Use of SignalScope as a quick and dirty general purpose scope

kevlar

I'm not an audio type but a EE trying to use the iPhone for quick and dirty instrumentation and demos.  It seems like a very handy tool for this kind of application.

Specifically for my case, I'm trying to see a pulse train that repeats every 1.3 seconds or so from some equipment.  But I'm having trouble manipulating the time axis in the waveform view, the longest I can get is about 200 ms.  What is the longest that it can be set to?  Could I convince you to increase it  :-) I don't mind seeing fewer samples.

Also a roll mode (signal slides from left to right as time passes) would be really cool looking on the iPhone.  Just a suggestion.

P.S. I'm seeing issues with running SignalScope while headphones (w/o mic) are plugged in.  Lot's of "nan"s on the time scale (nan=Not A Number).  I'd guess its probably from a divide by zero somewhere in the application.

benfaber

kevlar,

The longest time scale is indeed 20 ms/div, for a total of 200 ms. Longer time scales are planned for a future release (roll mode has already been implemented--see SignalScope on the Mac--but the iPhone requires further optimization of the code before it goes public). I would recommend using the Normal mode triggering to view individual pulses in your pulse train.

Regarding the nan values, can you give a repeatable series of steps to reproduce the problem?

Ben

kevlar

Sure,
  On an older iPhone (not 3G)

1) Setup SignalScope for audio device = Audio input, (Play Through is off)
2) Tap home button to goto iPhone home
3) Plug in headset (normal w/ no mic)
4) Tap SignalScope to start Application
5) Application comes up with "nan" as tick values on the time scale.  Also, on "Input Channels" I see a blank screen with nothing listed.

This happens with two different sets of headphones (one from Apple, one from Bose)

Hope this helps.

P.S.  Thanks for the advice about normal mode triggering.  Any rough idea when longer timescales might be available?  :-)

benfaber

When you are using standard headphones, the only audio input available to SignalScope would be the built-in mic. However, the audio input is automatically selected by the iPhone, and sometimes when you connect headphones, the iPhone doesn't even select the built-in mic. In this case, no input channels show up, because there are none, and the time scales shows "nan" because the current sample rate is effectively zero. A zero sample rate would indicate an infinite sample period, which is where the divide-by-zero comes from. At this point, all we can do is improve the labeling to tell that user that no input device is selected.

I have successfully used the headphone output, with iPod touch earbuds and other adapter cables, simultaneously with the built-in mic. It seems to be a matter of plugging the headphones in at the right time so the iPhone will select the built-in mic as an input. You might want to try working with your iPhone headset and then switching to the headphones to see what happens.

Ben

medengineer

Please don't flame me, I am a metallurgist, not an EE. 

I have just started playing around with trying to use the Signalscope
as a real scope around our lab.  Any help with the right
circuit to be able to input DC and AV voltages would be appreciated. 

Getting some initial data I hacked a mic cable and I am getting 2.7V DC output to the mic. 
At 4M ohms it seems to trigger the play/pause function of iTunes.  I have not gone above
that yet to see what impedance triggers that function.

Any suggestion on the circuit, without blowing up any chips in my phone. 

I can spin a circuit board once I know what to build.

Frequency And Sulphur

medengineer wrote Please don't flame me, I am a metallurgist, not an EE. 

I have just started playing around with trying to use the Signalscope
as a real scope around our lab.  Any help with the right
circuit to be able to input DC and AV voltages would be appreciated. 

Getting some initial data I hacked a mic cable and I am getting 2.7V DC output to the mic. 
At 4M ohms it seems to trigger the play/pause function of iTunes.  I have not gone above
that yet to see what impedance triggers that function.

Any suggestion on the circuit, without blowing up any chips in my phone. 

I can spin a circuit board once I know what to build.

I wish I could help, but I wanted to show a bit of respect for what you're doing.  I wish I had the skills to start playing around and experimenting like that.  So, I'm far from flaming, but instead respecting. 

benfaber

medengineer,

On the few iPhones I have measured, it appears that the headset mic input accommodates a sinusoid of nearly 10 mV rms. Anything above 14 or 15 mV will be clipped.

You won't be able to analyze DC voltages with SignalScope, using the iPhone's headset input.

Ben