iSTI Frequently Asked Questions
We have collected some frequently asked questions (and answers) regarding iSTI. If you are considering calling in support through iSTI@embeddedacoustics.com, please have a look here first.
Generating the STIPA test signal
Q: The sound level of the STIPA test signal seems to differ between sessions. Why is this?
A: The output sound level is controlled by the volume control buttons of your iOS device (iPhone, iPad, iPod). If you change the volume setting between measuring sessions, then the output level will be different. This means that you can use the volume controls to adjust the output level according to your needs.
Q: Is the STIPA test signal compatible with other STI measuring devices?
A: The STIPA test signal of iSTI complies with the 4th edition of IEC 60268-16. If you have another measuring device that is also 4th-edition compliant, then you can use the signal generated by iSTI with this device - and vice versa. Also, iSTI will correctly analyze the test signal by various 3rd edition generators.
Q: I want to connect a sound system to my iPhone for generating the test signal, but I still want to measure with my internal microphone. But as soon as I connect a cable to the headset jack, the internal microphone of my iPhone is switched off. How do I prevent this from happening?
A: Your iPhone decides which device to use for its audio input (internal or external) based on your iPhone's connections. If you connect with a three-ring 3.5mm TRRS connector, then your iPhone may expect an external microphone. Try connecting through a two-ring 3.5mm TRS connector (standard small headphone jack).
Carrying out STI measurements
Q: The current STI value is sometimes shown in white, sometimes in black. Why is this?
A: If the STI value turns black, this means that iSTI suspects that the current measurement may be unstable or invalid. This may happen while the measurement is still running; if so, this is not a problem. If the STI value still shows as black at the end of your measurement, you may want to duplicate your measurement, check your transmission chain and/or inspect the measurement details.
Q: I get high STI values even when no test signal is received. Is this an error?
A: If no test signal is received, then iSTI (and other STI measuring devices) still attempt to find the typical modulations featured by the STIPA signal in whatever signal is currently presented to the input. The way the STI model works, this leads to high STI levels - not to an STI of zero as one might expect. Embedded Acoustics is developing algorithms to reliably create warnings in this type of situation.
Q: The signal level in the 125Hz band seems low, and the signal-to-noise ratio seems poor in this band. Why is this?
A: Apple applies highpass filtering to the internal microphone as well as the headset jack. This filtering is implemented at such a low level of the iOS architecture that developers cannot change this. The most convenient workaround at the moment is using the analog input lines on the dock connector, but these are not used on iPhone 4 and iPad. In other words, if you are using iPhone 4 or iPad, then you have to live with a reduced dynamic range in the 125Hz band until digital microphone amplifiers become available for the dock connector. Valid measurements can still be done; you just need to be careful that the signal level in the 125Hz band is high enough to compete with the iPhone's internal noise.
Q: Why can't I store a measurement before the 20-second period indicated by the progress bar runs out? I want to do shorter measurements.
A: The standard says that the minimum duration of the STIPA test signal is 20 seconds. Shorter measurements are less reliable and show a larger statistical spread.
Storing and exporting data
Q: Is storing and exporting data supported in iSTI Lite?
A: No. You need the Professional or Standard edition for this.
Q: Can I throw away a measurement AFTER it has been stored?
A: Throwing away single measurements is not allowed by iSTI Professional, in order to comply with traceability requirements imposed by the various quality assurance standards in use with our customers. Once you store the measurement, it is permanent.
Q: Is there another way to identify a measurement than by its number?
A: Within a project, each measurement is assigned a number only - there is no name or other identifier, since it would be impractical to prompt for this information after each individual measurement.
Q: Can I export my project in any other way than through a CSV file by email?
A: Currently, no.
Q: How do I delete data that is no longer needed?
A: You can delete projects from the project browser. Swipe on the name of the project you wish to delete to reveal a "delete" button.
Sound level calibrations
Q: How do I change a calibration once it has been stored?
A: Simply delete the calibration and generate a new one with the same identifier and settings
Q: How do I delete calibrations?
A: In the calibration browser, swipe the calibration you wish to delete, in order to reveal the delete button. Deletion is only allowed if all projects linked to the calibration have been deleted as well. In other words, you cannot delete a calibration that is still used in measurements projects.
Sample rate ratio calibrations
Q: Why do I need to calibrate my sample rate? My old device did not require such a calibration.
A: Sample rate calibrations are not required by the standard, but still needed to obtain reliable results. Surprisingly enough, many CD players feature sample rates that are much higher or lower than the nominal sample rate. If you use such a CD player without calibrating its sample rate, then your STI measurements will be consistently too low -- whether you are using iSTI or any other STIPA measuring solution. However, only iSTI currently offers a solution for this problem.
Q: When calibrating to the internal STIPA generator, I get a deviation of 0.2. How is this possible? This should be 0.
A: Yes, a back-to-back sample rate ratio calibration with the internal generator should result in a deviation of 0. However, the measurement errors associated with the calibration algorithm may result in small deviations. This is not an issue for the reliability of your measurements; a deviation of 0.2 will not affect your measurement.
Versions, updates and compatibility
Q: Which versions of iOS are supported?
A: iSTI is supported with iOS 4.x; it may work on older iOS versions, but we do not test older configurations.
Q: Which iOS devices are supported?
A: Supported devices are iPhone 3GS and 4 and iPod Touch 3rd gen. or higher. On older devices (iTouch 2nd gen. and iPhone 3G), which lack sufficient processing power for STI analysis, it may be possible to run iSTI as a standalone STIPA generator. This is not guaranteed or supported.
Q: Is iSTI Professional compatible with iPad?
A: Yes and no - it will run, but the app does not take advantage of the added capabilities (such as screen real estate) offered by the iPad.
Q: Will I have to pay for updates of iSTI Professional?
A: No, all customers are eligible for free updates through the app store.
Q: I plan to use additional hardware (measuring microphone, connection cables, adapters) with iSTI. Which types are compatible?
A: Since all interfacing with these devices is currently analog, compatibility is not an issue. You will need to take care the hardware that you select is compatible with your iOS device. Please note that analog line inputs on the dock connector are not supported on iPhone 4 and iPad. More information is found in the cables and microphones section of this website.
Q: Are you planning to release iSTI on other platforms than iOS?
A: We have no immediate plans in this direction. In order for a hardware platform to qualify as the basis for a measurement device, it is important that the software and the hardware environment remain stable and predictable over time. There is only a limited number of hardware (audio) configurations to be prepared for when running an app under iOS; this is not the same for other mobile operating systems. This would make it much more difficult to guarantee the quality of measurements.
Q: Can you explain the differences between the different editions of iSTI (Professional, Standard and Lite)?
A: Please refer to the table presented on this page