The Minidisc FAQ Del 1

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Questions and Answers

What are the dimensions of a MD?

7 cm (2.75") x 6.75 cm (2 21/32") x 0.5 cm (3/16") The disc inside is 64 mm in diameter.

Is there an audible difference between modern MD and CD/DAT?

Since MD stores audio using a data reduction technique (see ATRAC, below) there are considerable differences between the audio signal from a CD and that of an MD digitally recorded from it. Whether or not these differences are audible however depends to a large degree upon who is doing the listening, most MD users consider the sound quality of modern MD equipment to be essentially the same as CD. But even those who can hear differences usually find them to be undisturbing and inconsequential, frequently being unable to say which is which. (Note that a proper A/B comparison of the two formats necessitates using the same DAC. If an outboard DAC is unavailable, or the MD unit (such as a portable) does not provide digital out, the MD unit's monitor mode can be used to do D/A conversion of external digital signals, thereby allowing comparison with a common DAC.)

A small double-blind test made by the ABX Company found that subjects did not find a difference between ATRAC processed music and its original, but could readily spot the difference when a special test signal was used for the comparison.

However, one significant difference with DAT is that differences with the original increase in each subsequent copy generation, even when recording digitally. Each time the MD is played, a full 16-bit data stream is regenerated from the compressed data. Each time it records, the input data stream is recompressed. The artifacts of the compression process build up from generation to generation. See ATRAC below.

Between MD and DCC?

Yes. DCC uses a lower-loss compression algorithm than MD (4: 1 vs. 5: 1), and thus could sound better. MD should be compared to DCC with the same DAC, for fair results. Also note that most tests compared the first generation Philips home DCC recorder with the first generation Sony portable MD recorder (MZ-1).

What is ATRAC exactly? How does it compare to PASC?

ATRAC (Adaptive TRansform Acoustic Coding) divides the 16 bit 44.1 KHz digital signal into 52 sub-bands in the frequency domain (after a Fast Fourier Transform). The sub-bands in the low frequencies are finer than the ones in the high frequency range. A psycho-acoustic transfer function that takes advantage of the masking effect and the absolute hearing threshold then removes enough information to reduce the data stream to 1/5th of the original size. Each channel receives that treatment separately (the Sony MZ-1 portable MD recorder features one ATRAC encoder/decoder chip per channel). PASC (Precision Adaptive Sub-band Coding) divides the digital signal into equally spaced sub-bands and removes less information (to only 1/4th of the original size). PASC is essentially the MPEG Layer 1 audio standard (can be decompressed with MPEG Layer 1 players after a trivial preprocessing step).

Both are data compression algorithms, used to store the information content from a stream of 16-bit samples in fewer bits. The purpose of compression is to reduce the rate at which the disk has to deliver or record bits, and to reduce the total number of bits stored. There are many compression algorithms. The ones used for computer data (for example in archiving programs) are lossless; the result of decompression is identical to the input.

PASC and ATRAC are both "lossy" algorithms. In order to get greater compression, they do not attempt to preserve every bit of the original data, but rather only the acoustically "important" bits. Considerable cleverness goes into finding the sounds masked by properties of the human auditory system, ones that you would not hear even if they were reproduced. By all accounts the two schemes do amazing well, considering they operate in real time.

What is the bit rate of MD audio after compression?

For a stereo signal it's 292162.5 bits/sec. ATRAC compresses 512 incoming 16 bit samples (1024 bytes) into one ATRAC ``sound group'' (212 bytes) giving an audio compression ratio of 4.83:1. Here is the math:

44100 samples/sec (incoming single channel rate) / 512 samples/soundgroup (giving 86.133 soundgroups/sec/channel) * 2 channels (giving 172.266 stereo soundgroups/sec) * 212 bytes/soundgroup (giving 36.5K stereo bytes/sec) * 8 bits/byte (giving stereo bits/sec) = 292162.5 bits/sec.

How does ATRAC compare with MPEG compression?

At what bitrate would an MPEG file be equivalent to a song compressed with ATRAC? ATRAC is 292kbit/sec, giving ``CD like'' audio fidelity. MPEG Layer 1 (i.e. PASC) gives transparent CD fidelity audio at 384kbit/sec, Layer 2 (i.e. Musicam) and Layer 3 give ``CD like'' fidelity at 224kbit/sec and 128kbit/s respectively. A user has compared ATRAC and MPEG Layer 3 and rates ATRAC far better.

VFQ has a system offering 18:1 compression, pitched as something of an alternative to MP3.

If I use digital connections between an MD player and MD recorder, why can't I make an exact copy of an MD? The compression of linear (PCM) digital audio into ATRAC format and expansion back again is an asymmetric process. Even if you are compressing and expanding data that has been through the compression and expansion cycle once before, the result of the operation will yield data that is slightly different from what you started with. This may seem strange, since fundamentally it could be done symmetrically. However, there are a few uncontrollable asymmetries in the process. One is the framing of input data. ATRAC starts by breaking the input signal into blocks of 512 samples. If it were possible to arrange that the sample block boundaries created during decompression be used when the signal was further recompressed, it would improve the symmetry of the process. Another is that ATRAC introduces noise, usually at an inaudible level, but in subsequent compression this noise is indistiguisuable from signal and can alter what ATRAC determines to be masked, thereby changing the number of bits allocated to each band. Also, mathematical round off error in the compression and decompression calculations may introduce noise as well.

The next question then becomes, why not copy the compressed data directly, thereby avoiding the asymmetry of compression and decompression? Unfortunately, consumer grade MD machines do not provide access to their compressed data, nor do they provide a way of directly recording compressed data, even if it were available. The S/PDIF digital interconnect only carries data in the linear (PCM) format. Professional machines however (such as the Sony MDS-B5 with direct ATRAC I/O) do allow exact bit for bit MD copies to be made.

What is the storage capacity of MDs, CDs, DATs, DCCs?

MD: 140MB in Data Mode [~160MB for 74 min. in audio mode]

CD-ROM: 650MB

CD Audio: 740MB

DAT: ~1GB

DCC: 175MB [60 min. tape]

What is the minimum allocation unit when recording on an MD?

About 2 seconds (4 seconds monoaural). Audio information on MiniDisc is organized in cluster units, which constitute the minimum unit for write or over-write of data to the disc. Each cluster contains 32 sectors of audio data, 3 sectors of link information (next cluster, previous cluster, and ??) and 1 sector of "sub data". Each sector is 2332 bytes, and 2 sectors are grouped together into a 4664 byte block holding 11 sound groups. Each sound group is 424 bytes and contains 11.6 msec of stereo data or 23.2 ms of mono data. A cluster is then 74624 bytes (32*2332), containing 176 sound groups (74624/424), for 2.0416 seconds of stereo sound (176*11.6) or 4.0832 seconds of monoaural.

The sound group (1/86th second) constitutes the finest resolution at which the audio stream can be edited. Older consumer grade MD decks have an edit positioning accuracy of approx. 60 msec (5 sound groups) but many modern machines allow edit positioning in units of a single sound group (11.6ms).

What is the MD disc space allocation algorithm?

According to MiniDisc Rainbow Book recommendations (a ``remark'', not a mandatory requirement) the virgin space that runs to the end of the disc (if any) is always the first allocated, followed (in ascending disc address order) by the remaining free segments located throughout the disc. Free segments smaller than 6 clusters (one cluster is slightly more than 2 seconds stereo) are not allocated until enough adjacent area is freed to make an open space of at least six whole clusters. (Thanks to David Tamkin for this investigation using a Sony MDS-JE520 MD deck.)

Why is MD capacity always the same (74:59 exactly) regardless of how much redundancy there is in the recorded music?

On a frame by frame (11.6 ms) basis, ATRAC is always trying to give as much resolution as it can to each spectral component that it deems important. However it is continually short of bits, and except for the case of zero input, or a very simple signal (e.g. a few pure tones), ATRAC is using all the bits per second it has available. If it had more bits per second available it could use them, and record the signal with better sound quality.

In Sony's AES paper on ATRAC they talk about the bit allocation algorithm, and how it in general allocates more bits than are available, so they have a method for subtracting an offset from the allocations in order fit within their bit budget for a sound frame.

In the zero or simple input case, there can be unused bits in some frames, but since there is an unvarying (i.e. fixed) relationship between 11.6 ms of audio and a 424 byte "sound group" on the disc, the system is not flexible enough to actually take up less disc space when it doesn't need it.

What else (besides compression) affects the sound quality of MD?

The entire rest of the sound reproduction chain is important. The digital to analog converter (DAC) is key to reproducing the sound from the decompressed data. The audio amplifier circuitry is also critical. And there have been reports of MD playback machines (not Sony) that were made unlistenable by poor-quality attached headphones!

Every component of the chain from the DAC to the eardrum is important to good sound reproduction. All elements other than the compression algorithm must be held constant before A: B comparisons are made, for example.

Is there any loss of information when I record from a CD?

There are two sources of distortion. One is the chain of components that brings the sound to the MD's input. If you go analog-to-analog, you introduce the CD's DAC and the MD's ADC chips, each with its own artifacts. However, you can bring the digital data stream directly to the MD; then the only source of differences is the ATRAC compression algorithm. The ATRAC encoder removes information from the audio material in order to store it on the MD (5: 1 compression with loss). To make better MD recordings from CD, connect the MD recorder to the CD player via a digital connection (if possible). Thus, the ADC (poor in the first generation machines) cannot affect the sound quality. Otherwise, when recording via the analog input, make sure to adjust the manual recording level on the MD machine so that the meter peaks just above -12 dB (on the Sony MZ-1, never enable the AGC for CD recording).

When I record from tape, microphone, or other analog sources?

Yes, because the MD's analog to digital conversion circuits are involved, in addition to the ATRAC compression. High-end MD decks frequently have sophisticated analog circuitry, offering improved recording and playback preformance over mid-range decks even when they share the same ATRAC chip.

I don't have the equipment to make a digital recording, will my recordings sound okay?

Generally, analog recordings sound fine. Do set the recording level manually to avoid the audible effects of the Automatic Gain Control (AGC) circuitry adjusting the level during very loud and very quiet passages.

Many have suggested setting levels manually when recording from analog sources. Any tips for how to best determine the correct level using the MZ-R3's and -R30's feeble little LCD bars?

You will have the correct recording level when the level meter is just between 4 and 5 bars. Verified with an MDS-503 from Sony, between 4 and 5 bars on the MZ-R3 record level meter will give you somewhere between -3 dB and 0 dB. Regarding the 'R30: A user connected a 303 to the R30 using a POC (optical) cable and compared the two meters. The result: Just consider the top bar to be 'digital over' and try to adjust your level in such a way that the second bar doesn't light up too often when recording from an analogue source (even less when recording live). That should do it. Compared to the MZ-1 the R30 meter is not very useful.

Is it worth it to get the equipment necessary to make digital recordings?

Digital recording provides the most convenience when copying CDs: no recording levels need to be set, track marks are copied from the CD perfectly, and analog to digital conversion artifacts (real or imagined) are completely avoided. The one problem with digital recording is that SCMS will prevent further digital copies to be made from the copied MD. Crutchfield's Tip of the Week is devoted to analog vs. digital recording to MiniDisc.

What's the difference between the 60 and 74 minute discs?

The pregroove wobble is at a higher linear frequency on the 74 minute blanks, causing the disc to spin slower (74 min: 1.2 m/s vs. 60 min: 1.4 m/s) and therefore record more information in the same linear space. There is also information encoded in the read-only TOC near the inner circumference of the disc telling the player how long the disc is. Originally it was difficult to make the 74' blanks, but nowadays the manufacturing costs of 60' and 74' discs are the same.

Will MDs of the future contain un-compressed data?

Sharp and Sony have both said that they will produce high density MD Data Drives with a capacity of about 650MB in 1997. Such capacity would allow a CD's worth of data to be stored uncompressed on an MD. However, new audio MDs made with such capacity would be incompatible with the existing hardware base (see below), and (assuming 16 bit linear recording), would not increase audio fidelity substantially, if at all. The main benefit of uncompressed recording would be the ability for professional users to make perfect MD duplicates. If the high density MD data drives become popular, it is possible that professional MD audio equipment will be made which uses MD data discs to store uncompressed audio.

Will my MD player be able to play them?

No. Twice the tracks and density -- implies major changes in the read-write mechanism, the laser optics, etc., plus, the decoder side of ATRAC includes no provisions for linear data.

How does a pre-recorded MD differ from a recordable one?

Physically, a pre-recorded MD is just like CD, using the same optical parameters, material, and production methods. Only the data contents are different (compressed ATRAC audio vs. 16 bit linear PCM audio). Unlike recordable MD, pre-recorded MDs do not have the magneto-optical coating layers or the lubricating layers. They are made with the same plastic-aluminum structure as CDs, so there is no way to record or erase anything on them. Prerecorded MD's are also read exactly like CD's (the player focuses a laser on pits and valleys within a transparent polycarbonate substrate backed by a coating of aluminum thus reflecting or dispersing the beam). Recordable MD's are similar, but a pre-groove replaces the pits and valleys and an MO coating replaces the aluminum one. When recording, a laser is focused from one side of the disc onto the pre-groove and heats a spot on the MO recording layer to its Curie point while a magnetic field from a head in contact with the other side of the disc aligns magnetic dipoles within (read magnetizes) the heated spot on the MO layer (the N/S orientation corresponding to 0s and 1s in the data). During playback the MD machine focuses the laser on the pre-groove again, but at lower power, and the data is read back by measuring changes in polarization of light reflected from the previously magnetized regions (the Faraday effect). In order to playback both pre-recorded and recordable media, all MD units have a dual function optical assembly which can, depending upon the disc type, detect changes in either reflectivity or polarization. In terms of audio quality, pre-recorded MDs are in theory no different than recordable MDs, although, as always, audio quality depends upon which version of ATRAC the discs are encoded with. Some users have noticed certain pre-recorded MDs that sound worse than a homemade copy of the same CD with a modern MD recorder. It's likely that the ATRAC encoders used for prerecorded MDs are improved over time just as they are in the consumer units. Finally, recordable MDs have an extra hole in their case, which is sensed by a microswitch within the MD unit that connects to the REFLECT pin of the microprocessor. It tells the MD player's microprocessor whether signal reflectivity is high (for pre-recorded) or low (for recordable).

Is there a difference in the disc layout of a recordable MD and pre-recorded MD?

On recordable MDs, 32 out of 36 sectors in every cluster (the smallest recordable unit) are used for storing audio data. Of the remaining 4 sectors, 3 are needed to accomodate the start up and run out of the error correction code, as stated in the MiniDisc IEEE paper: ``Because of the long interleaved ACIRC error correction code, three sectors must be used as "linking sectors." If the user changes or adds new data to the MD disc, two or three sectors for every start and end position of the new data need to be recorded.''. In other words, some space is wasted to accomodate re-recordability. On pre-recorded MDs, however, the data is fixed during mastering as one continuous stream, so these 3 extra sectors can be devoted to sub-data (but there is no documenation as to what this extra sub-data space could be used for). The Sony DADC MD handbook says that the maximum duration of a pre-recorded MD is 78:16.

How many times can I record on a recordable MD?

Sony claims a blank MD can handle up to 1 million recordings... Time will tell! In practice, the user Table of Contents (user TOC or UTOC) of an MD could become messed up by accident (defective MD recorder or physical damage) and render the MD unusable. The Sony portable MD recorder (MZ-1) allows to recover some unusable MD's in test mode (machine open and test jumper in place) via the TOC erase function: the TOC gets obliterated and that results in a "like-new" blank MD. Physical damage to the TOC cannot be repaired, but similar defects in other parts of the MD should be removed from the available space by creating short quiet tracks that include the problem and recording after labeling these as "bad".

How long can the data/audio be stored on a recordable MD?

Sony claims in their Magneto Optical FAQs that data may be stored with magneto optical technology for more than thirty years without loss or degradation. Once written to the disk, data are safe from the magnetic fields and heat found in normal environments. However, strong magnets placed directly against the MD can destroy data, as evidenced by informal experiments conducted by users. In contrast, a modern (ca. 1999) 'silver' pressed CD from a glass master is reckoned to last around 50 years or so from a chemical point of view. However a problem was found with the chemistry comprising early CD's (of around seven years and older) which caused degredation within a decade.

Do MDs skip during playback? Why not?

Rarely. A read-ahead buffer stores a few seconds of the audio material in memory during playback. The ATRAC decoder takes the data from the buffer, rather than directly from the medium. Thus, if the mechanism mistracks because of shock or vibration, the data continues to flow from memory while the MD machine recovers. If no more data remains available (when the disturbance lasts a long time), the audio material gets interrupted. Also note that the read-ahead buffer exists on every MD machine as part of the MD format.

Does the size of the music buffer vary from player to player?

Yes. Most MD machines feature a 10-second read-ahead buffer, but some only offer 3 seconds of memory, such as the first production run of Sony MZ-1 portable MD recorders and the Aiwa AMD-100.

Does MD have any special editing features for recording?

Yes. The MD format stores data like hard-disk or floppy-disk drives in computers. The TOC contains a list of starting/ending positions and names for each track, like the directory in computers. Tracks can be erased, divided, combined, moved [or just ``swapped'' on some early models] and named like the files in computers. For example, after recording 11 5-minute tracks on a 60-minute MD, 55 minutes are used and 5 minutes remain. If the user decides to erase track #8, the TOC gets updated, and now 50 minutes are used by 10 tracks and 10 minutes remain. If the user now decides to make a 7 minute recording, no problem. After pressing the rec. button, the MD recorder seeks the next "empty" spot on the medium (according to the TOC) and starts. The resulting track consists of two separate segments of audio material: five minutes at the end of the MD and two minutes out of the space where track #8 used to be. During playback of the 7 minute track, the read-ahead buffer keeps the audio material seamless while the mechanism jumps between the first chunk of the track and the second one... Now try that on any sequential medium (DAT, DCC, analog cassette)!

What happens if I bump my player when I'm recording?

Many users have reported problems if the MD recorder experiences shock and vibration while recording. Apparently the read-ahead buffer also works for recording, but strong disturbances could cause the laser to erase other spots on the MD, like the TOC or existing tracks, thus damaging previous recordings. To remain on the safe side, the MD recorder should not be subjected to shock or vibration while recording. MD drives certainly buffer during recording, otherwise MDs could not make recordings over discontiguous free blocks on the disc due to the dead time during interblock seeking. The bigger problem is, what happens when the recorder is jarred and the hot laser skids across already recorded material? One of the brochures for the professional MD units mentioned that they had a special circuit to cut the laser power when any shock occured, thereby avoiding overwriting [much] already written material. On the normal consumer portables, you could probably lose some material when a shock occurs during recording. In any case, they're able to recover and get back on track.

How flexible is the process where I put the titles of the tracks on the MD?

The user-interface for title entry on many portable MD recorders is limited, requiring a button push to cycle past every letter. Most current home decks have a remote with a non-querty character input method. Notable exceptions are the Aiwa portable that uses the volume thumbwheel on the remote to select each character, and some modern MD decks (chiefly Sony) that allow titling with a separately available keyboard-like remote or even a standard PS/2 keyboard in some cases.

Is there a limit to the length of the titles?

The titling capacity in the MiniDisc UTOC is as follows: There are 255 name blocks available, each one able to hold 7 characters, so the absolute maximum for all tracks combined is 255*7=1785 characters. The number of name blocks used by a track is: (title characters+6)/7 (rounding quotient down to nearest integer). The disc title is handled just like any other track title (the disc title is placed on track #0, which is actually the MD's free list). So, each track that has a title takes at least one of these blocks. This means that if you have 255 tracks, all must have titles of 7 characters or fewer, or some tracks must go without titles. It also means that MD does not have the capacity for a disc title plus 255 track titles, one track (or the disc) would have to have an empty title. In addition to these limits, certain units may not display or allow entry of long titles, for example Sony's MDX-U1 car player only displays the first 32 characters of a title.