Multimedia - Published on Wednesday, 26 October 2005 18:15 Written by tokig
SRC: SRC stands for Sample Rate Converter, and just as the name would suggest it is the part of the card that converts sample frequencies. The reason why this conversion is needed is that Creative's DSP and several of the purpose-specific components only work with sample rates of 48, 96 or 192 kHz.
Earlier Creative cards have employed quite crude methods of sample rate conversion. The result was fairly high levels of distortion and bad frequency response when the SRC worked with sample rates that were not factors or multiples of 48. This has been one of the major complaints regarding Creative's sound cards from audiophiles and home theater enthusiasts, and as such it is understandable that Creative has worked hard to solve its problems.
The SRC handles all sources (regardless of whether they are game sounds, analog input sources or MP3s) that do not have a sample rate of 48, 96 or 192 kHz. The sources are converted into 48 or 96 kHz (depending on the current "mode" and task at hand) in order for the DSP to be able to work with them. Badly executed sample rate conversion creates audible distortion of the sound and that is why Creative has put great effort into making the SRC as transparent as possible. To realize this, they have implemented a thing called X-Fi Hybrid SRC. The process consists of three steps, which you can see below:
Step one: the incoming sample rate is multiplied by two. (44.1 kHz in our example becomes 88.2 kHz, 44.1 * 2 = 88.2).
Step two: the doubled sample rate is multiplied by the target divided by the source multiplied by two to increase the sample rate to 192 kHz (88.2 * (2* (48 / 44.1)) = 192).
Step three: our sample rate of 192 kHz is divided by four to achieve a "DPS-friendly" sample rate of 48 kHz.
As you can probably see, step two is the complicated part of this puzzle. With a poly-phase FIR (Finite Impulse Response) filter, the most demanding calculation in the SRC process is made to achieve a sample rate of 192 kHz. The first step is however not without importance, the relatively simple process as well as step two helps to counteract what is known as imaging.
Creative has chosen not to give exact numbers on the amount of "taps" or what "order" the FIR filter consists of, but insists in its press documents that it is "equivalent" to an FIR filter with more than 100 taps. As a reference, a typical poly-phase FIR filter in a modern, relatively high quality SRC, has 64 taps. According to Creative's documentation, sound converted from 44.1 to 48 kHz has a frequency response of ± 0.0002d dB and THD+Nof -135 after one "pass" through the SRC. In practice this means that our DAPs, even the most extreme models, are quite a bit below this capacity, which means that the difference is pretty much inaudible.
Speaking of the amount of "passes", this is another interesting feature in Creative's SRC: if you want to use the SRC for something other than sample rate conversion, that is possible. For example, it can simulate a Doppler effect (when an object moves towards or away from the listener), pitch control, etc. To increase flexibility, for example to pitch several octaves at once, sound can be looped through the SRC a number of times. Another reason to convert sample rates is to smooth differences or "unevenness" in incoming sample rates.
With its approximate 7 million instructions per second, X-Fi's SRC has in principle as much calculation power as an older Pentium 4 in itself. In short, Creative has not only dealt with its problem with 44.1 kHz, it has also created new possibilities for pitch shifting and sample rate synchronization when the SRC got its overhaul.
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