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Thread: Transfer function measurement

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    Transfer function measurement

    Nick mentioned transfer functions in another post and I knew very little about it so I started trying to learn some more.

    I started talking to an engineer at Cross Spectrum labs and he told me that with RTA analysis from a USB microphone like the UMIK-1 you only get spectrum analysis which consequently has you miss out on measuring how the entire system is performing and how it may be coloring the original signal. For that, you need to measure the transfer function which compares the input signal to the measured response and plot that (called a transfer function). A useful video illustrating this concept by one of their engineers is at the following link:

    https://m.youtube.com/watch?v=uU8V5PkeoQg

    Through use of a DSP downstream from the source signal (which is being measured as well), you can aim for a transfer function that is linear/flat showing that you are indeed reproducing the original signal without coloration (or accurately); which apparently you aren’t doing (as well?) with a simple RTA analysis (at least that is my understanding).

    It made me wonder how many out there are measuring the transfer function when tuning and if people think it is useful. I have zero experience with this and am just learning but thought it was interesting.


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    Wave Shepherd - aka Jazzi Justin Zazzi's Avatar
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    Re: Transfer function measurement

    Quote Originally Posted by mauian View Post
    It made me wonder how many out there are measuring the transfer function when tuning and if people think it is useful. I have zero experience with this and am just learning but thought it was interesting.
    Wow, that's a fantastic video. Thank you for sharing!

    I have the luxury of using the same program that is in the video almost every day (SMAART). I find the transfer function measurement is incredibly useful for phase and time alignment between speakers and checking polarity too. I also love how SMAART can measure all of the things in real time so when I'm tuning a system I can work much, much faster.

    One of the things that makes SMAART (and other software like it) unique is it uses two channels for analysis (actually it can use as many channels as you want). This is sometimes called a dual-channel FFT. The transfer function needs both a reference channel and a measurement channel so you need at least two channels to work with which is why something like Room EQ Wizard does not do these things in real time.

    I think REW has the ability to do transfer functions by dividing one measurement from another. I'm not sure if it will extract the phase difference between measurements or not. It might be possible?

    I think REW is also doing a transfer function when you measure with the "sweep" mode instead of the RTA mode. REW knows what the reference signal is because it is creating it. REW also knows what the measurement looks like from the microphone. REW can then compare the two signals and create a transfer function which is what you get. It can find time delay and phase and group delay and so on. The bummer is REW does not do these things in real time. It does have a lot of neat features though and I freaking love that program too.

    Quote Originally Posted by mauian View Post
    Through use of a DSP downstream from the source signal (which is being measured as well), you can aim for a transfer function that is linear/flat showing that you are indeed reproducing the original signal without coloration (or accurately); which apparently you aren’t doing (as well?) with a simple RTA analysis (at least that is my understanding).
    Mostly, yes. Many of the same rules apply. A regular RTA cannot give you phase or timing information where a transfer function can, which is very powerful. The same concept of a "house curve" applies too since you can show that your system is reproducing the signal/music without coloration but a ruler-flat system is rarely the most enjoyable to listen to.
    Measure with mics, mark with chalk, cut with torch, grind to fit, sand to finish, paint to match.
    Updated Justin tuning sheet (Justin and Erica tuning companion for SMAART and REW)
    Do it for them.

  3. Back To Top    #3

    Re: Transfer function measurement

    Very cool. Thanks for your input on this, Justin. For someone to get up and running with this type of analysis in their car, is there a basic setup that you feel would be sufficient (hardware and software) for use with a laptop?

    Also, and videos or info on how to use a house curve when measuring the transfer function?


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    Wave Shepherd - aka Jazzi Justin Zazzi's Avatar
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    Re: Transfer function measurement

    The minimum amount of gear is getting really easy these days. A usb microphone costs very little money and the Room EQ Wizard software is free, or you can make a donation if you like.

    The calibration file for the microphone can make a difference. My first mic was one from Cross-Spectrum Labs and it was fantastic. I later replaced it with a laboratory-grade mic and the one from Cross-Spectrum (with the calibration file) matched very nicely. Also Herb is a great guy so I like to support his efforts. He has a USB mic from Mini DSP called the UMIK-1 I think.

    One step beyond the USB mic is a regular microphone with an XLR connector on it and an external sound interface, then add a loopback cable on channel2. Cross-Spectrum has the Dayton EMM-6 mic and you can pair it with one of many sound interfaces. I sometimes use the Focusrite Scarlett 2i2 and it's solid. Look at the gear recommendations on the right side of the REW homepage: https://www.roomeqwizard.com/

    There are two other threads on this forum right now about measurements. This one looks like it might be really interesting:
    https://www.caraudiojunkies.com/show...ds-quot-thread
    Measure with mics, mark with chalk, cut with torch, grind to fit, sand to finish, paint to match.
    Updated Justin tuning sheet (Justin and Erica tuning companion for SMAART and REW)
    Do it for them.

  5. Back To Top    #5

    Re: Transfer function measurement

    Thanks, Justin. Currently I use the umik-1 with REW and the helix dsp/RTA.

    I suppose my questions are:

    1) does measuring a transfer function in real time help achieve better results? (As opposed to RTA with the umik-1 that I am doing now)

    2) can u do real time transfer function measurement with the umik-1 and laptop or do you need something different (if so...what would be a good setup?) I’d like to try it out....

    I have fun doing and learning about this stuff (and if I’m being honest with myself I like buying new gear/toys) so excited to try it if it will help.

    Thanks again

    (Thanks for sharing that link on jtrosky and his tune log; I learned a lot from that thread and am currently doing a new tune as a result. Fun stuff!)


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  6. Back To Top    #6

    Re: Transfer function measurement

    Quote Originally Posted by mauian View Post
    Thanks, Justin. Currently I use the umik-1 with REW and the helix dsp/RTA.

    I suppose my questions are:

    1) does measuring a transfer function in real time help achieve better results? (As opposed to RTA with the umik-1 that I am doing now)

    2) can u do real time transfer function measurement with the umik-1 and laptop or do you need something different (if so...what would be a good setup?) I’d like to try it out....

    I have fun doing and learning about this stuff (and if I’m being honest with myself I like buying new gear/toys) so excited to try it if it will help.

    Thanks again

    (Thanks for sharing that link on jtrosky and his tune log; I learned a lot from that thread and am currently doing a new tune as a result. Fun stuff!)


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    EDIT: I see that’s a new post by jtrosky; I recognized his tune however from a previous similar post about his tume log/methods. ...my bad!


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  7. Back To Top    #7

    Re: Transfer function measurement

    People often poo poo on trying to do RTA on an iPad but for some reason it appeals to me.

    Something I was looking at was:

    This interface to use an XLR mic with the iPad:

    https://studiosixdigital.com/audio-h...ace-technical/

    The transfer function software add-on in the AudioTools app from studiosixdigital- $50

    With an XLR mic like the Dayton or others...

    (Not sure if anyone has experience with these)

    I also was considering the Scarlett Solo or 2i2. (Glad to hear you’ve used it and think it’s good)


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    Wave Shepherd - aka Jazzi Justin Zazzi's Avatar
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    Re: Transfer function measurement

    A real-time transfer function allows you the potential to work faster, but you'll also need lots of experience to make the best use of it.
    A "true" transfer function cannot be done with a USB microphone, it has to be done with analog equipment like an external sound interface such as the Scarlett. Room EQ Wizard can kinda do a transfer function with USB mics and it works pretty good too.

    Honestly, get comfortable with whatever tool is most appealing to you. Learn how an RTA works, get some experience playing with it and tuning things on your radio. Get some seat time and get a feeling for what influences what. Grab one of the many free RTA apps for your phone and sit in your car and figgle with the bass and treble knobs on your radio.

    It is fun to talk about the differences between this and that, and guide you towards building an ultimate measurement rig, but there are too many nuances that you won't understand yet. We can go deep into how a transfer function with real-time data capture can ... do whatever ... but you won't know how to make use of those more advanced tools until you get some familiarity with the fundamental tools.

    Oh! Go find the tuning guide that Andy Wehmeyer wrote at Audiofrog. It's really, really good for this. Read that whole thing a few times.

    Focus on the RTA for now using whatever microphone or app or software you understand the most. When you're ready for more try using Room EQ Wizard with an external sound card and build the loop-back cable so you can get a timing reference. Then experiment with how phase works, what group delay means, and so on.

    Read lots of tuning logs and ask specific questions about things you don't understand or want to learn. You can try my build log and my thread on the REW tuning companion spreadsheet if you like.
    https://www.diymobileaudio.com/threa...ective.135023/
    https://www.diymobileaudio.com/threa...wizard.206881/

    There are some good stickies on this forum and the other one. ErinH and SkizeR post quite a bit about measurements so seek out their posts too.

    After you're comfortable with all those things, then you can drop a thousand bucks on professional tools like SMAART and an array of microphones and I'll show you how deep the rabbit hole goes!
    Measure with mics, mark with chalk, cut with torch, grind to fit, sand to finish, paint to match.
    Updated Justin tuning sheet (Justin and Erica tuning companion for SMAART and REW)
    Do it for them.

  9. Back To Top    #9

    Re: Transfer function measurement

    Good advice for sure. Andy’s guide is fantastic and I’ve been learning all I can from your build log (and other logs) and anything Nick says on this forum and others. I definitely have learned a ton with lots of (mainly sweaty hours in the summer) tuning my car. I keep learning more with each attempt and with various posts from members. It’s definitely a rabbit hole...one I’m enjoying going down.

    Transfer functions might not be the next logical step in my learning. I feel like I’ve gotten pretty comfortable with REW and the helix RTA to match a house curve.

    What do you feel are the next steps to take that can be most helpful in taking a tune to the next level?

    Kyle Ragsdale posted some YouTube videos with band passed pink noise which I find has been very helpful in getting the imaging centered (and levels right) at all frequencies...an extra step after a tune matches a house curve.

    I feel like it’s those extra steps after an acoustic response is achieved via RTA that can really make a difference that I’m hoping to learn more about.

    Thanks!


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  10. Back To Top    #10

    Re: Transfer function measurement

    Oh, and thanks for the tuning companion spreadsheet. It’s awesome


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