AAX DSP/Native vs. TDM/RTAS
(as posted to the DAW-MAC mailing list 24 October 2011)
Posted by: Nathaniel Reichman on Sun Oct 23, 2011 6:51 am (PDT)
I was at AES on Friday, and despite some controversy over high upgrade
prices, and developers dragging their feet, it was completely evident that
Avid has put a tremendous effort into Pro Tools 10. I used systems there,
and they were snappy and powerful. The plug-in menus on the demo rig
showed that there are separate "DSP" and "Native"
menus, similar to RTAS and TDM now. However, Avid product specialists
where quick to point out that the underlying sonic code is identical, so
hopefully developers won't have a hard time making both versions of the new
AAX plug-ins (would love to hear from Frank Filipanitis here).
Posted by: Ben Cox on Sun Oct 23, 2011 6:56 am (PDT)
What I want to know is, does it also make it so that developers won't have
a hard time making plugins that only work in Native or DSP modes, and thus
charging separately for them? I'd love it if _all_ AAX plugins would run
in Native or DSP, and it was not possible for a plugin vendor to charge
separately for DSP vs. Native.
RTAS and TDM require two completely separate implementations due to the
fundamental differences in architecture; RTAS is written in C or C++ in a
floating-point environment, TDM is written in 56k assembly language in a
fixed-point environment. While the GUI and "glue" code could be
shared between the two, the actual processing code had to be written
completely separately for TDM and RTAS. This made ensuring bit-for-bit
matching and synchronization of changes difficult.
HDX changes the DSP hardware to a floating-point architecture and AAX
enables the possibility of writing a single implementation of the algorithm
in C that compiles to both the native and hardware-accelerated versions.
Note I said "enables the possibility". The realities of writing
an algorithm for the constrained architecture and feature set of the HDX
hardware and TI DSP makes it significantly harder than simply writing a
plug-in to run on the host. There are many differences to factor, ranging
from the straightforward (buffer size) to the complex (subtle differences
in mathematical operations btw Intel and TI). And while simple algorithms
can indeed run identical code on Native and DSP, many will still require
some conditional code that is different on Native vs. DSP.
In general however, it should be the case that if the algorithm runs as AAX
DSP it can run as AAX Native with little additional effort (though not
necessarily optimized for speed). The converse, however, is not true...
while it is MUCH more straightforward to adapt an AAX Native algorithm to
AAX DSP than it was to adapt RTAS to TDM, there is still a considerable
amount of effort and additional testing involved. So I expect we may see
some companies elect to do plugs that run as AAX Native but not AAX DSP, at
I hope that we won't see stratification in the market, with different price
points for AAX Native and AAX DSP+Native; I think it is much simpler to
have a single SKU and pricepoint for both. But the fact remains that
making a plug run on the DSP requires additional cost, effort, and support
-- so it either gets baked into the price of a unified (DSP+Native) plug,
or companies can attempt to recoup that additional cost only from those
customers using the DSP side, while keeping the Native side a little
I should also mention that attaining bit-for-bit matching between Native
and DSP may no longer be practical for many algorithms.... expect to see
manufacturers changing that bullet point to "matches to -96 dB"
or "difference below -110dB" or something to that effect.
One last note regarding "developers dragging their feet"... the
AAX spec was rolled out on a VERY aggressive schedule. Some vendors were
able to refactor their plugs for AAX quickly, others were not. Avid
provided a tremendous amount of support and encouragement to get as many
third-parties ready for the roll-out as possible, but the amount of effort
required varies tremendously from plug-in to plug-in. AAX also obsoleted a
graphics framework used by some plug-ins, so vendors whose products were
based on that not only have to refactor the processing code for HDX but
also completely redo the UI code. Some companies are also better
positioned to endure the painful parts of being on the bleeding edge, while
others need to wait for things to settle out and stabilize before
Again, while it is going to be a painful transition for many people --
users and developers alike -- I do believe that this is a good and
necessary move for the future.
Cool Stuff Labs, Incorporated