[time-nuts] Solving the right problem

[David Forbes]

Greg Burnett wrote:
> The question about the relationship between SPDIF (and AES/EBU) clock jitter 
> and audio DAC fidelity is of interest to those of us (who can hear these 
> differences).  For example, might there be a second order effect (with a 
> high sensitivity coefficient) regarding the effect of timing jitter on the 
> overall process of ADC / DAC audio processing (as some, here, have 
> suggested)? ...Or other mathematical relationships yet to be defined or 
> understood?
> 

I first read about this problem in 1991 in an audiophile magazine, in 
which the discussion was of how to send the clock from the CD transport 
to the DAC with low jitter. Much money was spent to get a decent clock 
to arrive at the DAC chip.

I chuckled to myself, because the audiophiles were trying to solve a 
problem that didn't need to exist. All you have to do is to generate the 
clock next to the DAC, then send it backwards through the digital signal 
chain to the CD transport. Or use a FIFO as an elastic buffer, whose 
output clock is the DAC clock and whose input clock is the CD transport 
clock.

The clock doesn't need to be very accurate at the CD since its job there 
is to read the bits from the disc with a decent eye pattern, not to 
produce picosecond-accurate sample timing as is required at the DAC.

The same is true of SPDIF and AES digital streams - why not send the 
clock backwards or use an elastic buffer?