Speex dev - Mar 2006 - ARM7 Speex decoder

Dear Jan

My intention is to read a piece at time of an .ogg file (stored in and external
Multimedia card) decompress it and render it on and LPC2000 DAC. Because I
don't have enough RAM memory to load the complete ogg file, I must read only
a piece of file at certain time, then decompress and render it, and repeat the
previous process until the end of the file. The problem is that I don't know
what is the minimum file block size that I must consider.

 I red,of course, the example that exist in the speex manual, however I have
some doubt. I got the FRAME_SIZE from the function

   /* Get frame size */
    speex_decoder_ctl(state, SPEEX_GET_FRAME_SIZE, &nbBytes);


repeat 

    lb = file_read(&file, nbBytes, (void *)cbits);
    /*Copy the data into the bit-stream struct*/

    speex_bits_read_from(&bits, cbits, nbBytes);
    /*Decode the data*/

     speex_decode(state, &bits, output);

until EOF

(I get the number 160) that I use it in my file_read function to get a block of
nbBytes (SPEEX_GET_FRAME_SIZE) bytes with which I feed the
speex_bits_read_from(&bits, cbits, nbBytes). Then I call the and the
speex_decode(state, &bits, output);

The problem is that I receive the error message 

warning: Invalid mode encountered: corrupted stream?

And the decode process doesn't continue.


I have the suspect that the fetched block is too small. What is the minimum
block side that speex_bits_read_from(&bits, cbits, nbBytes) accept ?


Thank you
Paolo

On Tue, Mar 21, 2006 at 09:56:04PM +0100, pberna@iol.it wrote:
> My intention is to read a piece at time of an .ogg file (stored in and
external Multimedia card) decompress it and render it on and LPC2000 DAC.
Because I don't have enough RAM memory to load the complete ogg file, I must
read only a piece of file at certain time, then decompress and render it, and
repeat the previous process until the end of the file. The problem is that I
don't know what is the minimum file block size that I must consider.
What you appear to be missing is that the speex data packets are 
encapsulated in an Ogg container. You must first parse the Ogg
header to extract the raw packets before passing them to libspeex
with speex_bits_read_from(). It's not clear from your description
that your file_read() function is doing all this.

speexdec uses libogg to do this.

Ogg pages produced by the reference encoder are around 4 kB, but 
by specification Ogg pages can be up to 65 kB in length. libogg
will internally buffer a full page before returning any packet
data for decode, so that's the minimum footprint you would have
to design for.

If you're in a memory contrained environment I'd recommend implementing 
your own ogg parser to pass the data to libspeex directly as it's read 
instead of buffering a whole page for checksum verification. You can 
either compute the checksum incrementally or dispense with it entirely 
and assume libspeex will detect any file corruption. Note that both of 
these are necessarily less robust against damaged files, but it should 
let you process data with an arbitrarily small read buffer.

A compromise would be to use a static 5 kB buffer and verify the crc
only if the page happens to fit, falling back to incremental parsing
otherwise.

HTH,
 -r