Foti and I fully agree on the desirability of using standard
off-the-shelf 192 kHz AES3 hardware to transport FM composite stereo
(“Nothing Propriety Here,”
RWEE, Dec. 11, 2013).
main point in my original article
was that the standard as currently implemented is a good start, and
it can be extended to accommodate the full FM baseband with little
complication and full backward compatibility simply by using the
right channel of the AES link, which is not used in the current
“complex modulation” (i.e., using in-phase and quadrature
signals) is extremely well known in the art and is the mainstay of
today’s DSP-based radios. Orban claims no proprietary rights to the
most applications, it is clearest to think of our proposal as simply
interleaving the even and odd samples of a 384 kHz-sampled signal
between the left and right audio channels of the AES link. Unless
there is baseband energy above 96 kHz, either the left or right
channel can be used by itself to reconstruct the signal, which is
what makes the system backwards compatible.
only significant objection of which I’m aware, regards the fact
that the proposed system has a Nyquist frequency of 192 kHz and thus
offers more bandwidth than is required to pass a 99 kHz stereo
is fine if the audio processor and transmitter are co-located and
connected with an AES3 cable, and the proposal requires such simple
DSP (interleaving and then de-interleaving samples) that it could
hardly be called DSP at all. But I imagine that STL manufacturers who
adapt this technique would want to reduce the bandwidth so that it
could be transmitted more efficiently over radio links or the
I were proposing a technique that used off-the-shelf hardware but did
not require backward compatibility with the 192 kHz system already in
place, I would have proposed 128 kHz quadrature modulation on a
standard AES link, which would accommodate a 99 kHz baseband with a
comfortable guard band before the 128 kHz Nyquist frequency.
STL DSP requirements for our proposed technique are minimal.
example, an STL transmitter could reconstruct the 384 kHz signal by
simply taking alternating samples from the left and right channels at
its input. It could then synchronously sample-rate-convert the signal
by a factor of 2/3 to 256 kHz and place alternating samples on the
left and right channels to create a 128 kHz quadrature signal, which
would be transmitted over the link. The receiver could trivially
reconstitute the 256 kHz signal (by de-interleaving) and
synchronously upsample it by a ratio of 3/2 to restore the original
192 kHz quadrature-sampled signal after re-interleaving.
of the generous guard bands, the required anti-imaging filters would
be very short and cheap. Moreover, the design techniques for
synchronous sample rate conversion are easy and well known.
course, one could imagine other, more complicated schemes that could
save a bit more bandwidth at the expense of a larger DSP load.
the bandwidth efficiency of the STL would be entirely at the option
of the STL manufacturer as long as the input and output followed the
President and Chief Engineer