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Rockbox mail archiveSubject: Re: MPEG Audio formatRe: MPEG Audio format
From: Bluechip <csbluechip_at_gmail.com>
Date: Sun, 15 Jan 2006 03:03:15 +0000 > > >>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. Excellent information. I notice you did not mention VERSION or CHANNELS ...Given a LAYER change will trigger a restart it seems logical to presume a VERSION change would also trigger a restart even if the change was Ver1/Lyr1 -> Ver2/Lyr1 Would you care to comment on CHANNEL changes ...even if your answer is "don't know" I would consider that to be helpful feedback. > > 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) :O A frame header can exist in the middle of an [a pair of] audio frame[s]?? Could you explain a little more on that - it seems very important to my util. If my file has only TWO [a pair of] audio frames (unlikely, but valid) ...the file can/will look like "AUDIO_DATA -- HEADER -- AUDIO_DATA" ...surely lack of a header at the start of the file will mean the first block of audio data cannot be found by the decoder? Or do you mean that there would be two [a pair of] audio frames which each contained half of the data required to recreate the original sound? Do you have a test file which I can throw at my util for testing please? (or instructions for making one) > 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. Perfect, thank you. >Jacob BC Received on 2006-01-15 Page template was last modified "Tue Sep 7 00:00:02 2021" The Rockbox Crew -- Privacy Policy |