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I know I can find other answers about this on SO, but I want clarifications from somebody who really knows MPEG-1/MPEG-2 (or MP3, obviously).

The start of an MPEG-1/2 frame is 12 set bits starting at a byte boundary, so bytes ff f*, where * is any nibble. Those 12 bits are called a sync word. This is a useful characteristic to find the start of a frame in any MPEG-1/2 stream.

My first question is: formally, can a false sync word be found or not in the payload of an MPEG-1/2 frame, outside its header?

If so, here's my second question: why does the sync word mechanism even exist then? If we cannot make sure that we found a new frame when reading fff, what is the purpose of this sync word?

Please do not even consider ID3 in your answer; I already know about sync words that can be found in ID3v2 payloads, but that's well documented.

eepp
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2 Answers2

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I worked on MPEG-2 streams, more precisely Transport Streams (TS): I guess we can find similarities.

A TS is composed of Transport Packets, which have a header, starting with a sync byte 0x47.
We also can found 0x47 within the payload of the TP, but we know that it is not a sync byte because it is not aligned (TP have a fixed size of 188 bytes).

  • The sync word gives an entry point to someone that looks at the stream, and allows a program to synchronize his process with the stream, hence the name.
  • It also allows a fast browsing and parsing of the stream: in a TS you can jump from a packet to another (inspect header, check sync byte, skip 188 bytes and so on)
  • Finally it is a safety measure that helps you to spot errors (in the stream during transmission for example or in the process if a bug caused a bad alignment)

These argument are about TS but I think the same goes with your case : finding a sync word within a payload should not be an issue because you should always able to distinguish payload and header, most of the time with a length information (either because the size is fixed like in TP or because you have a TLV format).

n0p
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    MP3 frames may be of variable sizes in an MP3 stream, and yes there is a documented formula to know a frame size from its header. But you still need to know where is this header. Say you connect to some MP3 stream source and what you first receive is the middle of a frame. How do you know if the next `fff` is a sync word or some payload? – eepp Aug 29 '14 at 18:22
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can a false sync word be found or not in the payload of an MPEG-1/2 frame, outside its header?

According to this, "frame sync can be easily (and very frequently) found in any binary file." See the section titled "MPEG Audio Frame Header"

I confirmed this with an .mp3 song that I chose at random (stripped of ID3 tags). It had 5193 sync words, of which only 4898 were found to be valid (using code too long to be included here).

>>> f = open('notag.mp3', 'rb')
>>> r=f.read()
>>> r.count(b'\xff\xfb')
5193

why does the sync word mechanism even exist then? If we cannot make sure that we found a new frame when reading fff, what is the purpose of this sync word?

We can be (relatively) sure if we are checking the rest of the frame header, and not just the sync word. There are bits following the sync which can be used to:

  1. identify a false positive or
  2. give you useful info

With .mp3, you have to use those useful bits to calculate the size of the frame. By skipping ahead <frame-size> bytes before looking for the next sync word, you avoid any false syncs that may be present in the payload. See the section titled "How to calculate frame length" in that same link.

JB0x2D1
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