The term DC is borrowed from electrical engineering. https://en.wikipedia.org/wiki/DC_bias https://en.wikipedia.org/wiki/DC_bias explains: The term origina

The term DC is borrowed from electrical engineering. https://en.wikipedia.org/wiki/DC_bias https://en.wikipedia.org/wiki/DC_bias explains: The term originated in electronics, where it refers to a direct current voltage, but the concept has been extended to any representation of a waveform. The term's use is extended to two-dimensional transformations like the discrete cosine transform used in JPEG. TF is defined in the previous demo page, http://people.xiph.org/~xiphmont/demo/daala/demo3.shtml http://people.xiph.org/~xiphmont/demo/daala/demo3.shtml. It means Time/Frequency Resolution Switching, a idea from Xiph's Opus codec.