An audio decoder for providing a decoded audio information on the basis of an encoded audio information, the audio decoder comprising: a window-based signal transformer configured to map a time-frequency representation of the audio information, which is described by the encoded audio information, to a time-domain representation of the audio information, wherein the window-based signal transformer is configured to select a window, out of a plurality of windows (310, 312, 314, 316, 318) comprising windows of different transition slopes (310a, 312a, 314a, 316a, 318a, 310b, 312b, 314b, 316b, 318b) and windows having associated therewith different transform lengths using a window information; wherein the audio decoder comprises a window selector configured to evaluate a variable-codeword-length window information in order to select a window for a processing of a given portion of the time-frequency representation associated with a given frame of the audio information; wherein the audio decoder comprises a bitstream parser configured to parse a bitstream representing the encoded audio information and to extract from the bitstream a one-bit window-slope-length information ("window_length") and to selectively extract, in dependence on a value of the one-bit window-slope-length information, a one-bit transform-length information ("transform_length"); and wherein the window selector is configured to selectively, in dependence on the window-slope-length information, use or neglect the transform-length information in order to select a window type (310, 312, 314, 316, 318) for a processing of a given portion of the time-frequency representation, wherein the transform length information determines a length of an MDCT kernel. a window-based signal transformer configured to map a time-frequency representation of the audio information, which is described by the encoded audio information, to a time-domain representation of the audio information, wherein the window-based signal transformer is configured to select a window, out of a plurality of windows (310, 312, 314, 316, 318) comprising windows of different transition slopes (310a, 312a, 314a, 316a, 318a, 310b, 312b, 314b, 316b, 318b) and windows having associated therewith different transform lengths using a window information; wherein the audio decoder comprises a window selector configured to evaluate a variable-codeword-length window information in order to select a window for a processing of a given portion of the time-frequency representation associated with a given frame of the audio information; wherein the audio decoder comprises a bitstream parser configured to parse a bitstream representing the encoded audio information and to extract from the bitstream a one-bit window-slope-length information ("window_length") and to selectively extract, in dependence on a value of the one-bit window-slope-length information, a one-bit transform-length information ("transform_length"); and wherein the window selector is configured to selectively, in dependence on the window-slope-length information, use or neglect the transform-length information in order to select a window type (310, 312, 314, 316, 318) for a processing of a given portion of the time-frequency representation, wherein the transform length information determines a length of an MDCT kernel.