Transition to AAX: A Real Programmer's Perspective
(as posted to the DAW-MAC mailing list 23 October 2011)
There's a lot of almost-correct or sort-of-right information floating
around regarding AAX, RTAS, and TDM. I'd like to try to set the record
straight about a few things.
Unified AAX architecture:
The AAX spec strives to unify development so that the same algorithm can
run natively or with DSP hardware acceleration on the new "HDX"
TI-based floating point DSP cards, but there will remain a distinction
between AAX Native and AAX DSP, with some (mostly third party) plug-ins
only supporting one or the other. While Avid appears to intend to offer
all of their plugs in one version only that runs on both, it is quite
possible to create a plug-in that is either AAX Native only, or AAX DSP
Bit depth vs. internal processing:
There seems to be much confusion regarding how bit widths apply to
different aspects of plug-ins.
TDM passes audio around using a 24-bit fixed-point pipe. Internal
processing can be 24 bits, 48 bits, 56 bits or more. I lost track of what
the mixer uses but it is very, very large.
RTAS passes audio around using a 32-bit floating-point pipe. Internal
processing can be 32 bits, 64 bits, 80 bits, or more. I don't know what
the native mix engine uses but again it's very very large.
AAX also uses a 32-bit floating-point pipe to pass audio around, for both
Native and DSP. Internal processing can be 32 bits, 64 bits, 80 bits, or
Application word size:
When folks talk about an OS or APPLICATION being 64-bit, what that refers
to primarily is the maximum size of a memory pointer, which places limits
on the maximum amount of memory an application can use. It has nothing to
do with the size of the data streams, or how efficiently the CPU can handle
32 vs. 64 bit float operations. A 32-bit application is limited to
directly addressing 4Gb of memory.
Whether a plug-in (or host) is "64-bit ready" or "runs as
64-bit" has nothing to do with the word size of the audio pipes
between plug-ins, which may well remain 32-bit float. Even if the audio
pipes between plug-ins are 64-bit, the internal calculations may still use
32 bit operations since they are considerably faster than 64-bit operations
(regardless of whether the host app or plug-in is "32-bit" or
Why a whole new format?
When the TDM spec was written in the early 1990's, the state of the art was
68000-based Quadras running System 7 and hard drives in the tens of Mb. It
had provisions for black and white monitors. RTAS was added as a branch of
that spec, and Windows compatibility was grafted in. Quite honestly, it is
amazing how long that spec lasted -- through the transition from 68k to
PPC, NuBus to PCI, OS9 to OSX, PPC to Intel. But facing continuing
aggressive evolution of OSX and fundamental structural incompatibilities
with operation in a 64-bit host application environment (among other
concerns), it was time to rewrite the plug-in spec to meet the needs and
capabilities of today's and tomorrow's systems.
Some question why Avid didn't simply adopt AU or VST -- a few simple
reasons are these: AU is a mac-only spec, and VST simply doesn't have the
power and flexibility to do everything RTAS/TDM and AAX are capable of.
Believe me, I am less than thrilled that the 17 years of expertise,
techniques, tools, and libraries I built around the RTAS/TDM spec are now
as useful as COBOL. But AAX was a necessary move on Avid's part, and not
one they took lightly.
Personally, I think it is remarkable that Avid made PT10 a transitional
product - one that runs BOTH the old (RTAS/TDM) and new (AAX) formats (with
the one limitation that TDM and AAX DSP can't coexist simultaneously). It
would have been much easier to simply make a clean break with the old.
"It's just a scam to make more money":
Some people seem to think third parties are dancing with delight at the
opportunity to force people to repurchase their plugs in a new format --
nothing could be further from the truth. This is a HUGE pain in the ass
for third parties, and plug-in manufacturers are stuck between a rock and a
hard place: they have to spend considerable money and effort to port their
software to the new spec, because no one is going to continue to buy the
old format. But on day one, the market size for the new format is exactly
zero. And existing customers often expect to get the new version for free
or close to it. It's an unpopular, very costly exercise for third party
It's not unlike having an expectation that since you bought an 8-track of
"Led Zep II" for $5 back in 1969, that Atlantic Records would
have provided you over the years a new copy -- at no cost to you -- in LP
format, then cassette, then CD, then DVD-Audio, then mp3, then SACD, then
So take a deep breath. Remember that the TDM//RTAS system you have today
works just as well and is just as capable as it was last week, and no one
is going to force you to stop using it. PT10+ and AAX have some real
advantages moving forward, and when those advantages overcome the cost
disadvantages for you, bite the bullet and upgrade. Just like you did
going from 68k to PPC, or NuBus to PCI, or OS9 to OSX. And give your
plug-in companies the benefit of the doubt; this change is hard and
expensive for them too.
Cool Stuff Labs, Incorporated