Hi, On Sat, Jan 14, 2006 at 07:42:05PM +0000, Bluechip wrote: > > >>Pretty much got a proper core written for an Audio file processor now. > >>I have another simple question about MPEG Audio files. > >>Am I ever likely to encounter an MPEG Audio file which will contain AUDIO > >>frames which differ in VERSION, LAYER, SAMPLERATE and/or CHANNELS? Eg. > >>Some frames are 48KHz and other frames are 44.1KHz ...or some frames in > >>Stereo, some in Joint-Stereo?
a change of LAYER or SAMPLERATE requires a restart of the decoder. > >>I have been unable to find any documentation which explicitly states > >>whether this should or shouldn't be allowed by the MPEG standard, and > >>nothing to say explicitly whether any players do or don't support this > >>type of change. > >>There is no reason why not, the file would have a valid structure - but > >>it seems unlikely. > >>This is relevant to creating the Xing/Xing or Xing/Info frame for VBRFix. > >>Thanks for any help, > >>BC > > > >Looking through the setup in Razorlame (a lame frontend for windows) there > >is the ability to allow different 'block' sizes between channels, although > >this is generally disabled. Another option give the ability to disable > >using 'short blocks', suggesting that the default is to allow them. Having > >stated this, I'm not too sure exactly what they are refering to with the > >term 'blocks'. It may mean the same as frame but I don't know. > > Thanks, I too use razorlame, and also have no idea what short or long > blocks are - and google isn't helping much either. LOL IIRC, the audio data that belongs to each MPEG-1 layer III audio header (in fact, the frame header can be located just in the middle of the audio data it belongs to) is parted into 2 halves AKA granules. Those in turn are parted into n (frequency) subbands, which can consist of either 1 long or 3 short blocks (windows) of samples (it's even possible to have long blocks in the lower and short blocks in some upper subbands). The former have a better frequency resolution, the latter a better time resolution (eg to reduce pre-echoes). Try a percussive sound like from a glockenspiel or triangle to hear the difference when short blocks are disabled. Jacob
