Re: Extracting Amplitude Data from Linear PCM on the iPhone
Re: Extracting Amplitude Data from Linear PCM on the iPhone
- Subject: Re: Extracting Amplitude Data from Linear PCM on the iPhone
- From: tahome izwah <email@hidden>
- Date: Thu, 30 Sep 2010 21:07:48 +0200
2010/9/30 David Weiss <email@hidden>:
>
> My questions are:
>
> 0. Linear PCM stores amplitude samples as 16-bit values. Is this correct?
This is one possible format, yes.
> 1. How is amplitude stored in packets returned by AudioFileReadPacketData()?
That depends. See the description under AudioStreamBasicDescription
and AudioStreamPacketDescription for supported word lengths.
> When recording mono linear PCM, isn't each sample, (in one frame, in one packet) just an array for SInt16? What is the byte order (big endian vs. little endian)?
No. A single mono sample frame is a single value. Again, the actual
format (word length, byte ordering) can be determined from the
corresponding ASBD that provides the context.
> 2. What does each step in linear PCM amplitude mean physically?
A 16 bit signal has a dynamic range of about 96 dB. The maximum
absolute value of a PCM signal is considered to be 0 dB digital full
scale, while a value of 0 corresponds to -∞ dB DFS
> 3. When linear PCM is recorded on the iPhone, is the center point 0 (SInt16) or 32768 (UInt16)?
For a signed 16bit value the center point is 0 (although this has been
debated in the context of the asymmetry of the format, which actually
covers a range from -32768 to 32767). I've yet to hear someone with
authority answer this question.
> What do the max min values mean in the physical wave form/air pressure?
They describe the maximum positive and negative air pressure that can
be measured using that representation and its corresponding digital
values.
> and a bonus question:
>
> 4. Are there sound/air pressure wave forms that the iPhone mic can't measure?
Well, I'd say the usual restrictions of band limited sampling and
discrete resolution apply (frequencies above Nyquist can't be
measured, neither can sound waves below the sensitivity, noise floor
and dynamic range of the device).
HTH
--th
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