NewLC - Converting from AMR to PCM16 does not work. - Comments

Re: Converting from AMR to PCM16 does not work.

andrew.hayes — Wed, 17 Aug 2005 00:18:07 +0200

Hi

You really don't seem to have understood what an AMR file is and how it is encoded. Or how to use MMF codecs and buffers. There's so many things wrong with this code. A little tutorial is in order:

Point 1:
You've new'ed a TPtr8 into existence, and then not put it onto the cleanup stack even though you've called leaving functions subsequently. Bad man! The TPtr8 should be an automatic variable in any case. I'm not sure why you've set the length and the maximum length as the same value either.

Point 2:
An AMR file (a .amr file you can play on any AMR file player) contains a 6-byte binary header, this is always {0x23,0x21,0x41,0x4D,0x52,0x0A}. If this is the type of file you're trying to decode and convert above, you need to remove the first 6-bytes before you even hit the AMR data you'll feed into the codec. You haven't done that above.

Point 3:
An AMR file consists of a number of encoded frames of audio data. Each frame represents 20ms of audio.

Assuming this is a normal AMR file (and not AMR-WB) the sound will decode into 8KHz, 16-bit PCM. 20ms of PCM of this type is 320bytes in size, do the math (8000Hz * 16-bits * 0.02sec = 2560bits = 320bytes). So, your output MMF buffer in your decoder should be 320 bytes in size, enough to hold 1 frame of decoded AMR. That's all you need.

These 20ms frames of audio are compressed in the AMR file. Depending on the encoding level used, frames will be the following sizes:

CMR Mode Frame size (bytes)
0 AMR 4.75 13
1 AMR 5.15 14
2 AMR 5.9 16
3 AMR 6.7 18
4 AMR 7.4 20
5 AMR 7.95 21
6 AMR 10.2 27
7 AMR 12.2 32

Each frame consists of a 1 byte header, then the rest of the frame is audio data. The entire frame is fed into the decoder (header too). The frame size can be deduced from the frame header. The top 4 bits of the header comprise the CMR, values 0-7 being valid for AMR, the top bit can actually be ignored, though it is used when AMR forms RTP payloads. The lower 4-bits of the header are reserved and are not used.

Possibly in the code above, you may want to read the frame header byte, deduce the frame size then read enough data to cover that one frame. Decode that frame into the waiting 320byte PCM output buffer then write that buffer out to another file?

None of this is necessary however as the MMF does all of this for you...

Point 4:
You're output MMF buffer is set to be 320bytes, enough to hold 1 frame of decoded AMR. OK

You're input buffer can be any size you like really as long as it's big enough to hold at least one frame of AMR.

How to use an MMF codec:
Calling ProcessL() on a codec with a source buffer and a destination buffer will (in this instance) decode 1 frame of AMR from the source into the destination. The processing results in a TCodecProcessResult. This will have one of the following statuses:

EProcessComplete - all of the source data has been used and the destination buffer is full. Read all new data into the source buffer and copy out the decoded frame from the destination buffer before calling ProcessL() again
EProcessIncomplete - the destination buffer is filled with one decoded frame, but there is still source data remaining. Copy the decoded frame out of the destination buffer. Check the number of bytes remaining in the source buffer, if it's less than the minimum AMR frame size (13bytes) copy in new data onto the end of the remaining data and call ProcessL() again
EDstNotFilled - should not happen in this case - decoding an AMR frame will ALWAYS produce 320bytes in the destination buffer
EEndOfData - not applicable to AMR decoding - once there is no data left in the file, the AMR stream has ended.
Other statuses are not used

Keep calling ProcessL() in a loop until all data in the file has been read and decoded. Copy out the decoded PCM frame each time to another file and everything should be OK.

Hope that helps.

Andy

Re: Converting from AMR to PCM16 does not work.

shagor — Wed, 27 Jul 2005 13:16:21 +0200

Quote

I wonder why you chose a value of 320 bytes for the input for AMR. In a packed format, the maximum size for a frame for AMR is 32bytes (for the hishest possible bit rate in AMR).

Symbian has a codec ported which is built in which does the AMR decoding? Which AMR decoder are you trying to use for the decoding?

Please read the above code, you can see, I am not trying to decode. What I am doing is just converting AMR to PCM.

Re: Converting from AMR to PCM16 does not work.

Tony Francis — Tue, 26 Jul 2005 12:26:44 +0200

Re: Converting from AMR to PCM16 does not work.

kesken — Fri, 22 Jul 2005 17:01:08 +0200

thanks for your kindness, i hope you'll get success

Re: Converting from AMR to PCM16 does not work.

shagor — Fri, 22 Jul 2005 08:44:30 +0200

Hello kesken,

I completely agree with you. There is no good example about converting AMR to PCM. Those code snippets you may find around about conversion, does not work. I don't know, why this should be so difficult. Anyway If I find any working solution, I will let you know.

Re: Converting from AMR to PCM16 does not work.

kesken — Fri, 22 Jul 2005 08:17:27 +0200

I'm also waiting for a good answer to this question desperately .
pleaaaaassee!
can't somebody just put a working example code(project) to newlc.
i think a tutorial is required about amr conversion issue.

Re: Converting from AMR to PCM16 does not work.

shagor — Thu, 21 Jul 2005 13:54:38 +0200

Quote

const TInt KAmrBufferSize = 320;
const TInt KPcmBufferSize = 10000;

320 is less than the actual amr file, so application crashes.

Re: Converting from AMR to PCM16 does not work.

yucca — Thu, 21 Jul 2005 12:22:32 +0200

try with thsese values:
const TInt KAmrBufferSize = 320;
const TInt KPcmBufferSize = 10000;

might just work a bit better.

yucca

Converting from AMR to PCM16 does not work.

shagor — Thu, 21 Jul 2005 09:23:30 +0200