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Posts tagged: distortion

‘Pipgate’, Radio Four’s Pips, what happened?

22 July 2014

‘The Pips’ are series six of short tone bursts transmitted on Radio 4, they are known as the Greenwich time signal and are intended to accurately mark the start of the hour. They have been transmitted since 1924, and originate from an atomic clock.

On the 21st July 2014 a listener wrote to the Radio 4 programme ‘pm’ to ask why the pips had been changed. The programme played the offending pips and the originals. (here is a link to the program, the item is at 28m 31s http://www.bbc.co.uk/programmes/b049y9pn)

Here is an ‘old’ pip:

and a ‘new’ pip,

You may think that ‘new’ pip sound harsher, by looking at the wave form and spectra we can begin to understand what has happened. Here are the two waveforms of the pips,

Waveforms of the two pips

Waveforms of the two pips

and the two spectra.

Frequency Spectra of the two pips

We can see from the spectra there are additional lines in the spectrum known as harmonics, comparing the two waveforms we can see that the ‘new’ pips appear to be similar to the older ones except that the peaks of the waveform have been flattened or ‘Clipped’ a little.

This clipping is a form of distortion, it occurs when the gain applied to the signal is to great or if there is a fault in a preamp and the amplifier is no longer able to properly replicate the signal at the input.  We can clearly hear the difference between the two signals and according to the concerned listener (and his cat) it has a very negative impact on the sound quality.  Denis Nolan, the network manager for radio 4, identified the fault as being due to a particular desk the signal was going through.

In our project we are writing an algorithm to perform a similar function to the upset listener, we don’t mean that our algorithm will write pithy letters to Eddie Mair, we want to build an algorithm to automatically detect when something like this has gone wrong and the sound is being distorted.  The way we are going about this is to simulate all sorts of types of faults on many different types of sounds, and then see if we can look for ‘features’ of the audio which seem to be very dependant on theses faults.  We can then build automated systems that look for occurrences of these features to locate them, and try and estimate how bad the error is from the features themselves.

Institute of Acoustics: Sound Recording Techniques – Our presentation.

26 March 2014

Today (Wed 26th March, 2014) Trevor is presenting some of our recent findings on the effect of distortion on perceived quality in music, as part of the Institute of Acoustics’ Sound Recording Techniques event.

Our talk is titled “How distortion affects the perceived quality of music: Psychoacoustic experiments” and slides for it can be found here (PowerPoint slides in .pdf format).

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.