Skip to toolbar

Posts tagged: iPhone

The Good Recorder iPhone app

14 February 2014

Good news! Today sees the launch of the project’s first ever app – The Good Recorder. Absolutely free and available now via the iTunes store, or click here.

What is The Good Recorder?

Screenshot 2014-02-14 15.39.36The Good Recorder is a sound recording app (currently only for iOS 7 devices) designed to help users achieve high quality audio recordings by monitoring for common recording errors and providing feedback about them. Currently the app incorporates findings and algorithms from our previous work with wind noise. The plan is to further develop the app with auto-detection of handling noise and distortion as our research in these areas progresses. Read more…..

Wind noise recordings – Validating a Wind Noise Detector

19 November 2013
Array of microphones used to capture wind noise

Array of microphones used to capture wind noise

After developing a microphone wind noise detector which is trained on simulated examples of wind noise (see my ICME conference paper),  rigorous proof of the algorithm’s success (or failure!) is required.  In fact the reviewers of this aforementioned paper suggested this.  To that aim I packed a car full with microphone stands, cables, preamps, and a number of recording devices and set off to collect some examples of wind noise.

The requirement for the location to collected these examples is that there is very low levels of background noise.  I found a location up upon Rivington Pike, north of Manchester.  There was a road which was closed for repair, ideal! as it means no traffic.  After a couple of false starts and some help from a kindly local man, I found a good location with, no road, rail, urban or air traffic noise.  I located a place away from trees, which can create a surprisingly loud level of rustling noise and set my microphones up.

Array of microphones used to capture wind noise

Array of microphones used to capture wind noise

Array of microphones used to capture wind noise

Array of microphones used to capture wind noise

I was using an Edirol R-44 to capture four channel of audio onto an SD card at 44.1 kHz sampling frequency.  I set up two measurement microphones, one with a wind shield, a sure SM58 dynamic microphone, a zoom H2 recorder and an iPhone taped to a stand.  Though one of my microphones sported a windshield, due to the particularly blustery conditions with 20 mph winds, wind noise was present on all recordings.  This made it all the more important that the background sound level was as low as possible as I intend to compute the wind noise level, assuming that the background noise level is negligible.

recording device used, 4 channels
recording device used, 4 channels
Calibration was carried out on the two measurement microphones by placing a calibrator on each, playing a 1 kHz tone at around 94dB and recording these sounds.  Now I can calibrate my recordings so that I can present data in the actual sound pressure levels recorded for these two microphones.  To calibrate the other devices is a little tricky, but a 1 kHz tone was played back over a loudspeaker at approx 1m distance and recorded on all devices simultaneously.  As I can now know the true sound pressure level from the calibrated measurement microphones, i can also compute the true level of this tone relative to the calibrated recordings and using this information calibrate the other microphones to within a few decibels.  To remove wind noise a narrow band-pass filter is applied centered on 1 kHz. Clearly there is some error due to the location of the microphones and and residual wind noise present within the pass-band, but this is not a significant problem.
Several hours later, and I am rather cold but have the data, now back to Salford set up my validation procedure.

Measuring a Portable Audio Device’s Response to Excessive Sound Levels

5 July 2013

One of the major issues that was raised from our survey is when a device gets overloaded when presented with excessive sound levels.  A common issue is recording the audio at a rock concert where the device is simply unable to cope with the sound pressure levels it is exposed to.  In order to understand how devices respond when placed in this situation an experiment was designed to attempt to capture the kind of non-linear behaviours that may occur.

excessive spl

The full report is accessible here:

http://usir.salford.ac.uk/29371/1/Distortion_of_Portable_Audio_Devices.pdf

The performance of a series of common devices was quantified including the; Cannon 550D, Edirol r44, Neumann U87ai via Focusrite 2i4, Shure SM57 via Focusrite 2i4, Zoom H2, Zoom H4, Google Nexus 4, Apple Iphone and a Sony camcorder (vx2000).

Most devices have some form of dynamic gain control to prevent signal clipping, but the implementations clearly differ considerably.  Some devices have many  settings for different situations indicting that there is no one particular method suitable for all cases.  The attack and release times of the measured systems range from 5 to 17 ms and 30 and 400 ms respectively.  Some devices may also demonstrate a nonlinear gain curve with no attack or release but which try to limit audible distortion by using a compression ratio of between 1.4 and 10.  While other systems have no protection and when presented with excessive sound levels will exhibit hard clipping.

 

What you told us about recording audio with mobile phones (and what your phone says about you…).

17 October 2012

Early on in the project we put a survey on the web to ask questions about where and how people make audio recordings, and what they make recordings of. We also wanted to know what issues people reported as having the biggest impact on audio quality in their recordings (you can still take part in the survey by clicking here, it only takes a couple of minutes). Three months on, over 150 people have taken part and we have begun to analyse the data. One of many interesting trends to emerge is a series of differences between iPhones and other brands of mobile. Read more…..