Mastering Quality Sound is a term used by the South Korean company Astell&Kern to describe 24-bit high-resolution audio files that use the FLAC format (a lossless compression format). Astell&Kern sell a range of high-end audio players that support a range of audio file types, including high-resolution FLAC files, and the players themselves incorporate microSD card slots.
Music files can be transferred from a computer to the player using microSD or microSDHC memory cards of up to 32 GB, or connecting via cable, but a small number of pre-recorded albums have been released on microSD cards marketed under the name MQS.
MQS microSD cards contains high-resolution audio files in FLAC format and these can be played back on many devices in addition to Astell&Kern’s players, although additional plugins or apps may sometimes be required on other devices. Astell&Kern products support 24-bit high-resolution audio with a sampling rate of up to 192 kHz (though the music on MQS microSD cards varies from 44.1 kHz to 192 kHz depending on the source) and claim that this gives up to 6.5 times the level of detail of a conventional CD-quality audio file.
Pre-recorded music has already been released on microSD cards, both under SanDisk brands such as Gruvi and slotMusic, and also on generic microSD cards. These formats used MP3 files rather than the higher quality FLAC though.