Of course, frequency response is not the only measure of sound quality. Background noise is actually a much worse problem with standard analog audio equipment.... However, when the samples are in numerical form, only a finite number of digits is available to represent them, and this limitation does indeed introduce noise. Such roundoff error is termed quantization error because a sample pulse, which can be any amplitude whatsoever, has been quantized to the nearest available numerical representation. Background noise due to quantization error is termed quantization noise.
