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A2DP support & sound quality & power usage of MS & Widcomm BT stack compared!

Now that I’ve made the stereo Bluetooth headphones (A2DP) support work with my Dell Axim x51v with the original Microsoft BT stack (see the tutorial here), I’ve thoroughly tested it and compared the sound quality and the power consumption of the two stacks.

First, the power consumption test, along with the CPU usage (all measured on the same x51v two times - with a hard reset in between. I've long been promising something similar to the AximSite folks in this thread):

As can clearly be seen, while device.exe, which is in charge of encoding the A2DP stream, consumes slightly less CPU time with the Widcomm BT stack, the latter still consumes about 13 mA’s more than the Microsoft stack. This, however, isn’t that big a problem if you also take into account the really superior sound quality.

What should you use to check out the sound quality yourself?

I really recommend for example the demo MP3 of Värttinä’s Oi Dai (linked from here); in there, you’ll clearly hear the quality difference between the A2DP mode of the MS and the Widcomm BT stack. Just make sure you try to listen to the generic distortion in the middle band when the girls start to sing.

BTW, use the first about five seconds to fine-tune the Subbands value with the Widcomm BT stack (see my earlier, related tweaking article here) – it’s mostly in the increased noise of strong lows (like at the beginning of this song) that the drastically decreased Subbands value has a really audible effect. With other kinds of music, the difference is very hard to hear.

Tweaking

I’ve played a bit with the MS BT stack parameters under HKEY_LOCAL_MACHINE\ SOFTWARE\ Microsoft\Bluetooth\ A2DP\Settings I’ve listed in my previous A2DP fine-tuning article. I’ve paid special attention to the following DWORD’s:

MinSupportedBitPool: it does have an impact on the sound transmission: while you can’t fine-tune the quality, a sufficiently high value (for example, 64; with 48, it still works) will make sure no connection can be made between the headphones and the PDA. That is, it’s useless to try to “tweak” this value.

UseJointStereo: the default 0 is “false”. When set to 1, it results in a mono transmission. With 2 (I’ve also tested with 3, with the same result), it seems it indeed uses joint stereo mode. It, however, hasn’t resulted in any kind of sound quality improvement. Conclusion: forget it too.

I’ve, in addition to trying to tweak BitPool, also tried to raise MaxSupportedBitPool , MinBitPool and, of course, BitPool – without success. I couldn’t achieve any sound quality improvement.

All in all, it seems the Microsoft BT stack is definitely worse, A2DP quality-wise, than that of Widcomm / Broadcomm and nothing (no tweaking) can be done to fix this.

Summary

There is not really competition between the A2DP quality of the two Bluetooth stacks. If you happen to have a WM5-upgraded Dell Axim x50(v) or x51(v), don’t hesitate: install the Widcomm BT stack on it if you want the best available sound quality. You’ll be presented definitely better sound quality, on the expense of slightly (but not much!) decreased battery life. And, of course, you get a lot of additional Widcomm goodies like Bluetooth PAN and the like.

Similarly, if you need to choose between, say, a Widcomm BT stack-based device and a MS-based one (and there's no known Widcomm hack for the latter), go for the earlier, if, otherwise, the two devices are the same.

This also means my sound quality problems with the HTC Wizard weren’t (also see my mourning here) because of the HTC Wizard hardware or the bad A2DP hack (see this for a more thorough elaboration on the hack) but because of the A2DP algorithm used in the MS BT stack. The official A2DP implementation of the really powerful Dell Axim x51v AKU2 isn’t at all better, quality-wise, than the A2DP hack for the Wizard.

I’ll try, of course, to push the Microsoft folks very hard to find the cause for the audibly lower sound quality.

More of my Bluetooth-related articles

The Bluetooth category in the Smartphone & Pocket PC Magazine Expert Blog