When iOS 4.2 was released, many of the DJ apps have been quick to respond and implement some form of access to the iPod library using the new API Classes code available. However, most DJ apps will want to store some key information about the audio separately (such as waveform detail, bpm and cue points). This is to ensure quick display and readiness next time the audio is loaded into the app. Ideally this storage of information must be as succinct as possible so the storage capacity isn't reduced dramatically as you play more tracks. For example, if I load 20Gb of tracks into my iPad and I really don't want those tracks duplicated anywhere as it will eventually eat up another 20Gb of space. This is where the app developers are more than welcome to confirm or deny the route taken to accessing the iPod library!
I only raise this as I've noticed that the Quixpin app seems to copy every audio file you play along with a '.peak' file (presumably holding the waveform detail) in a folder called 'Imported Tracks' which is clearly visible when connected to iTunes under the app sharing section. I'm wondering if this is an isolated case, since the other apps don't seem to show anything similar...or whether they're storing the files somewhere that the regular user can't access them!
As a test, I've currently got 42Gb available on my iPad. I've just loaded 60 tracks into the iPod Library which appear to be about 256.3Mb, that means I now have roughly 41.7Mb marked as available on the iPad. I've run the batch process in Sonorasaurus Rex to build the waveforms for the recently added tracks (all previous tracks had been done before I started this). When I go back to the iPad general properties section, it lists the available space as being about 41.4Gb....this suggests that the files are being duplicated on the iPad somewhere out my reach. Worth remembering next time you're loading tracks onto the iPad!
Processing Data (building waveforms etc):
Sonorasaurus Rex is unique at the moment in offering a batch process feature (on the iPad) to build the waveforms, whereas the other apps (like DJay and Quixpin) build them as the track is first loaded into the app. I prefer the batch load method simply because it reduces the number of tasks you're expecting the iPad to do whilst playing out somewhere. This must reduce the risk of the device crashing whilst playing out? However, imagine the length of time needed to build waveforms for your iPod library - especially if you've loaded several GB's worth of data. I guess my advice here would be, only load tracks into the iPod library if you're likely to use them in your DJ app of choice. Also load them in manageable chunks - a few albums at a time rather than your whole iTunes account!
Cue.Play.DJ are still operating on a non-iPod Library access method, where you load your tracks into their host based program to build the waveforms and do the bpm detection. Once done, you copy the tracks (and their corresponding data file) into the app sharing area which are then recognised by the app on the iDevice. My initial thoughts post iOS 4.2 were, "why don't they get iPod library access sorted soon?", whereas now I'm thinking the processing of the files on the host machine may not be such a bad idea after all and if all other DJ apps are duplicating the files, then Cue.Play.DJ is no different, albeit without the iPod library interface.
With iOS 4.2 the default access to the iPod Library appears to mimic a stripped down version of what you get through the iPod icon on the iDevice. This is either a simplified list by artist, track or album or (in the case of DJay) you can expand the library to cover the whole screen, displaying cover art as well. Unfortunately, I expect this doesn't give the developers much scope for adding columns to this display - for example BPM is a tag which can be captured/stored in iTunes and will be held with the track in the iPod library...so it would be great if BPM could be displayed in these lists as well - and if possible sorted by bpm if you wanted. Sonorasaurus Rex has its own custom library access screen, possibly because they started development of this before iOS 4.2 was announced. Therefore they can sort by bpm, artist or track...but you get no album artwork and the album title is not displayed. Maybe the next version of iOS would allow for easier access to this information?!
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