Re: FFT demo and some questions

On Thu, Sep 10, 2015 at 3:28 PM, Josh Nielsen <josh@joshontheweb.com> wrote:

> I'd like to throw in my vote for configurable sample rates as well.  I'm
> recording audio from the microphone via webrtc and I have to resample the
> stream if it isn't at the desired sample rate (44100).  This causes major
> performance issues if the default sample rate is too high.  It is also very
> unintuitive to figure out how to adjust the sample rate at the OS level for
> most people.
>

The last proposal was for allowing the AudioContext to run at a selectable
rate.  The output of the graph would get resampled to match the actual HW
sampling rate.

Selecting inputs and outputs is still being worked on, I think.

>
> On Fri, Sep 11, 2015 at 10:09 AM, Raymond Toy <rtoy@google.com> wrote:
>
>> First, this is the wrong list. This list is for issues with the spec. You
>> should probably be posting to public-audio-dev.
>>
>>
>> However,...
>>
>> On Tue, Sep 8, 2015 at 1:21 PM, Stephen C Phillips <steve@scphillips.com>
>> wrote:
>>
>>> Hi,
>>>
>>> I've only just joined the mailing list, so please accept my apologies if
>>> I ask questions already resolved - I can't find answers from my searches.
>>>
>>>
>>> First, I'd love it if you took a look at an application I have just
>>> published on my Morse code website to listen to the microphone and attempt
>>> to decode any Morse code sounds that are audible. It uses the Web Audio API
>>> of course and the FFT function in particular. I haven't seen many (any?)
>>> demos that have a practical use of the FFT so I did wonder if you'd like to
>>> add it to the demos list?
>>>
>>> The page is here: http://morsecode.scphillips.com/labs/decoder/
>>>
>>
>> That's a really neat demo with a nice explanation of what's going on!
>>
>>
>>>
>>>
>>> Secondly, in writing the application I came across some limitations with
>>> the API and wondered if they had been discussed:
>>>
>>>
>>> 1. It would be useful to set the sample rate.
>>>
>>> Chrome has it set to 48 kHz which of course gives a maximum sampled
>>> frequency of 24 kHz. For the Morse code decoder, a max frequency of 4 kHz
>>> would have been fine. The reason it might matter is that my application
>>> uses a 512 point FFT which gives a time-step of 10ms (between samples) and
>>> a frequency resolution of 93 Hz. if it was sampled at 6 kHz (say) then you
>>> could do a 64 point FFT to give the same resolutions while still having a
>>> max frequency higher than required - saving some processing time.
>>>
>>
>> Selectable sample rates (on output) have been considered but no one has
>> actually done the work to spec it. I don't think we've considered being
>> able to select the sample rate on the input side.
>>
>> Also, many OSes allow you to set the audio sample rate. Perhaps you can
>> change that on your OS for the rate that you want.  WebAudio should be able
>> to use the new sample rate.
>>
>>>
>>>
>>> 2. The time resolution of the analysis is limited by the fact that the
>>> FFT input data samples cannot overlap.
>>>
>>> Or at least I assume this to be so. My application uses the AnalyzerNode
>>> with a 512 point buffer and a ScriptProcessorNode of the same size. When
>>> the ScriptProcessorNode is called (every 512 frames) it inspects the
>>> contents of the AnalyzerNode to get the frequency-domain data..
>>>
>>> What I'd like, is to be able to do a FFT using a moving window of input
>>> data (i.e. a FIFO queue of samples) in order to get a higher time
>>> resolution. Combined with a slower sampling period (see (1)) this might be
>>> feasible.
>>>
>>> I'm wondering whether it would be fast enough to use a
>>> ScriptProcessorNode to do the FFT itself in this way (i.e. implement the
>>> FFT in JavaScript) but have not tried yet.
>>>
>>
>> You don't get higher resolution by overlapping frames.  You just get a
>> smoother resulting FFT.  The only way to get higher resolution is to use a
>> larger FFT and to sample fast enough.
>>
>> You could certainly do the FFT in Javascript, but it probably won't be as
>> fast as the FFT built into WebAudio, but you'd have more control over
>> exactly how you want to do your FFT overlapping and smoothing.
>>
>>
>
>
> --
> Thanks,
> Josh Nielsen
> @joshontheweb <http://twitter.com/joshontheweb>
> joshontheweb.com
>