PLL sound

The clock of the DAC is regenerated out of the digital data stream of the CD-player. Regeneration is done by a Phase Locked Loop (PLL) in the input receiver. However in this DAC design an additional PLL is implemented between the input receiver and the DA converters. This PLL has a very slow tracking of frequency changes. This is acceptable because the data stream is generated with very stable X-tal oscillator inside the CD-player.

The PLL exists of a Voltage Controlled X-tal Oscillator (VCXO) which is compared with the regenerated clock from the input receiver. This 'error signal', filtered with a low pass filter with a very low bandwidth (far below the audio range), controls the VCXO. The unfiltered 'error signal' is also available as an output; we called this output PLL sound. With an ideal CD-player and ideal PLL this signal should be a constant voltage. However after measurement and listening (!) we know better. At the output a very small signal is present with noise and signal that is some how correlated with the analogue music signal.

By connecting an amplifier with high gain to this output, differences between different players could be detected. It was even possible to determine differences between the original CD and several CD-ROM copies (brand as well as writing speed). We found a correlation between the sound quality of the CD-player (used as data source only for the DAC) and the signal at the PLL sound output. This feature makes it possible for the DAC owner to listen to the 'quality' of the CD-player and possibly to improve it. In fact the PLL sound output is some kind of subjective clock spectrum analyser.

See Jitter measurements on transports and CD-Rs for some examples