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Can't find mp3 frame header sync bits in node stream

Can't find mp3 frame header sync bits in node stream

Problem

I am almost at the end of my rope on this one. I am attempting to parse out mp3 frame data, but I am having a bear of a time locating any frames. I am using Node 0.10 and trying to make use of the newer stream api (although it doesn't work with the old one either)

I am looping through the buffer looking for the 11 bit Sync word, but i never find it.

 var stream = fs.createReadStream('./myAudioFile.mp3')

 stream.on('readable', function(data){
    var chunk, header;

    while (null !== (chunk = stream.read())) {
        for (var i = 0; ( i + 4 ) 

Am i missing something here?

Problem courtesy of: monastic-panic

Solution

You're getting bitten by a quirk of Javascript bitwise operators. Let me demonstrate.

> 0xffffffff & 0xffe00000
-2097152
> 0xffe00000
4292870144

The result of a Javascript bitwise operation is always treated as a signed 32-bit integer. Since your constant is an unsigned 32-bit integer, it'll never match.

One potential fix is to use ~~ to perform the same mangling on your constant:

if ((header & 0xffe00000) == ~~0xffe00000) {

Another would be to just read in 16-bit integers instead, since the sync word will fit into that size anyway.

Unrelated but worth mentioning:

  • You're actually going to need to use chunk.readUInt32BE(i) here, not LE. Right now you're finding instances of FE FF in the stream, not FF FE like you want.

  • In the rare instance that a sync word spans two chunks, your code misses it.

Solution courtesy of: duskwuff

Discussion

View additional discussion.



This post first appeared on Node.js Recipes, please read the originial post: here

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Can't find mp3 frame header sync bits in node stream

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