Increasing the output voltage makes the sound louder, that is the purpose of a volume control, which basically implements a variable overall gain. In the case of the Xonar STX, a full scale analog output from the PCM1792 DAC always results in a 20 Vp-p output on the headphone jack (if it is enabled), but the digital signal that is converted by the DAC is attenuated by the gain and volume controls. Here is how it is implemented in the open source ALSA driver:
Code:
static void update_pcm1796_volume(struct oxygen *chip){ struct xonar_pcm179x *data = chip->model_data; unsigned int i; s8 gain_offset; gain_offset = data->hp_active ? data->hp_gain_offset : 0; for (i = 0; i < data->dacs; ++i) { pcm1796_write_cached(chip, i, 16, chip->dac_volume[i * 2] + gain_offset); pcm1796_write_cached(chip, i, 17, chip->dac_volume[i * 2 + 1] + gain_offset); }}
As it can be seen in the
datasheet, registers 16 and 17 are the digital attenuation control that allows for 0 to 120 dB attenuation in 0.5 dB steps. "hp_gain_offset" is 0 dB in the 600 Ω mode, -6 dB in the 300 Ω mode, and -18 dB in the 64 Ω mode.
