Re: convert kAudioSessionProperty_CurrentHardwareOutputVolume to dB value
Re: convert kAudioSessionProperty_CurrentHardwareOutputVolume to dB value
- Subject: Re: convert kAudioSessionProperty_CurrentHardwareOutputVolume to dB value
- From: Kevin Dixon <email@hidden>
- Date: Tue, 17 Jul 2012 14:40:06 -0700
Sorry -- more accurately, I was measuring *amplitude* of a 1kHz 0dB
sine wave (integer PCM, peak values at +/-32767).
When CurrentHardwareOutputVolume was reporting 1.0, I measured a
steady signal with peak amplitude of -6dB on the headphone output of
the device. I used this as a baseline.
Using MPVolumeView, I adjusted CurrentHardwareOutputVolume to where it
reported 0.711712 (if this were linear gain, this should indicate
approximately -3dB attenuation). I then measured a steady signal,
which the peak amplitude had decreased to -22dB.
The difference between my measured amplitude at
CurrentHardwareOutputVolume = 1.0 and CurrentHardwareOutputVolume =
0.711712 was 16dB.
Most of this is irrelevant to the question at hand. How do I translate
the value retrieved from CurrentHardwareOutputVolume to the actual
attenuation the system is applying to the output of my RemoteIO unit?
-Kevin
On Tue, Jul 17, 2012 at 10:53 AM, Brian Willoughby <email@hidden> wrote:
>
> On Jul 16, 2012, at 17:08, Kevin Dixon wrote:
>>
>> So it seems that kAudioSessionProperty_CurrentHardwareOutputVolume
>> returns arbitrary volume units instead of linear gain.
>> For example when i get the value 0.711712, I convert to dB with
>>
>> 20.0f * log10(volume)
>>
>> returning ~ -2.95 dB.
>>
>> However when I measure a 0dB FS tone from my headphone output, with
>> the volume at 0.711712, the observed power is more in the neighborhood
>> of -16dB
>
>
> -16 dB what?
>
> Decibels are relative, therefore, citing a -16 dB measurement is incredibly
> vague when there is no reference value. Is that -16 dBm? -16dBV? -16 dBmV?
>
> With the volume at 1.000000 (0 dB), what power do you expect to measure?
>
> Also, don't forget that there are potentially other volume settings in the
> path, and they must all be 0 dB if you want to measure one of them
> accurately.
>
> Brian Willoughby
> Sound Consulting
>
>
>
>> Can anyone clarify this situation? It is imperative that I know the
>> actual attenuation so my algorithm can react correctly.
>
>
>
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