Re: Core Audio Beginner Help
Re: Core Audio Beginner Help
- Subject: Re: Core Audio Beginner Help
- From: Phil Hystad <email@hidden>
- Date: Sat, 24 Dec 2011 12:17:52 -0800
Ariel,
Thanks for the pointers. I will take a look. Unfortunately, my experiments will need to wait 2 or 3 days.
phil
On Dec 24, 2011, at 11:23 AM, Ariel Elkin wrote:
> Start by taking a look at how to setup an Audio Session and the Remote IO Audio Unit:
> http://www.subfurther.com/blog/2010/04/30/what-you-missed-at-360idev/
>
> On those slides, take a look at Chris' example of a ring modulator to process microphone input at sample level.
>
> Unlike iOS4, iOS5 provides Audio Units that can perform DSP filters and effects, including high-pass, low-pass, reverbs, compressors, etc. Their details are here:
> http://developer.apple.com/library/ios/#documentation/AudioUnit/Reference/AudioUnitParametersReference/Reference/reference.html
>
> And as you said, the Core Audio documentation tells you everything about Audio Units, except how to get them to work. And understanding the related sample code is as beginner-friendly as solving the graph isomorphism problem in polynomial time.
>
> But there are indeed Core Audio programmers on this list, and even magnanimous ones too, who care to show how to use the force:
> http://lists.apple.com/archives/coreaudio-api/2011/Dec/msg00020.html
>
> On 23 Dec 2011, at 9:02pm, <email@hidden>
> wrote:
>
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>> Today's Topics:
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>> 1. Re: "early" anti-alias rolloff on the iPhone 4S ADC (Keith Lazuka)
>> 2. Core Audio Beginner Help (Phil Hystad)
>>
>>
>> ----------------------------------------------------------------------
>>
>> Message: 1
>> Date: Thu, 22 Dec 2011 13:26:45 -0800
>> From: Keith Lazuka <email@hidden>
>> Subject: Re: "early" anti-alias rolloff on the iPhone 4S ADC
>> To: Andrew Smith <email@hidden>
>> Cc: coreaudio <email@hidden>
>> Message-ID:
>> <email@hidden>
>> Content-Type: text/plain; charset=ISO-8859-1
>>
>> I tried that. Unless there's a problem with my methodology, the
>> anti-alias filter's transition band starts at 19khz regardless of
>> whether the iPhone is sampling at 44.1 or 48.
>>
>> -keith
>>
>> On Thu, Dec 22, 2011 at 12:52 PM, Andrew Smith
>> <email@hidden> wrote:
>>> Hi,
>>>
>>> Why not run the iPhone at 48k? Presumably that will push the filter knee
>>> upwards.
>>>
>>> Best regards,
>>> Andrew Smith
>>>
>>> Hello,
>>>
>>> Our application relies on a relatively flat frequency response around
>>> 20kHz. This is no problem on the iPhone 4 and the 3GS, but on the
>>> iPhone 4S we are seeing severe attenuation in this band. I did some
>>> measurements, and it appears that the transition band for the iPhone
>>> 4S ADC anti-alias filter is more gradual than the iPhone 4 but it starts
>>> significantly lower (about 2700Hz lower).
>>>
>>> This image shows the magnitude frequency response curves for both the
>>> iPhone 4 and the 4S given an input sine sweep from 0kHz to 24kHz over
>>> the course of 10 seconds:
>>> http://dl.dropbox.com/u/441716/iPhone_4s_early_rolloff/iphone_4_vs_4s_sine_sweep.png
>>>
>>> Test Setup:
>>> I generated a sine sweep from 0-24kHz using Baudline running on an
>>> iMac. Baudline was attached to the iPhone-under-test via an Edirol
>>> UA-25 USB audio capture box sampling at 48khz. I connected the output
>>> of the Edirol to the iPhone's linein using an iRig harness and then
>>> connected the iRig's/iPhone's lineout to the Edirol's linein, completing the
>>> loopback. The iPhone-under-test was sampling at 44.1kHz and was
>>> running a custom pass-through app using the RemoteIO audio unit with
>>> the PlayAndRecord audio session active and Measurement mode enabled.
>>>
>>> (While in this test I did a full duplex test, I have also done a
>>> separate test of
>>> just the iPhone 4S's ADC and saw similar results.)
>>>
>>> Now I understand that there are no guarantees regarding the iPhone's
>>> frequency response above 20kHz, but I was wondering if there was a way
>>> to configure CoreAudio such that the anti-alias filter transition band
>>> starts higher? I'm already setting the kAudioSessionMode_Measurement
>>> flag, which--according to the headers--should minimize the effect of
>>> system-supplied DSP. But are there any other flags that need to be set
>>> to get closer to the raw input from the ADC? ?Or am I out-of-luck?
>>>
>>> I also tried running the iPhone 4S at 48khz sample rate, but despite
>>> the increased distance from Nyquist, the anti-alias filter transition
>>> band starts at the same frequency as when I ran the iPhone 4S at 44.1khz.
>>> The only difference I saw with 48kHz is that the filter's transition
>>> band is about 2x wider, resulting in gentler rolloff, but still not
>>> gentle enough
>>> for our application.
>>>
>>> Any help would be much appreciated.
>>> Thank you,
>>> Keith Lazuka
>>>
>>>
>>>
>>
>>
>> ------------------------------
>>
>> Message: 2
>> Date: Fri, 23 Dec 2011 09:58:45 -0800
>> From: Phil Hystad <email@hidden>
>> Subject: Core Audio Beginner Help
>> To: email@hidden
>> Message-ID: <email@hidden>
>> Content-Type: text/plain; charset=us-ascii
>>
>> I have been reading the Core Audio documentation at the Apple Developer site and I am not sure I know any more today then when I started. Are there any Core Audio programmers on this list or is there a better spot.
>>
>> Basically, I am planning to experiment with various DSP filters on an input audio signal. So, take in the audio analog signal which then goes through ADC via the hardware then I want the stream of numbers. Of course, there are a lot of details on the way.
>>
>> What I need is help in pointing me in the right direction. That is, what are the important parts of Core Audio where I need to focus my attention.
>>
>> Thanks,
>> phil
>>
>>
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>> End of Coreaudio-api Digest, Vol 8, Issue 392
>> *********************************************
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