Is the incoming sample in a standard DPCM already quantized? I began learning about DPCM and i've got one issue I am not sure about:

Is the incoming sample m[k] (see image for reference) already quantizied or not? I think it is but with an (assumed) very fine quanitization. So the Quantiser block is doing a requantization with a more coarse quantization, right?
No it is assumed that m[k] is not quantized, as the main purpose of DPCM is to quantize the difference between samples rather than quantize the whole samples.

ofcourse if m[k] is an image it must been quantized as the video sensor in any digital camera is a digital sensor by definition so it is doing some means of quantization.