One weakness MiniDisc has when compared with DAT is that perfect copies cannot be made (without professional equipment) due to the asymmetric nature of the MiniDisc audio compression system. Every time the signal goes through the compression/decompression process, it changes slightly. After several generations these effects become noticable.

One could argue that whether or not there is loss from generation to generation is a moot point since SCMS (Serial Copy Management System -- present on all consumer digital audio equipment) allows only a single generation of digital copying.

However professional machines exist which do not obey SCMS, and so the question remains, how bad are the effects of this generational loss? In order to discover this, I conducted an experiment which produced samples of audio copied up to 100 times.

I generated these samples as follows:

1. Digitally copied from CD (Panasonic MASH SL-S490 portable player) to MiniDisc (Sony MDS-503 MD Deck), a short audio sample (Donald Fagen: Kamakiriad, Trans-island Skyway (track 1), 1:34-1:54)

2. Digitally copied this segment back and forth from the MDS-503 to a Sparcstation 2 via a Townshend Computer Tools "DATlink" (workstation SCSI digital audio interface) 100 times (over the optical connector), saving each intermediate copy.

A selection of the resulting generational copies are given below. (You may need to equip your Unix, Macintosh, or Windows system with an MPEG audio player to hear these.)

• 1st generation MD copy
• 2nd generation MD copy
• 3rd generation MD copy
• 4th generation MD copy
• 5th generation MD copy
• 6th generation MD copy
• 12th generation MD copy
• 25th generation MD copy
• 50th generation MD copy
• 100th generation MD copy

I have chosen not to do subjective evaluations of these samples for the following reasons:

• It is a very serious undertaking. I have talked to several researchers here at ATR about the science of subjective testing, and if one wishes to perform a statistically valid test, it requires a good deal of work.

• The effects of compression vary greatly depending upon the source material. Tests conducted on one musical sample will not hold for others. Selecting the samples themselves is a difficult task.

• The few subjects I have tested have slowly become attuned to compression artifacts, causing a sample that was originally judged to be transparent to show artifacts on subsequent presentations. For me this calls into doubt the entire premise of a subjective evaluation of MiniDisc generational loss, and prompts the question of what, precisely, I would like it to prove.

Eric Woudenberg, April 1996

Addendum

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