Re: "early" anti-alias rolloff on the iPhone 4S ADC
Re: "early" anti-alias rolloff on the iPhone 4S ADC
- Subject: Re: "early" anti-alias rolloff on the iPhone 4S ADC
- From: Sean Costello <email@hidden>
- Date: Wed, 21 Dec 2011 14:18:12 -0800
My guess is that the antialiasing in the 4S is probably inherent in the codec used for the AD convertor. Most modern AD/DA convertors have the antialiasing filters implemented in hardware. If this is the case, this will be impossible to bypass via software.
Sean Costello
On Dec 21, 2011, at 2:12 PM, Keith Lazuka wrote:
> 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
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