H04SSTEREOPHONIC SYSTEMS In this subclass, the following term is used with the meaning indicated:"stereophonic systems" covers quadraphonic or similar systems In this subclass, it is desirable to add the indexing codes of H04S2400/00 and H04S2420/00.In this subclass non-limiting references (in the sense of paragraph 39 of the Guide to the IPC) may still be displayed in the scheme. H04S1/00 H04S1/00Two-channel systems H04S5/00, H04S7/00 take precedence H04S1/002Non-adaptive circuits, e.g. manually adjustable or static, for enhancing the sound image or the spatial distribution control circuits for electronic adaptation of the sound field H04S7/30 H04S1/005For headphones H04S1/007in which the audio signals are in digital form data reduction aspects thereof based on psychoacoustics G10L19/02 H04S3/00Systems employing more than two channels, e.g. quadraphonic H04S5/00, H04S7/00 take precedence H04S3/002Non-adaptive circuits, e.g. manually adjustable or static, for enhancing the sound image or the spatial distribution control circuits for electronic adaptation of the sound field H04S7/30 H04S3/004For headphones H04S3/006in which a plurality of audio signals are transformed in a combination of audio signals and modulated signals, e.g. CD-4 systems H04S3/008in which the audio signals are in digital form, i.e. employing more than two discrete digital channels data reduction aspects thereof based on psychoacoustics G10L19/02 H04S3/02of the matrix type, i.e. in which input signals are combined algebraically, e.g. after having been phase shifted with respect to each other H04S5/00Pseudo-stereo systems, e.g. in which additional channel signals are derived from monophonic signals by means of phase shifting, time delay or reverberation  H04S5/005of the pseudo five- or more-channel type, e.g. virtual surround H04S5/02of the pseudo four-channel type, e.g. in which rear channel signals are derived from two-channel stereo signals H04S7/00Indicating arrangementsControl arrangements, e.g. balance control H04S7/30Control circuits for electronic adaptation of the sound field H04S7/301Automatic calibration of stereophonic sound system, e.g. with test microphone H04S7/302Electronic adaptation of stereophonic sound system to listener position or orientation H04S7/301 takes precedence H04S7/303Tracking of listener position or orientation H04S7/304For headphones H04S7/305Electronic adaptation of stereophonic audio signals to reverberation of the listening space H04S7/301 takes precedence H04S7/306For headphones H04S7/307Frequency adjustment, e.g. tone control H04S7/301 takes precedence H04S7/308Electronic adaptation dependent on speaker or headphone connection H04S7/40Visual indication of stereophonic sound image H04S2400/00 H04S2400/00Details of stereophonic systems covered by H04S but not provided for in its groups H04S2400/01Multi-channel, i.e. more than two input channels, sound reproduction with two speakers wherein the multi-channel information is substantially preserved H04S2400/03Aspects of down-mixing multi-channel audio to configurations with lower numbers of playback channels, e.g. 7.1 -> 5.1 H04S2400/01 takes precedence H04S2400/05Generation or adaptation of centre channel in multi-channel audio systems H04S2400/07Generation or adaptation of the Low Frequency Effect [LFE] channel, e.g. distribution or signal processing H04S2400/09Electronic reduction of distortion of stereophonic sound systems H04S2400/11Positioning of individual sound objects, e.g. moving airplane, within a sound field H04S2420/13 takes precedence H04S2400/13Aspects of volume control, not necessarily automatic, in stereophonic sound systems H04S2400/15Aspects of sound capture and related signal processing for recording or reproduction H04S2420/00Techniques used stereophonic systems covered by H04S but not provided for in its groups H04S2420/01Enhancing the perception of the sound image or of the spatial distribution using head related transfer functions [HRTF's] or equivalents thereof, e.g. interaural time difference [ITD] or interaural level difference [ILD] H04S2420/03Application of parametric coding in stereophonic audio systems H04S2420/05Application of the precedence or Haas effect, i.e. the effect of first wavefront, in order to improve sound-source localisation H04S2420/07Synergistic effects of band splitting and sub-band processing H04S2420/11Application of ambisonics in stereophonic audio systems H04S2420/13Application of wave-field synthesis in stereophonic audio systems