Contrary to popular belief of aptX sound quality, in some cases it can produce worse audio quality than SBC with a standard 328k bitrate.
SBC dynamically allocates quantization bits for frequency bands, acting on a «bottom-to-top» basis. If the whole bitrate was used for the lower and middle frequencies, the upper frequencies are «cut off» (silenced).
aptX quantizes frequency bands with the same number of bits constantly, which makes it a constant bitrate codec: 352 kbps for 44.1 kHz, 384 kbps for 48 kHz. It can't «transfer bits» to frequencies that need them most. Unlike SBC, aptX will not «cut» frequencies, but will add quantization noise to them, reducing the dynamic range of audio, and sometimes introducing crackles. SBC, on the contrary, «eats the details» — discards the quietest areas.
On average, compared to SBC 328k, aptX makes less distortion in music with a wide frequency range, but on music with a narrow frequency range and a wide dynamic range SBC 328k sometimes wins.
Let us consider a special case, a piano recording. Here's a spectrogram:
