From Famitracker Wiki
DPCM (Delta PCM) is an analog-to-digital and digital-to-analog signal conversion technique usually used for transmission of voice information where quality is not of primary importance. Only an increase or decrease of the signal amplitude from the previous sample is sent whereas a no-change condition causes the modulated signal to remain at the same 0 or 1 state of the previous sample.
2A03 sound chip has single DPCM channel, which allows to play short digital samples with very low quality. DAC resolution of DPCM channel is 6 bit. The DPCM channel lack of volume control, and it's output level also affect to volume of the triangle channel.