HELP ME - DECODER MPEG (LAYER I)!!!

HELP ME - DECODER MPEG (LAYER I)!!!

Post by Mateusz Barans » Wed, 26 Jul 1995 04:00:00



  I'm looking for simple way to extract MPEG Audio frames from file
frame by frame (Layer I). Currently I'm using John Fulmer's
aencode based on ISO sources. It produces file with frames
separated by sequences of zero bytes min 8 bytes long.
  Is it guaranteed by any standard (MPEG Audio file format)?

Frame start detection based on syncword (bin 1111 1111 1111)
recognition won't do - there are many syncword bit pattern
occurences in frame body.
                                MATTHEW

 
 
 

HELP ME - DECODER MPEG (LAYER I)!!!

Post by Patrick De Sm » Fri, 28 Jul 1995 04:00:00


:   I'm looking for simple way to extract MPEG Audio frames from file
: frame by frame (Layer I). Currently I'm using John Fulmer's
: aencode based on ISO sources. It produces file with frames
: separated by sequences of zero bytes min 8 bytes long.
:   Is it guaranteed by any standard (MPEG Audio file format)?

: Frame start detection based on syncword (bin 1111 1111 1111)
: recognition won't do - there are many syncword bit pattern
: occurences in frame body.
:                               MATTHEW

I've never really studied the bitcontense of an MPEG-stream, but to my
knowledge the syntax was designed to avoid "illegal" syncwords
(1-sequences are avoided when coding bitalloc, scalefactors and in
subbandsamplesquant.). So, something else is wrong ???

Also, normally when (de)coding MPEG audio (layer 1/layer 2) a fixed
bitrate is used; get the bitrate, check the padding bit and CALC. the next
framestart/syncword-position.

Why use layer 1, when layer 2 performs (much) better ?


feel free to contact me by e-mail, using THE ABOVE address.
(i wrote a thesis on MPEG-audio parameterevaluation and a (partial)
DSP56002 realtime encoding implementation)

 
 
 

HELP ME - DECODER MPEG (LAYER I)!!!

Post by Norley L » Fri, 28 Jul 1995 04:00:00




>  I'm looking for simple way to extract MPEG Audio frames from file
>frame by frame (Layer I). Currently I'm using John Fulmer's
>aencode based on ISO sources. It produces file with frames
>separated by sequences of zero bytes min 8 bytes long.
>  Is it guaranteed by any standard (MPEG Audio file format)?

>Frame start detection based on syncword (bin 1111 1111 1111)
>recognition won't do - there are many syncword bit pattern
>occurences in frame body.
>                            MATTHEW

You touched the nerve of MPEG audio standard. Unfortunately, unlike MPEG
video, the 12 bit syncword is not exclusive in the audio bit stream.
Therefore, the best way to detect the syncword is to use the combination
of header decoding and syncword pattern.

The frame size can be calculated from bit rate and sampling frequency, as
well as the layer (see MPEG spac section 2.4.3.1).  Therefore, the sequence
should be:
  start -> syncword detect -> frame size calculation ->
  get to the end of frame -> next syncword.

Hope it helps.

Norley Liu

 
 
 

HELP ME - DECODER MPEG (LAYER I)!!!

Post by Mark Garlang » Mon, 31 Jul 1995 04:00:00





>:   I'm looking for simple way to extract MPEG Audio frames from file
>: frame by frame (Layer I). Currently I'm using John Fulmer's
>: aencode based on ISO sources. It produces file with frames
>: separated by sequences of zero bytes min 8 bytes long.
>:   Is it guaranteed by any standard (MPEG Audio file format)?

The grammer does not require any zeros before the sync word.

Quote:

>: Frame start detection based on syncword (bin 1111 1111 1111)
>: recognition won't do - there are many syncword bit pattern
>: occurences in frame body.
>:                           MATTHEW

  Parsing the layer 1 grammer is VERY simple.  Take a look
at the standard 11172, page 14 and 15 describes the syntax.
Or you can look on page 30, it gives the formula for calculating the
distance between two syncwords.

Quote:>I've never really studied the bitcontense of an MPEG-stream, but to my
>knowledge the syntax was designed to avoid "illegal" syncwords
>(1-sequences are avoided when coding bitalloc, scalefactors and in
>subbandsamplesquant.). So, something else is wrong ???

  In the Bitalloc and scalefactors portion of the file, the syncword
can not appear in a valid stream.  But in the samples, it is possible
for the sync pattern to appear. It is not too common since the syncword
must also be byte aligned.

                                   Mark

 
 
 

HELP ME - DECODER MPEG (LAYER I)!!!

Post by Christian Voelk » Tue, 01 Aug 1995 04:00:00


Yohoo!

 MB> Frame start detection based on syncword (bin 1111 1111 1111)

No. Syncword for frame recognition (I, P, B frames) ist 0x00 0x00 0x01 0x00
Then there will come 10 bits of frame count, then three bits of frame type.
(001 for I frame, 010 for P frame, 011 for B frame)

Unn wech.....
  Christian

 
 
 

HELP ME - DECODER MPEG (LAYER I)!!!

Post by Wilfred Li » Wed, 09 Aug 1995 04:00:00


: Yohoo!

:  MB> Frame start detection based on syncword (bin 1111 1111 1111)

: No. Syncword for frame recognition (I, P, B frames) ist 0x00 0x00 0x01 0x00
: Then there will come 10 bits of frame count, then three bits of frame type.
: (001 for I frame, 010 for P frame, 011 for B frame)

: Unn wech.....
:   Christian

The guy asked about the audio part (infered by the 'LAYER I'). So the
syncword is 1111 1111 1111.

Wilfred

 
 
 

1. Any MPEG Layer 3 audio decoder chips available?

I've been trying to find Layer 3 audio decoder
chips for a long time.  From the responses I've
received, it appears that in order to be able
to purchase/use any of these chips a licensing
agreement is necessary.  IOW, I can't just run
down to a chip retailer, purchase a chip, plop
it into a socket and have something useful.

Is this accurate?  Is there an easy way around
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Thank you.

--kyler

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