On Sun, 27 Nov 2005 13:01:22 +0000 (GMT), Jim Lesurf
wrote:

In article , Don Pearce
wrote:
On Sun, 27 Nov 2005 09:39:52 +0000 (GMT), Jim Lesurf
wrote:


I was surprised some time ago to be told that some DACs *don't* reclock
the input data stream. Indeed, there is a review of one in this month's
'Hi Fi World' where the reviewer claims the DAC he is reviewing makes a
'feature' of not doing so. That said, such reviews often contain
nonsense, so he may be wrong... :-)

Well, as you know, the data arrives serially, and completely out of
order,

I would not have said "out of order" as the sequence is defined and I
assume is followed. Hence the bits arrive "in sequence".


The sequence is defined, but it is also definitely out of order in
that it won't fill up a DAC from one end. The order is not LSB-MSB, or
even anything close.



so the correct bits must be gathered together and assembled into
a word before they can be presented to the DAC. That happens at a clock
edge, so I think that means the DAC must be reclocked to function.

This depends on the meaning of "reclocked".

For example, the SPDIF signal is equivalent to a bitstream XORd with a
frequency that defines the frequency and phase of the transmitter's bitrate
and sample rate clock.

You can recover this by manipulating and filtering the input stream, and
hence get a new 'clock' which is simply derived from the input stream.
Nominally, there would be no need to have anything like a PLL or LO in the
reciever. The output to the DAC could then be timed from this 'recovered
clock'.

I would not tend to call the above 'reclocked'.

The only way I would say the DAC was not reclocked was if it simply
used the next received data edge to trigger the DAC as it filled up. I
don't think any current machines do this.

However a system that has a LO and realigns the timing to this, perhaps
altering the LO using a PLL guided by the clock info in the input I would
called 'reclocked'.

I know that the Meridian DACs I prefer do reclock with a (dual) PLL system.
I don't know about other DACs, and have been told on occasion that some of
them don't reclock.

How would they work?

d

Pearce Consulting
http://www.pearce.uk.com