X-Fi Processing Elements: Exploring the SRC

Sample rate conversion is one of the causes of poor audio reproduction in current sound cards. So, why make such a big deal out of sample rate conversion on the X-Fi? This time, Creative has implemented an SRC that generates very low noise and distortion, and the Audio Ring allows data that doesn't require sample rate conversion to bypass the step altogether. For the conversion of a 997Hz signal from 44.1kHz to 48kHz, the SRC demonstrated -136dB THD+N and +/- 0.00025dB pass-band ripple. Creative says that this is 300 times the quality of the SRC step in previous generation SoundBlaster products. These excellent results allow data to be passed multiple times through the SRC without any significant distortion of the data, making the SRC a key part in effects processing.

Pitch shifting can now easily be done on the SRC. Multitasking the sample rate conversion hardware for effects further allows creative to justify the efforts that they put into its development. One thing that Creative says can be dynamically applied to sound in realtime through the SRC is the Doppler effect. This could allow game designers, for instance, to apply a Doppler effect to all moving sound in a scene relatively easily. Sound synthesis can also use the SRC to change the pitch of a sound easily. The SRC also fits into 3D audio processing.

In pro audio applications, the SRC can be used to avoid the necessity of an external clock for syncing audio signals. Synchronized audio is still possible on X-Fi, but audio from multiple sources can be mixed with very good results using the SRC. As we have said before though, applications that do not require sample rate conversion will be able to bypass the SRC all together.

In order to create such a high quality SRC, Creative created a three-step hybrid filter. First, the sample rate is linearly increased by a factor of 2 using fairly simple math - no complex filters are needed for this step. The next stop along the way to sample rate conversion is a poly-phase FIR filter that up-samples its input to 2*(desired_sample_rate / original_sample_rate). This gives us data with a sample rate that is 4 times the desired sample rate. The final step is to reduce the sample rate by a factor of four (which is also an easy computation). Aside from increasing the quality of the sample rate conversion, increasing the sample rate to 4x the desired final rate also serves as an effective anti-imaging filter. The highest frequency that can be represented in sampled digital audio is half the sample rate (as per Nyquist). Imaging is a "ghost" of the audio signal in the range between the sample rate and half the sample rate. Assisting with anti-imaging is a by-product of the X-Fi SRC.

The above is an example of the SRC employed to convert a 44.1kHz signal to 48kHz. The X-Fi has 256 sample rate converters in its SRC engine. The order of the poly-phase FIR was not given, though Creative maintains that their entire system gives better results than a single order 64 poly-phase FIR filter. The SRC engine also has a tunable cache that can adapt to different latency requirements, as well as its own DMA engine.

The X-Fi Audio Ring: Powerful and Flexible X-Fi Processing Elements: The Quartet DSP
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  • DerekWilson - Wednesday, August 31, 2005 - link

    That's not a game port, its a digital connector to the break out box. The breakout box has optical in and out, spdif out, RCA out, quarter inch out, and problabaly a few things I'm forgetting right now.

    There is no daughter card connection. The thing that looks like it is a dell case front panel connector. For whatever reason.
  • Saist - Tuesday, August 30, 2005 - link

    I'm trying hard not to be the wet blanket, but why in the world is everyone so gung-ho about creative products? I know I'm not the only one whose suffered from scratchy sound, static generation, swapped speaker channels, and dropped speaker channels with Creative cards. I've lost count of the number of games I've played where a sound issue has been specifically traced to a Creative driver set or Creative hardware problem. I don't want to even think about leaving the "well supported" windows world and looking towards systems that use ALSA, ARTS, or OSS sound systems. The lack of documentation makes setting up and running creative cards a pain in the rear.

    Quite frankly, given what I've seen of Creative's products and experienced, I'll stick with my Via Envy and wait for Via Envy2 if I'm going to upgrade. Just doesn't seem to be any sense to stick to Creative's path which invariably either winds up with less than desirable products and higher than desirable prices.
  • flexy - Tuesday, August 30, 2005 - link


    i take 10 ! :)

    a) any REAL pro will probably NOT get a creative soundcard

    b) a gamer/casual PC enthusiast will probably NOT spend $400 on a soundcard.

    c) i had to stop reading the article and laughed when i read "$400"...and i laughed even more when i read that " Unfortunately, there are not a great many games out there that support X-Fi yet. On our list are Doom 3 and Battlefield 2. We tested both of these games and attained good results. We weren't able to create accurate and repeatable sound tests, but from our subjective analysis of gameplay, we couldn't really discern a quality difference between older hardware and the X-Fi."

    in other words: You did NOT see (hear) any difference between a Audigy 2 and this card - besides the sad fact that there's barely a game out which supports X-FI.

    What (please ?) is the point of this card/review ? Makeing us PC enhusiasts want to spend $400 on a soundcard which has no real-life use at all - or convince the *real* audio-professionals that now Creative is a contender in the "pro-market" ?
  • DerekWilson - Wednesday, August 31, 2005 - link

    Creative is a contender and active member in the pro market with their EMU line.

    I wouldn't use a consumer product for professional applications. I also woulnd't use a pro card for gaming.

    I don't recommend the Elite Pro at the $400 price point. But to people who want a gaming card with excellent sound quality and lots of recording features (and have money to burn), the Elite Pro fits their needs.

    to be clear, games that support EAX do support the X-Fi ... just not X-RAM -- the defining performance feature. It would be more accurate to say that no game exploits all the features of X-Fi.
  • JNo - Wednesday, August 31, 2005 - link

    Unfortunately Derek, you failed to make it VERY clear that the bottom of the range x-fi card (which most gamers would probably be happy with) is $130, which is very much more in people's spending range for an everyday soundcard. Obviously you didn't get to test it, having the elite pro instead, but because the basic model is very similar with only very slightly worse DACs (only audiophiles can tell), no extra RAM (no/v little impact today), no remote or break out box, some intelligent guesses could have been made as to its value.
  • AnnihilatorX - Tuesday, August 30, 2005 - link

    "The SoundBlaster X-Fi Elite Pro is the best non-pro sound solution for audio listening, features and recording"
    Ironic isn't it lol
  • PenGun - Tuesday, August 30, 2005 - link

    What's the deal with the 1 K spikes in Dynamic Range and THD? The Gina at least is smooth. The creative stuff is all over. You should be aware graphs are useful but are not a good indicator of how a card sounds.

    PenGun
  • SDA - Tuesday, August 30, 2005 - link

    "The SoundBlaster X-Fi Elite Pro is the best non-pro sound solution for audio listening, features and recording. "

    That's like saying that a Prescott is the best non-low-wattage, non-AMD solution for games. Yes, whoopee, but what if you don't have stupid limitations?

    To make myself clearer: a pro sound solution at the same price point or below would offer better sound quality, more features, and better recording capability.
  • Googer - Wednesday, August 31, 2005 - link

    quote:

    The SoundBlaster X-Fi Elite Pro is the best non-pro sound solution for audio listening,


    I Love the irony of this statement.
  • Eskimooo - Tuesday, August 30, 2005 - link

    Wherever I search web before spending any larger sum there are always people complaining about the stuff they have never seen an/or have no clue about.
    Why do you post opinion like that and give no example? What is the point?
    I mean I am reading reviews and I am trying to make a well informed decision about how to spend my money. If you draw a comparison, do it really, so that it is a valid point.
    So what is the soundcard that would give me better quality of sound at recording, playback and more features at the same time at the same price?
    I did not have opportunity to listen to music played back with system using X-Fi so I am looking for opinions from those who did before I eventually decide to order it online, too. Thanks to Derek this review. I have read about X-Fi enough to believe it is worth the price and that it will serve me well for a good few years like the Live card did.

    Enlighten me, cus I am looking better feature set

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