Thursday, February 04, 2021

Cordova-plugin-ble-central without BACKGROUND_LOCATION permission

Happy New Year.

I have been developing a Cordova app on Android that uses Bluetooth Low Energy (BLE).  To accomplish this, I have been using Don Coleman's cordova-plugin-ble-central.  This is a neat plugin with a pretty simple API that lets you do serial communications over BLE. It is compatible with both Android AND iOS.  It's installable with NPM, but I recommend you get it directly from his Github.  The one on NPM seems to be broken on newer Android devices.  The issue  is that Google now requires ACCESS_FINE_LOCATION permission if you are using bluetooth, and the one on NPM is older and hasn't been updated to request this permission.  But that's not really what this post is about.

My app is essentially a remote control for a light.   All I am trying to do is communicate over bluetooth when the app is in the foreground.  However, the Don Coleman plugin demands the BACKGROUND_LOCATION permission (presumably this would be required if I was trying to continue to send/receive data notifications while the app is in the background).  The problem is that this permission comes with some fairly hefty declaration requirements if you are trying to get your app into the Play Store.  For example, you have to make a video demonstrating the feature in your app that makes use of this functionality.  As I stated earlier, I have no such feature so it will be impossible for me to get my app to pass review.  

The only solution I could see was to fork Don's plugin and remove the BACKGROUND_LOCATION permission.  So far it seems to work.  If you have a similar problem, perhaps you can benefit from this version of the plugin as well.  A couple of things:

  1. In case it doesn't go without saying, if your app is in the background, you will not be able to do Bluetooth communication using this version of the plugin.
  2. I have only made changes to the Android side. I don't know if the iOS side still somehow requests BACKGROUND_LOCATION permission.  If it does, I will ultimately need to address that as well since my app is going to be available for both platforms.

So here it is: High Tech Harmony's cordova-plugin-ble-central without BACKGROUND_LOCATION

To use it, go to your Cordova build folder of your project and do the following:

(only if you have Don's plugin already)
cordova plugin remove cordova-plugin-ble-central 

cordova plugin add

cordova clean android

cordova build android

Friday, June 19, 2020

Read this if your DJI Spark is "Hopping" (erratic behavior)

My DJI drone has had this issue with my DJI Spark where, when I try to take off, it will  sort of hover, but then "hop" up and down.  It heads toward the ground and then seems to recover.  When it is heading toward the ground, the obstacle avoidance alert is going off.

Searching the internet, I found forums littered with a gazillion about "Fly Away" issues. I do NOT consider this is a FLYAWAY issue.  It seems like there are so many issues that are blanket described as "Fly Away", and while I suppose it is possible you could lose your drone as a result of this, it is most likely just going to hit the ground straight below, worst case.  I can technically fly it around, but it is fighting me the whole time.  The obstacle avoidance alert is intermittently on and off. The drone will ascend okay, but it descends VERY slowly (which is kind of scary if you get any sort of real altitude).  Also, at random it will just start heading vertically downward even though I didn't command it to.  Again the obstacle avoidance sensor beeping when the problem is occurring. Sadly I was not able to get this anomaly on video for a better description.

This issue cropped up seemingly out of nowhere. One day it flew fine, I put it away, next season I took it out, and it flew like crap.  I could occasionally get a problem-free flight out of it, but I never had any real confidence in it.  So for the last 2 years, I have been basically not flying my DJI Spark drone, due to this weird behavior.

After months of pondering (and letting my warranty lapse), I contacted DJI.  I described the issue and they recommended I send it in  to DJI for paid repair.  On the repair estimate, it said that the drone had crashed:
"After carrying out the damage assessment, we found that the unit has physical impact damage, unfortunately the damage that is not caused by product malfunction is Non- warranty repair; We'll either repair it or replace it with a product that's new or equivalent to new in both performance and reliability after payment has been received. For more information, please visit ( - DJI North America"
The bit about impact damage was BS. I am the only one who has ever flown this thing, and I baby it.  The most I have ever done is buzzed a wall.  It has never impacted anything.  My guess is this is what they write on anything that gets shipped to them that doesn't fall under "warranty repair".  But I had no other viable options so I told them just to go ahead.

Weeks and $150 + shipping later, I got my drone back with a slip saying they replaced the vision system.  And lo, it did fly correctly... For a few (gentle!) test flights.  Then one day I took it out to fly and it started doing the hopping thing again.  I tore my hair out, ready to scrap this noisy money pit.  One last time I refocused my Google search terms to include "hopping", and that's when I hit the jackpot.  I found a handful of postings on DJI's forum of people who seemed to have the exact same problem. While there were a ton of the usual unhelpful responses (i.e. "Get rid of it! Sell it to me!"), I saw a couple of postings saying that the issue was solved by replacing the props. At first I just wrote this off.  Why would props make any sort of difference as to how the craft would fly (other than, maybe, FLY or NOT FLY)?  Okay maybe "FLY but be LOUD"...  But this post got my attention.  after a while, I realized I had the following options:
  1. Throw away drone?
  2. Buy a full set of genuine DJI props, and then throw away drone after proving it makes no difference.
So I did the latter.

And it worked.


So naturally I now have some new questions.  How the hell is this behavior really caused?  One idea I had is that excess prop vibration somehow causes the obstacle avoidance sensor to falsely trip. 

Then there is the issue of the earlier "repair".  This was obviously the problem all along. It seems that DJI was more than happy to take my money to "fix" the craft, when the only issue was the props.  Think about it, there must be thousands of people who have experienced this problem, and I'm sure DJI customer service has enough data to recognize this as being a prop issue right off.  But either they didn't, or they chose not to tell me.  I would give them the benefit of the doubt, but then they did tell me they found "crash damage".

Anyway, lesson learned.  If your unit is out of warranty, think twice before sending it in to DJI for repair.  The only guarantee appears to be that you will pay them a hefty repair fee.

Tuesday, May 19, 2020

Defuddling JamKazam Vol. 2: Don't believe everything you read

I feel that one of the most important things to get out of the way are the truths of many, many claims and misnomers that are floating around on social media - no doubt a common issue with very rapid adoption of a complex tool.  We'll do this style.  Here are some, in no particular order:

Claim: "You Can't Use WiFi with JamKazam"
Status: Partly true
Well, you CAN use WiFi.  But it's better if you use an ethernet cable.  For starters, the nature of WiFi is that it tends to deliver packets via different physical transmission paths, which causes them to arrive in the wrong order.  On the other side, these packets have to be queuing them into a buffer (temporary holding tank for data), and reassembled into a contiguous stream.  Then they are released for you to hear. Ethernet cables require less of this action to be required, so JamKazam uses a smaller buffer, and therefore you end up with lower latency.  Now consider that most people have total shit WiFi setups at their homes, and using WiFi becomes an even worse choice.  Bottom line is, use an ethernet cable.

Claim: "You need the best internet plan available"
Status: Very unlikely
JamKazam is, contrary to what seems to be a rampant believe, not very bandwidth intensive.  It is the equivalent of a video conference call.  If someone is using solid audio gear, and having problems with latency or "choppiness" (which is a very vague description), it is most likely their computer or networking gear that is at fault.  It is very likely that the computer or router is on the edge of what it can handle in terms of load.

Claim: (Windows user) "You need to download ASIO4ALL"
Status: Very unlikely
Your audio interface hopefully has a native ASIO driver available from the manufacturer's website. This is what you should use.  If they don't, you can try ASIO4ALL, which is a third-party, lower latency wrapper for Windows' standard (high latency) WDM drivers. In some cases it works, but in most cases it doesn't seem to work very well.  It is definitely a second choice.

Claim: "A Chromebook is a viable cheap laptop option"
Status: False
While a Chromebook is, in fact, a laptop, Even if the processor power was enough, it does not run Windows or MacOS, which as of this writing are the only supported operating systems.  Chromebooks run ChromeOS, so, no go. If you are looking for the cheapest machine you can buy new that still has enough power, it's probably going to be a mini desktop computer, like the Intel NUC (bring your own monitor/keyboard/mouse).

Claim: "You need USB 3 (aka USB 3.1) ports"
Status: False
USB 2.0 max throughput = 480 Mbps
Even if you run 192KHz @ 24 bits < 10 Mbps
The latency of USB 3 is not improved over USB 2.0.
Even if you are using a 100 Mbps USB Network Adapter, JamKazam will not come close to saturating your connection.

Claim: "You need a (quad-core Intel Core i7 or other crazy CPU) if you want JamKazam to work as well as it can."
Status: False
You do need a real bit of processor power.  Practically speaking, I'd say, minimum dual core Core i3. Preferably a quad core Core i5 or better.  But to be fair, the amount of peak processor power you actually need for JK varies with how you are using it.  I've actually tested it with a Core 2 Duo from 2011, and it was "enough processor power to have a jam session".  Just barely.  As you add musicians, and use features like the video stream or Jam Tracks, that is when you will exceed your processor power, and when that happens, the result is NOT pretty.  Basically it's a sea of loud sparks and crackles in yours and everyone else's headphones.

Claim: "Turn off your video, it adds latency"
Status: Partially True
The video can have no impact, or a lot of impact, depending on a lot of factors.  The specific claim that it adds latency seems to be a little bit of a generalization, as though latency is the only evil one experiences on JK.  In fact, what it really does is add a ton more data that JamKazam has to coordinate which, if you or anyone in the session is on the edge, will very definitely result in a variety of problems - latency being only one.  Playing with the video on is more fun, but it's best to assume it is going to be a luxury and not a guarantee.  If everyone is getting good results, then congrats and enjoy - but it seems to vary with each session and the participants.

More parts coming soon!