Audio Equipment

MiniDisc (MD)

Over the past couple of years we have received many inquiries regarding the use of MiniDisc (MD) recorders for fieldwork. Standard MD recorders utilize an audio compression algorithm known as ATRAC (Adaptive TRansform Acoustic Coding) that is based on psychoacoustic principles. The input signal is divided into sub-bands that are then transformed into the frequency domain using a variable block length. Transform coefficients are grouped into non-uniform bands to reflect the human auditory system, and then quantized on the basis of dynamic sensitivity and masking characteristics. ATRAC compresses compact disc audio (16-bit 44.1 kHz stereo) to approximately 1/5 of the original data rate with minimal loss in sound quality. This technology permits 74+ minutes of stereo sound to be recorded on a small 64mm optical disc.

Audio compression distorts sound, but in such a way that humans normally can't detect. The compression method used in MD recorders has frequency and timing constants built-in that are intended to make the compression inaudible to human ears, but which may make it audible to other species. For instance, humans can't hear a sound at a certain frequency if there is another, louder sound at a nearby frequency, so the compression algorithm will leave out the former sound. We can't hear the difference, but it's possible other species might and any scientific analysis of the compressed sound would have to work with a distorted version. Additionally, compression algorithms typically can't track very fast changes in overall loudness correctly, since humans can't hear such changes very well. Other species (e.g. many songbirds) do have significant variation in amplitude over very short time periods in their vocalizations. If you try to record such species with a MD, you'll likely introduce some distortion.

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.

Newer machines that utilize ATRAC 3.5 & 4.5 versions are considerably better. When recordings made on these machines are compared to similar recordings created by high-quality analog cassette recorders the results typically favor the MD version. The total lack of tape hiss, wider frequency bandwidth of the MD system, and lower overall distortion levels all contribute to favoring the MD. Low-level high-frequency information that the MD ATRAC system might have masked would have been buried in tape hiss on the analog cassette version so frequency masking is not an issue when these two formats are compared.

Two popular current generation MD recorders are the HHB PORTADISC MDP500 ($1,399), and the Sony MZ-N10 ($345).

The HHB unit is a professional, rugged, field device that utilizes ATRAC version 4.5. It features XLR style microphone input connectors, 48-volt phantom power, low frequency filters, 6-second pre-record buffer, and a 40-second memory buffer to produce glitch-free recordings, large backlit metering display, and a USB port for up/downloading audio data files directly to both Macintosh and Windows computers. Both optical and coaxial digital inputs and outputs are provided as well as analog line outputs. Field powering is accomplished via rechargeable NiMH, standard AA alkaline batteries, or via 12-15-volts DC external power.

The Sony unit is a small compact Walkman style device that utilizes ATRAC DSP-Type-S. It features a small, but useful backlit display and manual record level control. Skip-free "G" protection is provided as well as date/time stamping. Microphone inputs and headphone connectors are 3.5mm stereo mini- jacks. Field powering is accomplished via standard alkaline AA style or re-chargeable batteries.