[audio iphone maths] how to calculate the number of Packets for X seconds?

Question

Hi guys,

i'm tring to understand the Audio * things for iPhone

currently i'm reading: core audio overview

here i got a question:

from apple example codes:

- (void) calculateSizesFor: (Float64) seconds {

    UInt32 maxPacketSize;
    UInt32 propertySize = sizeof (maxPacketSize);

    AudioFileGetProperty (
        audioFileID,
        kAudioFilePropertyPacketSizeUpperBound,
        &propertySize,
        &maxPacketSize
    );

    static const int maxBufferSize = 0x10000;   // limit maximum size to 64K
    static const int minBufferSize = 0x4000;    // limit minimum size to 16K

    if (audioFormat.mFramesPerPacket) {
        Float64 numPacketsForTime =
            audioFormat.mSampleRate / audioFormat.mFramesPerPacket * seconds;
        [self setBufferByteSize: numPacketsForTime * maxPacketSize];
    } else {
        // if frames per packet is zero, then the codec doesn't know the
        // relationship between packets and time. Return a default buffer size
        [self setBufferByteSize:
            maxBufferSize > maxPacketSize ? maxBufferSize : maxPacketSize];
    }

    // clamp buffer size to our specified range
    if (bufferByteSize > maxBufferSize && bufferByteSize > maxPacketSize) {
        [self setBufferByteSize: maxBufferSize];
    } else {
        if (bufferByteSize < minBufferSize) {
            [self setBufferByteSize: minBufferSize];
        }
    }

    [self setNumPacketsToRead: self.bufferByteSize / maxPacketSize];
}

i understood almost everything, but i just did not get WHY this:
Float64 numPacketsForTime = audioFormat.mSampleRate / audioFormat.mFramesPerPacket * seconds;

i thought something like this would work: numPacketsForTime = seconds * packetsPerSecond

so, packetsPerSecond = audioFormat.mSampleRate / audioFormat.mFramesPerPacket ????

could you help me with the maths?

thanks!

Answer 1

Apple's formula is:

audioFormat.mSampleRate / audioFormat.mFramesPerPacket * seconds;

combining your formulas gives:

seconds * audioFormat.mSampleRate / audioFormat.mFramesPerPacket

which is exactly the same formula with the order changed. Note that:

seconds * audioFormat.mSampleRate / audioFormat.mFramesPerPacket
= seconds * (audioFormat.mSampleRate / audioFormat.mFramesPerPacket)
= (audioFormat.mSampleRate / audioFormat.mFramesPerPacket) * seconds
= audioFormat.mSampleRate / audioFormat.mFramesPerPacket * seconds

You seem to understand it but from different directions. 8)