Network Audio Players/Servers

Along with other very progressive audio brands – Chord and dCS in particular – Mola-Mola has its own, custom DAC solution executed in DSP. There are no off-the-shelf DAC chips inside the Tambaqui... Mola-Mola's software upsamples all incoming data to 3.125MHz, truncating the wordlength to 5-bits while using a 7th-order noise-shaper to retain full dynamic range right up to 80kHz. Each 5-bit digital 'word' is sufficient to describe one of 32 possible pulse widths that, in turn, describe the amplitude of the audio signal on an (over)sample-by-sample basis. The pulses vary in steps of 10nsec (the system clock is 100MHz) right up to a full width of 320nsec (0.32µsec). This Pulse Width Modulated (PWM) signal is fed into a 32-stage shift register, clocked at 100MHz, so a composite of 32 pulses ends up reproducing the full PWM signal every 10nsec.

The 32 outputs of the register are summed together so that the final DAC output is the moving average of the PWM signal over consecutive blocks of 32 clock cycles (ie, one PWM cycle), updating every 10nsec. The PWM signal is 'conditioned' by a comb filter whose teeth coincide exactly with the 3.125MHz repetition rate. Mola-Mola could have used the signal from any of the 32 outputs alone and simply low-pass filtered it. Instead, the moving average technique not only overcomes any slight mis-match in the summing resistors but it also removes the PWM carrier that could potentially demodulate clock jitter down into the audio band. This is innovative stuff... PM