The doc comments in that file you linked is taken directly from Apple's spec. In those docs, you'll find these two important things:
"The duration of the audio in the file is [the number of valid frames] divided by the sample rate specified in the file’s Audio Description chunk."
OK, cool, but how many valid frames are there? There are two possible ways to know:
- If the CAF has a packet table, it must include the number of valid frames. Perfect.
- The only CAFs that are allowed to NOT have a packet table are those with constant packet sizes:
"Note that, as long as the format has a constant number of frames per packet, you can calculate the duration of each packet by dividing the mSampleRate [frames per second] value by the mFramesPerPacket value."
That tells you the duration per packet, but because packets are a constant size, the number of packets is just the audioDataSize / bytesPerPacket. The latter value is included in the Audio Description. The former is often embedded directly into the file, but it's permitted to be -1 with the audio data as the last chunk, in which case its size is totalFileSize - startOfAudioData
It breaks down like this:
- If there's a Packet Table Chunk, use it and the Audio Description:
seconds = validFrames / sampleRate
- Otherwise, packets must have constant size:
framesPerByte = framesPerPacket / bytesPerPacketseconds = framesPerByte * audioDataSize
The library you've got reads the Audio Description chunk, but I don't think it reads the Packet Table. Also, I'm not confident it calculates the audio data size if the chunk is -1. Maybe it does both/either, in which case, you can use the information above.
If not, you can just parse the file yourself, especially if you only care about the duration. The file starts with a short header, then is split into "chunks" (aka TLVs).
Here's a sample implementation you can use as a starting point or to modify the library you linked:
func readCAF() {
buf := []byte{
// file header
'c', 'a', 'f', 'f', // file type
0x0, 0x1, 0x0, 0x0, // file version, flags
// audio description
'd', 'e', 's', 'c', // chunk type
0x0, 0x0, 0x0, 0x0,
0x0, 0x0, 0x0, 0x20, // CAFAudioFormat size
0x40, 0xe5, 0x88, 0x80,
0x00, 0x00, 0x00, 0x00, // sample rate
'l', 'p', 'c', 'm', // fmt id
0x0, 0x0, 0x0, 0x0, // fmt flags
0x0, 0x0, 0x0, 0x1, // bytes per packet
0x0, 0x0, 0x0, 0x1, // frames per packet
0x0, 0x0, 0x0, 0x2, // channels per frame
0x0, 0x0, 0x0, 0x3, // bits per channel
// audio data
'd', 'a', 't', 'a', // chunk type
0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, // size of data section (-1 = til EOF)
// actual audio packets (in theory, anyway)
0x0,
0x0,
0x0,
0x0,
0x0,
0x0,
}
fileSize := len(buf)
br := bufio.NewReader(bytes.NewBuffer(buf))
type cafHdr struct {
Typ [4]byte
Version uint16
_ uint16
}
type chunkHdr struct {
Typ [4]byte
Sz int64
}
type audioDescription struct {
FramesPerSec float64
FmtId uint32
FmtFlags uint32
BytesPerPacket uint32
FramesPerPacket uint32
ChannelsPerFrame uint32
BitsPerChannel uint32
}
type packetTable struct {
NPackets, NValidFrames, NPrimingFr, NRemainingFr int64
}
const FileHeaderSz = 8
const ChunkHeaderSz = 12
const AudioDescSz = 32
const PacketHdrSz = 24
fileHdr := cafHdr{}
if err := binary.Read(br, binary.BigEndian, &fileHdr); err != nil {
panic(err)
}
if fileHdr.Typ != [4]byte{'c', 'a', 'f', 'f'} || fileHdr.Version != 1 {
panic("unknown file format")
}
remaining := int64(fileSize) - FileHeaderSz
audioDesc := audioDescription{}
packetTab := packetTable{}
var audioDataSz int64
readChunks:
for {
hdr := chunkHdr{}
if err := binary.Read(br, binary.BigEndian, &hdr); err != nil {
panic(err)
}
remaining -= ChunkHeaderSz
switch hdr.Typ {
case [4]byte{'d', 'e', 's', 'c'}: // audio description
if err := binary.Read(br, binary.BigEndian, &audioDesc); err != nil {
panic(err)
}
hdr.Sz -= AudioDescSz
remaining -= AudioDescSz
case [4]byte{'p', 'a', 'k', 't'}: // packet table
if err := binary.Read(br, binary.BigEndian, &packetTab); err != nil {
panic(err)
}
hdr.Sz -= PacketHdrSz
remaining -= PacketHdrSz
case [4]byte{'d', 'a', 't', 'a'}: // audio data
if hdr.Sz > 0 {
audioDataSz = hdr.Sz
} else if hdr.Sz == -1 {
// if needed, read to EOF to determine byte size
audioDataSz = remaining
break readChunks
}
}
if hdr.Sz < 0 {
panic("invalid header size")
}
remaining -= hdr.Sz
// Skip to the next chunk. On 32 bit machines, Sz can overflow,
// so you should check for that (or use Seek if you're reading a file).
if n, err := br.Discard(int(hdr.Sz)); err != nil {
if err == io.EOF && int64(n) == hdr.Sz {
break
}
panic(err)
}
}
var seconds float64
// If the data included a packet table, the frames determines duration.
if packetTab.NValidFrames > 0 {
seconds = float64(packetTab.NValidFrames) / audioDesc.FramesPerSec
} else {
// If there no packet table, it must have a constant packet size.
if audioDesc.BytesPerPacket == 0 || audioDesc.FramesPerPacket == 0 {
panic("bad data")
}
framesPerByte := float64(audioDesc.FramesPerPacket) / float64(audioDesc.BytesPerPacket)
seconds = framesPerByte * float64(audioDataSz)
}
fmt.Printf("seconds: %f\n", seconds)
}
