It's not that the sound files wouldn't play - they did. But quite often they would "clip" during playing; so after a second or two of sound, the audio would stop, but the module would remain in a "busy" state, and refuse to play any more sound files. The problem was particularly noticeable if we interrupted one file that was already playing and tried to play a second file.
Unfortunately, there were a lot of changes between the module we had working (running off an Arduino Pro Micro at 3.7v with a lipo battery) and the one that gave us so much trouble today (an Arduino Pro Mini, off a changeable 3.3v/5v laptop usb supply).
- We were using a different SD card - although both were branded Sandisk, it's quite possible that one, neither or both actually were! The working sd card was 2Gb, the non-working one was 1Gb.
- We were using a different power supply - the working module was powered off a 3.7 lipo, the non-working one was problematic at both 5v and 3.3v
- We were using different sound files - though even when we copied the .ad4 files off the working sd card onto the non-working one, the problem remained.
Clipping has already been identified as a possible problem with these modules.
Because the module uses the data instruction 0xFFFF to stop the audio playing and values 0xFFF0-0xFFF7 to change the volume, any clipping in the audio data might accidentally fire the "stop" command
So we made sure that all our sound files had no clipping in them. But the problem remained.
We tried copying the working .ad4 files onto the non-working sd card - but the problem remained.
We tried powering the non-working device off a lipo battery - but the problem remained.
We tried playing each sound file in it's entirety, instead of interrupting the file and trying to play a different one, mid-way through. Success was more frequent. But every now and again the module would "lock up" and get stuck in the busy state.
It was starting to look like we had two identical-looking modules, potentially with different firmware versions handling how the audio data is read and played back off the SD card. And that's when Steve had a bit of a breakthrough - a full, complete, FAT-format of the card, then copy the files onto the disk in one go (instead of copying them one-at-a-time).
When we did this, the audio playback worked as expected.
The more we pondered why this was, the more we were reminded of when we created our own audio module a few years back. Parsing FAT tables was quite arduous. And most of the time, a file was written to disk using consecutive sectors. So when the file was written to a blank, virgin disk, it would be written onto sector one, then two, then three etc.
We spent quite a bit of time making our playback routines work with a FAT-based file system. It was tempting at the time to simply find the start of the file and assume that is was written on consecutive sectors - it would have saved a lot of messing about every 512 bytes! But while we went out on a limb to ensure that our playback routines would work with files anywhere on the disk, it appears that the WTV020-SD modules don't....
So if you've copied files onto your sd card and then deleted them (in doing so, the 8.3 file format name is simply prefixed with a ? character) then it's quite possible that any subsequent file might get written to sectors one, two, then nine, forty-seven etc. The FAT table does the job of bringing all the file fragments together.
But if your playback routine assumes that the file continues on consecutive sectors, it's easy to see why playback would fail. And it would also explain why the WTV020 module doesn't like playing back files that have been copied onto a disk after other files have been deleted off or moved around on it.
So, in short, to get your sound modules to work consistently well, always format the SD card fully before copying your files to it. Not a "quick format" (we tried that, it didn't work, since quite format simply blanks down the FAT table but leaves the data still written on the sectors). But a full, slow, 3-4 minutes to blank a 1Gb disk type format.
Do that and everything should be golden!