music. The best effect is obtained while listening to audio in the environment with constant noise (in trains, automobiles, planes), or in environments with fluctuating noise level (e.g. in a metro). Improvement of signal intelligibility in condition of ambient noise allows users to hear audio well and preserve hearing ability, in contrast to regular volume amplification. Natural mode Natural mode is characterised by subjective effect of balance of different frequency sounds, regardless of level of distortion, appearing in the reproduction device. It is also regardless of personal user's ability to perceive specific sound frequencies (excluding obvious hearing loss). The natural effect is obtained due to special sound processing algorithm (i.e. "formula of subjective equalisation of frequency-response function"). Its principle is to assess frequency response function (FRF) of mediaplayer or any other sound reproduction device, in accordance with audibility threshold in silence (subjective for each person), and to apply gain modifying factor. The factor is determined with the help of integrated function to test audibility threshold: the program generates tone signals (with divergent oscillations – from minimum volume 30–45 Hz to maximum volume appr. 16 kHz), and user assess their subjective audibility. The principle is similar to in situ audiometry, used in medicine to prescribe a hearing aid. However, the results of test may be used to a limited extent as far as FRF of sound devices depends on reproduction volume. It means correction coefficient should be determined several times – for various signal strengths, which is not a particular problem from a practical standpoint. Sound around mode Sound around mode allows for real time overlapping of music and the sounds surrounding the listener in their environment, which are captured by a microphone and mixed into the audio signal. As a result, the user may hear playing music and external sounds of the environment at the same time. This can increase user safety (especially in big cities and busy streets), as a user can hear a mugger following them or hear an oncoming car. Controversy Although these issues are not usually controversial within digital audio players, they are matters of continuing controversy and litigation, including but not limited to content distribution and protection, and digital rights management (DRM). Lawsuit with RIAA The Recording Industry Association of America (RIAA) filed a lawsuit in late 1998 against Diamond Multimedia for its Rio players, alleging that the device encouraged copying music illegally. But Diamond