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+From - Mon Nov 5 09:19:09 2001
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+From: Rob Leslie <rob@mars.org>
+To: mad-dev@lists.mars.org
+Subject: Re: [mad-dev] Some information about programming with MAD (in synchronous mode)
+In-reply-to: Your message of "21 Oct 2001 16:13:19 EDT."
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+Date: Mon, 22 Oct 2001 01:31:30 -0700
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+
+Joe Drew wrote some good info on the MAD high-level API that I hope will be
+helpful to others.
+
+By way of clarification, MAD also has a low-level API which does not use
+callbacks. You can control the entire decoding process yourself more or less
+as follows:
+
+ /* load buffer with your MPEG audio data */
+
+ mad_stream_buffer(&stream, buffer, buflen);
+
+ while (1) {
+ mad_frame_decode(&frame, &stream);
+ mad_synth_frame(&synth, &frame);
+
+ /* output PCM samples in synth.pcm */
+ }
+
+This is vastly simplified, but it shows the general idea. mad_frame_decode()
+decodes the next frame's header and subband samples. mad_synth_frame() takes
+those subband samples and synthesizes PCM samples.
+
+It is also possible to call mad_header_decode() before mad_frame_decode().
+This just gives you the frame's header info, in case that's all you want, or
+perhaps to help you decide whether you want to decode the rest of the frame.
+
+As Joe mentions, each of the stream, frame, and synth structs needs to be
+initialized and "finished" before and after use:
+
+ struct mad_stream stream;
+ struct mad_frame frame;
+ struct mad_synth synth;
+
+ mad_stream_init(&stream);
+ mad_frame_init(&frame);
+ mad_synth_init(&synth);
+
+ /* ... */
+
+ mad_synth_finish(&synth);
+ mad_frame_finish(&frame);
+ mad_stream_finish(&stream);
+
+You can work with just a struct mad_header instead of a struct mad_frame if
+you only want to decode frame headers.
+
+Joe writes:
+> MAD always outputs 32-bit (well, mad_fixed_t) little-endian data. Take
+> this into account when outputting samples to the sound card.
+
+This isn't quite right: the mad_fixed_t type is not necessarily little-endian.
+It's the same endianness as the native integer types. Also, it's only
+guaranteed to be *at least* 32 bits wide.
+
+The fixed-point sample format is important to understand, and I recommend
+reading the comments in libmad/fixed.h. The thing to remember when converting
+MAD's fixed-point integer samples to 16-bit PCM (or whatever) is that MAD
+encodes samples as numbers in the full-scale range [-1.0, +1.0) where the
+binary point is placed 28 (MAD_F_FRACBITS) bits to the left of the integer.
+However, you need to be prepared to handle clipping as some numbers may be
+less than -1.0 (-MAD_F_ONE) or greater than or equal to +1.0 (MAD_F_ONE, aka
+1 << MAD_F_FRACBITS).
+
+> Information on the other (error, filter, message) functions would be
+> appreciated, though I think in knowing this information anyone should be
+> able to puzzle it out.
+
+In the high-level API, the error callback function is called whenever a
+decoding error occurs. The error number is in stream->error.
+
+The filter callback function is called after decoding a frame, but before
+synthesis. Here it is possible to modify the frame's subband samples, for
+example to perform a uniform attenuation/amplification, or to do other special
+processing in the frequency domain.
+
+The message callback function is only used with MAD_DECODER_MODE_ASYNC, and is
+called whenever the parent process sends a message via mad_decoder_message().
+This callback can generate a reply by overwriting the message buffer that is
+passed to it. (The size of the reply must be the same or smaller than the
+message.)
+
+Cheers,
+ -rob
projects / vlc / blobdiff