mad-dev May 2001

mad-dev@lists.mars.org

15 participants
14 discussions

madplay in BeOS
by whee 21 May '01

21 May '01

I decided on trying BeOS again and was wondering if anyone had written an output module using the media server (or whatever). Everything compiles fine, but I don't see a way to actually generate sound without having madplay decode to a .wav and then playing that. It should be pretty simple to do and I was going to try it myself, but ended up confusing myself with the internals of mad while looking at the existing audio_* files. I hope someone's bored enough and has BeOS. :D -whee

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Re: Low level programming interface question
by Rob Leslie 09 May '01

09 May '01

Hi Lionel, > If I put aside the licensing issues ( :-) ), my biggest problem is to really > understand how the low-level MAD interface is working. After looking at the > WinAMP plug-in source, I somewhat understood how it works (except for the > timers, where I am completely lost). My understanding for now is : > > - read some data from the stream > - call mad_stream_buffer These steps: > - call mad_frame_decode > - call mad_synth_frame > - send the data to the sound device should be repeated until MAD returns the error MAD_ERROR_BUFLEN. > - copy the data remaining in the buffer (ie the 'next_frame' pointer) to > the start of my input buffer > > For a stand-alone player, this works wonderfully. The problem comes when I > try to incorporate this in my AVI player. Indeed, when I added some printfs > to my small player, I saw that MAD always : > > - output the same amount of data (0.006 seconds of sound) > - always 'progressed' of the same amount of data in my stream Each MPEG audio frame generally contains the same number of PCM samples, and (in the case of constant bitrate streams) will usually consume the same amount of data from the stream. Each call to mad_frame_decode() and mad_synth_frame() will decode only a single frame. > On the other hand, in my AVI stream, for each video frame, the amount of MP3 > data is not AT ALL constant.... Moreover, it is sometimes smaller than the > amount of data read by MAD => I get (as soon as the first video frame) a > decode error from MAD (I did not check exactly what error it was). You will have to demultiplex the MPEG audio data from the video data and stream this independently to MAD. > I could do 'buffering' (ie concatenate some sound AVI chunks) but then I > would completely lose my synchro (and I do not want to rely on timers or to > retrieve the synchro, it would start to get bad with long films). You will probably have to deal with the synchronization somehow. MAD's timers can help you keep track of how much audio data has been decoded. They might seem intimidating but they're really quite simple to use. I'll post some details in a follow-up message. > PS: I hope this is the right mailing list, I did not know if I should send > this to the devel list or the user list :-) I think mad-dev is probably more appropriate; I've adjusted the Reply-To header but you might need to subscribe before you can reply. Cheers, -rob

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Resampling 44.1k to 48k
by Ira Weiny 09 May '01

09 May '01

I have an application which needs to decode mp3's to 48kHz. As most mp3's are 44.1 I am going to have to resample. I am using libmad to decode the mp3 just fine by starting with the minimad source. I then integrated the resampling algorithim in resample.c. I noticed from the comments that this was not the best resampling algorithim available. Has anyone else done something better with fixed point math? Ira

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MAD timer API
by Rob Leslie 08 May '01

08 May '01

The basic timer datatype is mad_timer_t; it represents a unit of time. You can initialize a timer by setting it either to a zero constant or to some other explicit value: mad_timer_t timer; timer = mad_timer_zero; /* or */ mad_timer_set(&timer, seconds, frac_numer, frac_denom); Timers have a precision of 1/352800000 second, so you can use the frac_numer and frac_denom arguments to specify a fractional value as a rational number. The resulting timer value, in seconds, is: seconds + (frac_numer / frac_denom) You can add one timer value to another with: mad_timer_add(&timer, timer2); You can also multiply a timer value by an integer: mad_timer_multiply(&timer, scalar); Timer values can be either positive or negative; to change the sign of a timer: mad_timer_negate(&timer); To get the absolute value: timer2 = mad_timer_abs(timer); Or, to determine the sign: sign = mad_timer_sign(timer); /* < 0 negative; = 0 zero; > 0 positive */ In general you can compare any two timer values: comp = mad_timer_compare(timer1, timer2); /* comp < 0 : timer1 < timer2 */ /* comp = 0 : timer1 = timer2 */ /* comp > 0 : timer1 > timer2 */ Finally, there are methods for extracting the value of a timer. To convert a timer value to selected UNITS (see libmad/timer.h for unit constants): count = mad_timer_count(timer, UNITS); /* rounded down */ If you assume a basic unit of seconds, you can get the remaining fractional part of a timer in arbitrary terms with: frac_numer = mad_timer_fraction(timer, frac_denom); The precision of the timers is such that frac_denom can be any valid MPEG audio sampling frequency to get sample-granular results. Finally, you can get a string representation by supplying a sprintf-style format and selecting various units: mad_timer_string(timer, dest, format, UNITS, frac_UNITS, subparts); For example, to get HH:MM:SS.DDD you could use: mad_timer_t timer; char dest[13]; mad_timer_string(timer, dest, "%02lu:%02u:%02.%03u", MAD_UNITS_HOURS, MAD_UNITS_MILLISECONDS, 0); (MAD_UNITS_HOURS implies the next two format specifiers are for minutes and seconds.) There are also selectors for video frame units, and even drop-frame time coding is supported: mad_timer_string(timer, dest, "%02lu:%02u:%02u;%02u", MAD_UNITS_HOURS, MAD_UNITS_29_97_FPS, 0); After each call to mad_frame_decode() (or mad_header_decode()), frame.header.duration is filled with a timer value representing the playing time of the audio in the frame. You can accumulate these times with mad_timer_add() to keep an accurate time position as you play the audio. -- Rob Leslie rob(a)mars.org

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mad-dev May 2001