-equalization is generally preferred because it does not increase the reproduced noise. However, pre-equalization can run into the hazard of compression -- particularly for program materials with large amounts of high frequency material. Therefore, most commercial recorders and readers incorporate both pre and post equalization.

Noise sources [18]

Bulk-erase noise: There is a noise source due to the size and orientation of the particles in the magnetic coating. The signal to noise ratio for a band of wave numbers from k to dk (k = 2/) is given by:

Thus, the signal to noise is proportional to the track width (w), the volume fraction of the coating occupied by particles (p such that the maximum = 1) and the orientation (F(theta) = 3/8 for random and 1 for perfect). The signal to noise ratio is inversely proportional to the mean size of the particle (s) and the standard deviation of the particle size distribution sigma. Therefore, decreasing the track size, increasing the number of particles, and decreasing the particle size; will improve the bulk signal to noise.

Bias noise: The noise from a bulk erased tape increased when moved over an ac biased head. Essentially, what is happening is that the bulk-erase noise induced in the record head is re-recorded back on the tape -- resulting in a net doubling of the noise flux power.

DC noise: A large direct current in the record head will cause the noise level to increase by 10 dB or more at longer wavelengths. This increase has been attributed to inhomogeneities in the coating process. DC bias noise is common in particulate coatings, but very rare in film coatings.

Surface noise: Certain long wavelength noise is due to variations in head to tape spacing caused by inhomogeneities in the tape surface, tape stiffness changes, and surface treatment.

Print-through: Print through is an interference effect unique to magnetic tape systems. The problem occurs when magnetically induced information in one layer of the tape induces magnetic changes in the next layer of tape wound on the reel. The interference creates a very distinct pattern of signal echos of ascending strength which precede the signal (pre-prints) and signal echos of descending strength which follow the signal (post-prints). The worst offenders for print-through are magnetically hard materials, particularly cobalt.

Companding

Dolby, DBX and other companding techniques.

The basic idea in companding is relatively straightforward. Low intensity signals are increased in amplitude before being recorded on the tape. Higher intensity signals are left alone. When the tape is read, the process is reversed. The differential companding signal is added to the input on recording, and subtracted on playing.

When this technique was first developed, the initial systems showed a tendency to "breathe". In other words, the noise developed a characteristic modulation due to the companding algorithm. This tendency can be significantly reduced by breaking up the differential companding signal into several frequency ranges. (For type A Dolby, these are 0-80 Hz, 80 Hz - 3 kHz,3 KHz high pass and 9 kHz high pass.)

Companding techniques have proved especially valuable for cassette players. Type B Dolby is commonly used in cassette players. In type B Dolby, the low level, high frequency signals are boosted by 10 dB during the record process (and attenuated by the same amount during the reproduce). This does tend to compress high frequency signals however. This tendency has been addressed bythe HX type B -- where the ac bias is reduced a bit for high frequency signals. This increases the dynamic range admittedly at the cost of some linearity.

Duplication

AC biased magnetic recording can handle frequencies 10-100 times larger than those of the highest audio frequencies. As a consequence, audio programs can be duplicated at high speeds. This is exceptionally important when considering the consumer applications for audio products.

Recording

The Erase Head

During recording, the tape first moves over the erase head, where all traces of earlier programming is removed. The erase head must also leave the head in the maximum random condition for lowest possible noise. Therefore, the erase head gap length is large to provide a strong erasing field. The erase head is also fed with an ac signal in order to minimize noise.

The Record Head

The record head is usually fabricated with a gap size on the order of the thickness of the magnetic coating. This provides a compromise between low and high frequency response. Additionally, the signal going into the record head is usually altered in two major ways from the original signal. First, an ac component is been added to the signal to linearize the response (see the next section). Secondly, the frequency spectrum is been altered to compensate for losses that occur in the recording process for short wavelength (high frequency) signals. This compensation process (called pre-equalization) has the ultimate goal of giving the tape recorder a flat frequency response up to its maximum design frequency.

The Reproduce Head

The reproduce head is usually fabricated with a small gap in order to maximize short wavelength (high frequency) response. The inductive pick-up generates a signal which is the derivative of the flux, so the signal must be integrated. Additional equalization must be added (called post-equalization) to compensate for losses that occur in the reading process for short wavelength (high frequency) signals-- again with the intent of providing the recorder with a flat frequency response.