AstroNuclPhysics Nuclear Physics - Astrophysics - Cosmology - Philosophy


1. Introduction. Development of electroacoustic recording

Everyone who deals with sound recordings - their recording, reproduction, editing, archiving - is from time to time in connection with the development of electronics faced with the questions: "What medium (carrier) to record on?"; "Which instrumentation is best for my range of genres?"; "From what sources to record?"; ... etc. In this article I will try to share my experience in this area using minidiscs.

The first widespread electroacoustic medium was a gramophone record , which remained until about the 1950s. practically the only carrier for individual (home) use - tape recorders were used only in radio and recording studios. With the development of low-current electronics, classical coil tape recorders first began to expand , and from the end of the 1960s, cassette tape recorders began their path of mass expansion , partially displacing gramophone records. They eliminated two main disadvantages of records: 1.Inability to record. 2. Susceptibility to mechanical damage, wear and loss of quality during repeated playback. With the gradual improvement of the technology of tape production and the increase of the technical level of tape recorders (such as the introduction of Dolby B, C noise reduction), the acoustic quality of gramophone records (new and undamaged) was practically achieved.

The definitive end of the era of gramophone records was marked by the introduction of laser digital CDs in the 1980s and their widespread use during the 1990s. Compact CDs outperform turntables in all respects - with significantly lower noise levels, incomparably greater dynamic range, damage resistance, no wear and tear, smaller dimensions and much easier handling.
Note: Even now, there are "rock" proponents of gramophone records, claiming that the sound from a CD is not as natural as from records. However, these opinions are subjective (perhaps the question of the habit of faint noise and crackling) and they are not based on the truth in well-recorded CDs.

A certain disadvantage of compact discs was initially the inability to make their own recordings on this medium - in that they resembled the original gramophone record. However, around the mid-1990s, CD recording ("burning") devices began to expand , mostly as part of PCs. In addition to single-disc CDs (type R), R / W types have been developed to allow re-recording ("rewriting") similar to magnetic media.

Simultaneously with the optical laser recording and playback of the CD system, the recording of digitized sound on a magnetic tape - the DAT system - was also developed . Despite some of its advantages, however, it has not become more widespread; used eg for professional field recording. A certain disadvantage here is the slow access to data typical of magnetic tape systems.

The system that combines high quality digital recording with the operability and flexibility of recording and reproduction (and in addition allows you to insert descriptions and provides a number of editing options) is the MiniDisc . This system was developed by the well-known Japanese company Sony at the turn of the 80's and 90's.

2. Principle of the MiniDisc system

The MiniDisc (MD) system is based on the principle of magneto-optical recording of digitized audio data. The medium for recording and reproduction is a polycarbonate rotating disk with a diameter of 64 mm provided with an active layer with special ferromagnetic and optical properties. The surface of the minidisc consists of three layers: an iron-cobalt-terbium-chromium alloy as a recording layer, aluminum-titanium as a reflective layer and a silicone protective layer. The central hole of the minidisc has a pressed metal center , which ensures precise centering and also reduces the mechanical stress on the disc at the point of application to the drive shaft in the device. The minidisk itself is encapsulated in a 7cm x cassette6.75 cm, 0.5 cm thick, with sliding caps covering the holes for the optical laser and magnetic recording head. This cartridge protects the disc very well from dirt and mechanical damage.

Fig.1. Principle of magneto-optical recording of digitized data on a minidisk

The principle of recording is schematically shown in Fig.1. The optical and magnetic heads are placed opposite each other, between them the minidisc disc itself moves tightly. The semiconductor laser emits a constant (unmodulated) light beam with a power of about 5 mW during recording - this power is automatically adjusted when the media is inserted. Upon impact with the surface of the disk, the beam heats the locally active layer to the so-called Curie temperature (which is the temperature at which the material loses ferromagnetic properties and becomes a paramagnetic substance - for materials used on MD it is only about 180 oC). After the beam passes, the surface temperature immediately drops below the Curie point and the ferromagnetic properties are restored. If a signal is applied to the magnetic head located opposite the other side of the disk, a trace of this signal will remain in the active layer in the form of a change in magnetic orientation due to the magnetic field . This change in magnetization then leads to a change in the polarization of the reflected light beam during reading-playback (due to the so-called Faraday effect).
The magneto-optical Faraday phenomenon consists in the fact that when polarized light passes through a transparent optical medium in the presence of a magnetic field, the plane of polarization of this light is rotated . This rotation increases with the intensity of the magnetic field and is most pronounced whenlight passes through the substance in the direction of the magnetic field . The direction of its rotation is the same as the direction of the current in the excitation electromagnet. When the beam bounces back and forth, the angle of rotation adds up. The electro-optical variant of this phenomenon - the Kerr effect - consists in the twisting of the polarization plane of the light passing through the substance under the action of an electric field.

When reading (playing, reproducing), the laser beam (about 10 times less intensity than when writing) hits the surface of a rotating minidisc, where places with different magnetic orientations reflect the polarized beam differently; such a polarization- modulated reflected beam is then converted in the photodiodes of the reading head into an electrical signal reproducing the original recorded signal - Fig.2.

Fig.2. Principle of optical reading of digital electroacoustic data from a minidisc. The reflected beam, polarized modulated according to the differently oriented magnetization of the recorded track on the surface of the minidisc, first passes through a polarization analyzer (divider). The so-called Wollaston birefringent prism is used, which is a combination of two crystals (mostly CaCO 3 limestone ) optically bonded at an angle of 45 o. In this prism, rays with different polarizations separate from each other from the original direction and emerge at different angles. The photodiodes A and B, on which the angularly scattered rays impinge, then give a signal of different strength depending on the angle of polarization. By subtracting these signals, we get, after adjustment, a digital signal reproducing the originally written bit combination in the magneto-optical recording according to Fig.1. Another pair of photodiodes is used to track.
Note: The optical reading head of the minidisc contains another four photodiodes detecting reflected rays in the case of using recorded (non-rewritable) minidiscs, where the shape of the recording is similar to that of a CD - aluminum reflective layer with a signal stored by wells. We do not mention this type of minidisc in our communication (focused on recording).

This system at low power consumption (several mW) optical and magnetic recording head allows high recording density , the possibility of direct local overwriting of a precisely defined area on the disk, high number of transcription cycles (virtually unlimited, at least 1 million is specified), very low write / read error , low sensitivity to mechanical perpendicularity of the recording head.
In addition, a large solid-state buffer is used for recording and reproduction, which ensures continuous playback without interruption even if the head temporarily loses track (due to locally damaged disk space or mechanical shock or vibration). In the event of this fault, the data is played back from the memory and the control system in the meantime finds the place where the signal was lost, starts reading again from the surface of the minidisc and the buffer is filled with new correct audio data. The size of the buffer usually ranges from 2MB for desktop devices (corresponds to about 6sec. Recording) to about 16MB for MD walkmans (corresponds to about 40sec.); for portable devices, a larger buffer memory is required due to the possibility of frequent shocks and vibrations.

So simply put:
Recording on MD is magneto-optical, reading is purely optical similar to CD.

3. Signal digitization, ATRAC, Long Play mode

The electrical signal applied to the magnetic recording head (Fig. 1) is not a common electroacoustic signal as in analog recording, but a signal digitized by an analog-to-digital converter. From the original time domain (waveform electroacoustic signal - on the horizontal axis of the graphical representation is a time of the vertical signal amplitude) the signal using the discrete Fourier cosine transform is converted into the spectral domain (in graphic form, the horizontal axis frequency harmonics kosinusovky The vertical axis shows the amplitude its representation). The resulting spectral coefficients are quantized and converted to digital bit information - a combination of logic "0" and "1", which are already conducted to the recording magnetic head in the form of electrical pulses (Fig. 1). By the mechanism mentioned in the previous paragraph, these bit combinations are recorded in the active layer of the minidisc as places with a locally changed magnetic orientation.

When reading, the laser beam hits the surface of the minidisc, places with different magnetic orientations differently reflect the polarized beam, which is converted in the reading head into an electrical signal reproducing the original written bit combination (Fig. 2). This is then converted to spectral coefficients using a digital-to-analog converter and back to the time domain using an inverse cosine Fourier transform using a tone generator - a usual electroacoustic signal (as a mixture of harmonic functions at appropriate frequencies) is synthesized to reconstruct the recorded music.

To reduce the data flow , the MiniDisk uses an original algorithm called ATRAC (Adaptive TRansform Acoustic Coding). *) Discrete cosine transformation takes place separately in three frequency bands (approximately 20Hz-5kHz, 5-12kHz, 12-20kHz) and spectral quantization "digitization") the size of the transformation blocks adaptively changes according to the nature of the signal ...... It is based on psychoacoustic propertieshuman hearing (such as frequency response of volume or time-spectral resolution). The algorithm used leaves only "acoustically significant" bits, omitting "redundant" bits that are not reflected in the resulting acoustic signal. With the 16-bit quantization and 44kHz sampling frequency used, the data flow is effectively reduced by about 5-fold.

The structure of the recorded audio data divides the recording into blocks, each of which consists of 36 sectors - 32 sectors carry audio information, four are used for auxiliary data and for the addresses of the previous and next blocks. The recording on the disc can be divided into 255 separate tracks (recordings, tracks). All this information about the contents of the minidisc is recorded in the so - called " Table Of Contens " ( TOC) - carries information on the start and end addresses of songs, song titles, recording mode information (such as LP2 / 4). This information table is played back in the working memory of the device when the minidisc is inserted, it is updated during recordings and edits, and the contents of the TOC are saved in the appropriate place of the disc (near the center) when the disc is removed - the recorder releases its own disc. Any breach of the TOC will result in loss of access to the uploaded data.

4. Recording to a minidisc in Long Play 2x / 4x mode

The LP4 compressed mode offers the tempting way to record four times more audio on a single minidisc, ie 320 minutes on an MD80. But what about the sound quality? - What reduction in sound quality "paid" for this significant increase in recording capacity? If we perform electroacoustic measurements , we find the following facts:

Although electroacoustic measurements provide objective data on the recording and reproduction characteristics of the system, practical listening experience is also important (although it may be subjectively influenced). I can summarize my experience as follows :

  1. When listening in normal residential conditions (two two-way speakers, reasonable volume, normal level of acoustic background), the listener does not know whether music or a word is being played from a CD or a minidisc in LP2 / 4 mode.
  2. When listening from headphones or good speakers in an environment with a low level of acoustic background, we already know a certain influence compared to reproduction from a quality CD:
    - There is no such "transparency" of sound in the subtlest sounds, echoes, reverberations.
    - The sharp "penetration" of the sound in places of sudden changes (onset) is somewhat smoothed out.
    - The stereo effect is somewhat weakened and, conversely, the traces of dynamic "crosstalk" between the two channels are intensified (it manifests itself again mainly in headphones).

To at least partially eliminate these differences and subjectively improve listening , I tried to slightly filter (emphasize) the signal during recording with a graphic equalizer in the lower and especially the upper part of the frequency spectrum: low frequencies in the 20-60Hz range amplified by about 3dB, high frequencies in the 6- 15kHz amplified by about 3-6dB, leaving the middle frequency unaffected. This enriches already during recording those marginal parts of the spectrum which can be most reduced during data stream compression.

5. Advantages and benefits of minidisc

The main advantages of minidiscs can be summarized in the following points :

  1. Provides high-quality digital audio recordings analogous to CDs.
  2. Small dimensions, resistance to damage, ...
  3. Quick data access - instant search for recordings ...
  4. Wide range of editing options - operative division, merging, deleting, moving any sections of recordings, inserting and editing texts ...
  5. Standby for playback and recording from a wide range of audio sources - analog and digital inputs ...
  6. Possibility to connect to a computer ...
  7. In Long Play mode, we can record more than 5 hours (320 minutes on MD80) on one minidisc.
  8. Miniature portable MD Walkmans easily allow you to not only listen in the field, but also record live in digital quality via microphones.

So we can say that the minidisc surpasses classic audio cassettes in all respects , which it can replace at a higher level. In general, the minidisc is very advantageous wherever we need to record well and operatively from various sources of electroacoustic signal, including the microphone.

6. Disadvantages of minidisc

  1. So far, a small expansion of minidiscs may limit compatibility with other phonoamateurs. If we send an audio cassette or CD, for example, by post to our colleague, we can be sure that he will play it successfully. It only makes sense to send a minidisc to someone who has the appropriate player.
  2. Arbitrarily blocking repeat playback from one minidisc to another via digital input / output can be an annoying limitation when working more seriously with a minidisc library - for example, when we want to sort thematically similar songs on one minidisc by copying from several other minidiscs; if these songs have been digitally recorded, they can paradoxically only be copied via analog input / output. The manufacturer is not to blame for this mistake - this was forced on him by the mafia practices of the "copyright protection" associations, which, even at the cost of hampering technical progress, try to maintain and increase the unjustifiably high profits of show business.
  3. A minor technical error is the difficulty in editing (deleting and merging) two nearby places in the recording, when the message "Impossible" appears unjustifiably and it is no longer possible to complete the editing work of this place.

7. Suggestions for improving MD recorders

Based on my experience using the Minidisc system, I would like to make two modest suggestions that could further improve this system ....:

I myself have no possibility to promote these proposals in any way, but maybe one of the readers who should have these possibilities can be inspired ...

8. Conclusion - singing of minidiscs

In conclusion, I would try to supplement the objective technical analysis described above and confront it with my own experience with the use of minidiscs.

The main mini-disc device I use for recording and editing is the MDS-JB980 (SONY product), which (along with the MDS-JA333ES) is the highest model of desktop mini-disc recorders. It has all the recording, control, editing, filtering and reproduction functions that have been introduced in MD so far, as well as all analog and digital inputs and outputs, including connection to a computer. I have the device connected via analog and digital inputs and outputs as part of a larger electroacoustic apparatus ("towers") composed of components of the same class as the MDS-JB980. Includes: receiver with amplifier connected to 6 speakers including subwoofer (Dolby Surround Prologic system), cassette recorder (with 3 heads, 3 motors and pre-magnetization calibration), CD player and digital equalizer. Working with this device is very elegant and flexible , I am completely satisfied with it.

As another minidisc device, I use the MDS-JE470 , which has a smaller spectrum of functions and inputs and outputs, but the quality of the recordings is essentially identical to that of the above-mentioned type. I use it as a 2nd device and when playing each other from one minidisc to another. Again with very good experience with the exception of a nonsensical artificial restriction preventing repeated digital transcription.
It is also a miniature battery-powered MD walkman MZ-R900 for use in the field . I use it sporadically only for recording and reproduction. I basically perform all editing on desktop devices which are incomparably more perfect for these tasks.

Finally, they are two "mini towers" CHC-CL5MD and CMT-M100MD equipped with minidisks. I use them to reproduce already recorded minidiscs, although they also allow recording, but without manual control and without checking the level of the recorded signal. As these microsystems contain in compact form a radio tuner, cassette recorder, CD player and minidisc, they are the ideal means for normal listening to recordings from all common types of media; their advantage is easy and fast operation without the need to know the properties and interconnection of individual components.

Using these devices, I successfully recorded, edited and provided texts for several hundred minidiscs. I recorded from a radio, a CD, other minidiscs and especially audio cassettes. I "digitized" practically my entire library of about 800 cassettes into minidiscs; I saved the original cassettes "deep in the archive". This brought a drastic improvement in clarity , speed of access to individual recordings and their parts (without tedious rewinding of tapes), easier handling and complete reproducibility playback (compared to older cassettes, where uneven winding of the tape could lead to fluctuations, different tape recorders played different tapes with different characteristics, there was a "copy" between adjacent turns of the tape and similar non-standards).

I can conclude this article by stating my unequivocally positive experience with the use of minidiscs. Their strengths certainly outweigh the minor shortcomings mentioned in paragraph 6.

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AstroNuclPhysics Nuclear Physics - Astrophysics - Cosmology - Philosophy

Vojtech Ullmann