10
Special Design Features
Congratulations on your purchase of the Nº36 Digital Audio Processor. The Madri-
gal design team is confident you will enjoy the outstanding performance of the
Nº36 for many years. In case you are interested in technical details, what follows
is a brief outline of some of the key technologies in your new processor.
An “intelligent” FIFO
Unlike previous processors which were highly dependent on the quality of the
digital signal they were fed, the Nº36 Digital Audio Processor delivers outstanding
performance with even less-than-ideal digital signals.
“FIFO” stands for “First In, First Out.” It describes a simple buffer in which the
digital information is stored temporarily on its way to being converted to analog.
Just as a large water tower can provide a steady source of water to a small town,
despite hour-to-hour variations in the supply of water from the well, a FIFO can
provide a steady, consistent source of digital data to the converters which are re-
sponsible for changing that data into music. Even if there is significant “jitter” (in-
consistencies in timing) in the incoming digital information, the output of the
FIFO is controlled by a special clock with tremendous accuracy. The result largely
eliminates the jitter and allows the musical information to be reproduced cleanly,
without jitter-induced distortions.
The trouble with most FIFOs lies in their behavior when the incoming signal is
poor enough to cause the “water tank” to overflow or to be emptied. Normally, a
FIFO would then have to “invent” false data to fill the gap, throw away excess
data, or revert to non-FIFO operation. None of these approaches is acceptable, as
they all represent serious performance compromises.
Of course, one could simply use an
extremely
large buffer. Unfortunately, this so-
lution is a poor one.
A larger buffer implies a longer delay between when information goes in and
when it starts coming back out. With laserdiscs, for example, you must keep the
in/out delay small so as to keep the soundtrack synchronized with the picture on
the screen. An oversized buffer would make every movie’s audio out of step with
its video, an unacceptable situation. Of course, one could bypass the FIFO for
movies, at the cost of losing all of its distortion-reducing benefits.
Madrigal engineers have developed a proprietary buffer management scheme
which reduces reproduced jitter to less than 20 picoseconds while maintaining
the synchronization of sound and picture in movies. It employs a buffer large
enough to absorb the jitter found in transports of reasonable quality, yet small
enough to have imperceptible delay. The rate at which data is released from the
FIFO buffer is controlled by software to track the
long-term
data rate of the in-
coming signal, allowing the buffer to absorb all the short-term variations which
cause sonic degradation. This approach yields a “smart” FIFO buffering scheme
which rejects virtually all incoming jitter without requiring an enormous buffer
and suffering the consequent audible delay. It also avoids the sonic penalties asso-
ciated with the various strategies used when a buffer overflows or empties.
The “smart” FIFO operates at both 44.1 kHz and 48 kHz sampling rates. The Nº36
reverts to non-FIFO (recovered clock) operation for 32 kHz sampling rates (a
proposed but rarely used standard for digital satellite transmission). It also reverts
