Despite its attractive costs and cheap removeable media, Sony's MiniDisc system introduced in the early 1990s has had a bad rap in wildlife sound recording circles. This was a result of the compromises of the original system, and the difficulty of getting recordings off the recorder in a digital format. HiMD was a revolutionary step change - Sony revamped the format at the beginning of 2004, and incrementally loosened the transfer restrictions on the software over the next year. It is time to set aside any prejudices about the original format and take a fresh look. For a practical operational perspective, you can read Doug Ireland's review of a HiMD deck under fairly intensive field use.
New readout technology means the largest discs can now hold 1Gbyte of data, opening the way to an hour and a half of uncompressed 44.1kHz 16 bit stereo recording. Combined with low-noise microphone preamplifiers in many of the HiMD recorders, MiniDisc is worth a second look and has much to offer the newcomer and those with a constrained budget the possibility of genuinely excellent results in return for less than ideal ergonomics. Many recordists dismissed the format after seeing the problems of the original MD system launched in the early 1990s. It's worth taking a look at what those issues were, and how Sony have addressed them in HiMD. Three myths need to be debunked -
Although common now in ubiquitous MP3 recorders and digital note takers, MiniDisc was the first time digital audio compression had been applied to field recording. One of the greatest reservations holding the original MD format back from serious wildlife recording was this enforced use of lossy audio compression, a result of the small disc capacity of about 1/5th of a CD. This caused Cornell's Labs of Ornithology to issue an edict that there were serious reservations about using the format for wildlife sound recordings, based on a seminal paper "Techniques for Audio Recording Vocalizations of Tropical Birds" by Gregory Budney and Robert Grotke
Cornell MD doctrine 
"We tested a popular early-entry portable field machine that utilized ATRAC 2 and found it to be unacceptable for natural sound recordings especially recordings that might be subjected to scientific analysis."
The British Library had similar reservations 
"...However, the format uses a data reduction process to reduce digital storage space, which is designed to be transparent to human hearing. Whether this subtly damages recordings of wildlife is a controversial topic; however, you should be wary of using MD if your recordings might be used for scientific analysis later."
These reservations no longer apply to uncompressed recordings made with HiMD. On its own, however, this would not have elevated HiMD to the levels of a practical medium for the wildlife sound recordist
Two other quiet revolutions had to happen. The second was Sony rolling back the iniquitous copyright restrictions that hamstrung the original format, preventing recordists digitally transferring their MD recordings to computer. Sense finally prevailed, and though you still have to use Sony's proprietary SonicStage software, you can digitally transfer a bit for bit copy of your uncompressed HiMD recordings to your hard disk. Old recordings made on the previous generation of machines remain trapped on their media  from digital transfer by Sony's copyright shenanigans, but HiMD is refreshingly free of these political restrictions as of SonicStage 3.4
The last, evolutionary, change is that gone are the noisy microphone preamplifiers of the minidisc decks of yesteryear. Sony's first generation HiMD, line, as tested by Rob Danielson of the UW-Milwaukee Film department in comparison with a Sound Devices 722 showed that the unsung heroes of the HiMD revolution are the Sony design engineers who designed in low-noise preamplifiers into the first-generation HiMD line. This allows the machines to do creditable service even with low-noise microphones of the Sennheiser MKH-series or Rode NT1-A, when used with an appropriate outboard phantom power supply. From an engineering point of view, HiMD offers superior low-noise performance microphone inputs that some of the cheaper CF recorders on the market at the time of writing (2006/Q1) The gain profile of the recorders is also well suited to wildlife recording, compared with some devices aimed at musicians who work with much higher sound levels which need less amplification. For the Sony HiMD MZ-NH700 deck tested, the lowest input level capable of driving the MD to a fully modulated 0dBFS recording is -66dBu.
HiMD is a quiet revolution which offers real quality at an entry-level price for the wildlife sound recordist, who can concentrate his budget on good quality microphones to get started in the field.
Whilst the ultimate performance that can be achieved can hold its own with much more expensive gear, HiMD is not for everyone.
On the upside, however, HiMD offers advantages that low/midrange CF field recorders don't :
The conclusion is that MiniDisc can offer the cash-strapped recordist recording at 44.1k/16bits a better potential signal quality compared to the low-end solid state CF recorders targeted at the musician/garage band market where sound levels are much higher than nature recordings.
you can read more about the measurements of