HDX with Pro Tools 11 is the best choice so far because it is the most powerful, accurate, and versatile system offering great software and hardware integration for tracking with AAX-DSP 64-bit plugins at an extremely low latency as well as extra processing power for mixing. HDX does not support old TDM plugins. Using HDX with Pro Tools HD 10 is still a very good option and offers a lot more plugin choices, but Pro Tools 11 will give you better sound quality as well as improved stability and processing power.
If you can find a bargain on the secondhand market, HD Accel is a potential alternative to HDX since it is a proven and reliable system; however, Avid will stop supporting it at the end of 2016. Like HDX, it will give the same processing advantages but with TDM plugins. AAX and AAX-DSP are not supported at all, and as we will discuss later, its audio resolution is not as well-developed as other solutions.
HD Native offers lower recording latency by offloading the routing processes to the PCIE card. This is beneficial for overall system performance while mixing or recording, but it will not allow any plugin processing to run on the chip, making this option a purely native platform. HD Native is therefore perfect if you would like to to perform mixing on your powerful desktop or laptop while being able to track at low latency.
If you already have an interface, you might want to keep it. A Pro Tools HD license will open with any interface. Pro Tools 10 with Complete Production Toolkit will give you the same benefits as any HD 10 user. Opting for Pro Tools 11 would also be very beneficial in this situation because of the overall performance and the new dual buffer option, which will still be able to track at acceptable latencies. Using a third-party interface will raise the overall latency slightly and Automatic Delay Compensation will not compensate for external inserts and external input monitoring automatically; we will later look at how to readjust the timing manually instead.
Recording in a Digital Audio Workstation (DAW) can be subject to latency as the audio buffer takes time to be computed inside the digital domain resulting in a constant trade-off between increased stability (higher H/W buffer sizes) and lower latency (lower H/W buffer sizes). A very fast computer or more optimized and integrated DSP are very well equipped to handle low latencies. As we begin using native plugins while recording, the stress on the CPU can become too much for low buffer sizes. Increasing the playback engine's buffer will add more audio and MIDI latency, so an HDX or HD Accel system can help in solving this problem by allowing you to use DSP plugins instead. For this purpose, you need to use AAX-DSP or TDM plugins. These plugins will have much lower latencies than their native versions but at a higher price tag. One more thing to keep in mind is the sound difference between RTAS and TDM since the internal mathematics are different. We will learn more about their limitations later on, but as a general rule, working with the same plugin types will reduce audio representation changes and system usage.
So is it worth investing in an HD system? There are two different schools in the digital audio industry between DSP-accelerated systems and Native ones. Digital audio has been here since the advent of the modern computer. At first, CPUs were too slow to allow fast enough processing at low latencies, so DSP assistance was the way to go. Nowadays, CPUs have become so fast that additional DSP assistance is not necessarily the best solution as it can also add latency and complexity to the system. In many situations and projects, if you have a fast computer, you can probably do without the extra hardware.
Note
HDX cards are not the only way to get extra processing power. Universal Audio also uses DSP cards to run their plugins, adding power but a significant increase in latency, branding them as mixing plugins. This illustrates that it takes time for the signal to leave the CPU, get processed by the card, and come back to the CPU. Their latest Apollo soundcard solves this problem, embedding low latency DSP on the soundcard, and allows plugin processing at the time of recording.
If you understand their architectures, accelerated solutions still have many advantages over native solutions, but as CPUs become faster, the gap becomes smaller each year. The main advantage of an HDX or HD Accel system is their ability to run many more plugins using AAX DSP or TDM plugins at lower latency and higher track counts than a native-only system. They also allow you to track at low latency all the time because their cards handle audio processing. Your computer's CPU will also have a lot more resources for audio processing, so you should achieve greater performances, higher plugin counts, and improved stability on smaller systems.