The GedLee metric - a new way to measure distortion?
Distortion measurements do not always tell the whole truth about what equipment sounds like. Should we pay more attention to some distortion products than others?
Hi-Fi News a few months ago had an interesting article on the measurement of distortion and the correlation of such measurements with subjective experience.
Distortion occurs in all electronic and digital audio systems. The shape of the waveform of a signal is altered leading to the production of new frequencies from the frequencies present in the signal.
Distortion is measured by inputting a sine wave, which is the simplest signal consisting of only one frequency, into the equipment under test. If this is 1 kHz, then the harmonic distortion products generated will be 2 kHz, 3 kHz, 4 kHz... all the whole-number multiples of 1 kHz up to and beyond the limits of human hearing. The level of the distortion components is then expressed as a percentage of the whole signal. A figure of 0.1% is considered adequate, but even lower figures are desirable.
But there is a problem. It is possible for a piece of equipment to measure very well, yet still not sound entirely clean. The suspicion is that not all distortion products are equally offensive to the ear.
Listening tests conducted by a number of researchers tend to show that higher-order harmonics are more troublesome than lower order such as 2nd, 3rd etc. This implies that it would be helpful to weight the individual harmonics in the measurement according to their nuisance value.
This is not new science. As early as 1937, according to the Hi-Fi News article, the Radio Manufacturers' Association of America suggested that higher order harmonics should be progressively more heavily weighted. The BBC refined this in 1950 and again in 1961.
The problem was however that in these early days of electronics it was extremely difficult to measure the levels of the various harmonics accurately. Even today, this is not a standard facility on most test equipment.
However, Earl Geddes and Lidia Lee have suggested that not only is this essential, but that distortion products at low levels also be more heavily weighted, and have come up with the 'GedLee metric' which is basically a formula for calculating a figure from measurements that will correspond closely to the subjective experience of distortion.
This is a worthy enterprise. If there could be one figure that all manufacturers could quote and could be relied upon, it would be very much easier to make comparisons between different equipment.
However, the chances of such a common system being adopted are not good. For one thing, manufacturers whose current products do not fare well under the GedLee metric will not be inclined to use it. For another, the GedLee metric is not the 'god equation' of high fidelity. There could well be better ways of measuring the subjective effect of distortion.
Perhaps the ultimate solution would be to publish tables of harmonic spectra, then users could calculate their own figures based on whatever metric they think is best for their purposes.
“It isn't about equipment and software -it's all about knowing how to use it”
An RP reader asks: "While recording how do I hear my vocals and music through my headphones and studio speakers at the same time?" Read more... 
Your mic might be cardioid, omnidirectional, figure-of-eight or hypercardioid, but what difference will it make to your recording? Read more... 
Made any good mixes lately? Who said they were good? Only you? Hmmm... Read more... 
If you buy CDs at flea markets or car boot sales, you may be buying counterfeit, pirate or bootleg products. Does this matter? What do you need to know? Read more... 
One of the most difficult things in popular music is to find a great groove for a song, particularly for a band playing non-sequenced instruments. But there is so much to think about, where do you start? Read more... 
Some things about Apple's iPad are great. Some things are, er, missing. But how useful is it in the studio right now? Read more... 
Mike Leader of Leader Cinema systems, manufacturers of world class large-scale systems for both music industry and motion picture industry professionals, tells you more about subwoofers than you thought you could possibly want to know... Read more... 
Get your de-essing wrong and either your vocal will still be sibilant, or its high-frequency content will sound mangled and tortured. But how can you get de-essing EXACTLY right? Read more... 
A Record-Producer.com visitor asks whether a good analog-to-digital converter is necessary. Or will any old converter do? Read more... 
An RP reader wants to capture his hihat in high fidelity. So what is the right mic for the job? Read more... 
Take a trio of rhythm guitar, bass and drums. Add a vocal, add some overdubs. Oh, and someone find a lead guitarist for the solo... Read more... 
As Roger Waters of Pink Floyd once said, "It doesn't have to be loud. We just like it loud." Well, some people would like it even louder. Is this possible? Read more... 
When stereo sounds so much better and more lifelike than mono, why would you ever want to record a vocal with a single mic? Read more... 
Whoever designs microphones clearly thinks that they all should be microphone-shaped. But isn't it about time we had something more appropriate to the way we use them? Read more... 
If an offer seems too good to be true, then it's probably a scam. Would you fall for this one? Read more... 
The Neve Melbourn 12-2 mixing console is a classic in miniature, but now quite expensive to buy on the used market. But might this be an alternative? Read more... 
An RP visitor enquires whether the Genelec 8240A with digital inputs and DSP is suitable for the home recording studio. Hmm... Read more... 
How can I get my music up to a level that is acceptable for radio? My music always seems to be a few decibels lower than the accepted level. Read more... 
I am a singer songwriter and producer of my own material. Hardware/Software:- Cubase 3SX, Yamaha 01X DAW controller/MLAN Firewire card, Sontronics Helios Microphone...... Read more... 
Take a 300-year old bass line, a cello player with just seven days' experience, and some highly uncool-looking musicians... and make a recording that is really top of the pops for weird! Read more... - Can a pianist's wrong note played in 1962 be fixed in 2013?
A wrong note that has resonated for more than half a century is fixed - without the aid of a time machine. - Are certain mic/preamp combinations special?
An RP reader finds that a certain combination of microphone and preamp works much better than he expected... - Why haven't you had a Hot 100 hit yet?
Anyone with a home recording studio now has the means to create a hit. So why haven't you yet? - Can guitarists be trusted with their own equipment?
What sounds good to the electric guitar player doesn't necessarily sound good to the audience. A live sound engineer explains... - What is your main concern if your interest is voice over?
So you're setting yourself up in the voice over business. What should you think about before anything else? - Can you print a vinyl record on a 3D printer?
Amazingly, the answer to this question is - almost - yes! - An interesting microphone setup for violinist Nigel Kennedy
If you're miking one of the finest violinists in the world, should you point the mic at the double bass instead? - To mic or not to mic the backline? That is an interesting question raising fascinating further possibilities
An RP reader fears losing cabinet resonances if the backline of his band is miked up through the PA. But perhaps it is innovation in live sound that has been lost over the years. - When working in a new theatre, always find out where the tielines go
Any theatre would have tielines from the stage to the sound control booth. But from one side of the booth's window to the other? - Top producer 'steals' song as part of massive publicity stunt
A major producer has 'stolen' a song from a pair of unknowns. A crime, or normal everyday life in the music business? - If AKG makes a USB microphone, does it mean that the time has come for USB mics?
AKG makes some of the best mics in the world. AKG also makes a USB mic. A great USB mic? - Would you record vocals like this?
Two singers, one microphone. Could it cause an Internet sensation? - Hum - Can you hear it, even if you can't hear it?
If a tree falls in the woods, does it make a sound (if no-one is around to hear it?) If you can't hear hum, then is there no hum? - Preparation for mastering: Don't do any mastering yourself
Are you planning on getting your tracks mastered? Then the first rule of preparation is not to master the tracks yourself. - A hum generator plug-in? We want one!
You can get a plug-in to emulate virtually any analog audio process these days. But what about hum? Where's the plug-in for that? - An inside view of the weirdest recording session ever, at the BBC!
Take a 300-year old bass line, a cello player with just seven days' experience, and some highly uncool-looking musicians... and make a recording that is really top of the pops for weird! - Would you put your head inside a piano? No? Then why place a microphone there? [with video]
![Would you put your head inside a piano? No? Then why place a microphone there? [with video]](http://www.record-producer.com/i2/julia-lezhneva.jpeg "Would you put your head inside a piano? No? Then why place a microphone there? [with video]")
Sometimes unusual microphone placements can capture an interesting sound. In this instance, the sound is completely wrong. - When is a click not a click? When should you fix a click, and when should you leave it alone?
Clicks occur for all kinds of reasons - imprecise editing, random noises, gremlin infestation. But when do you need to do something about them? - Should you make decisions as you record, or keep your options open until later?
There are so many decisions to be made during the process of recording. Should you make them one at a time as you go along, or let them pile up so that you have a mass of decisions to make in the mix? - Is Pro Tools 11 really the 'new standard for audio production'?
Audio production - it's all about the DAW isn't it? Well that's what marketers would have you believe.
Earlier discussion on this topic...
Ian Bell, Northrepps, UK
Measuring harmonic distortion is in principle straightforward. You feed in a sine wave to the amplifier under test. At its output you connect a very sharp notch filter to remove the original sine wave. Anything left is distortion.
In practice it is not so easy. You need to start with a very low distortion sine wave which is itself a non-trivial design exercise. Then you need a very sharp notch filter. If you want to measure to 0.1% distortion then this filter must attenuate the original sine wav by at least 60dB, for 0.01% it is at least 80dB - whilst leaving any harmonics unaffected - another non-trivial design exercise.
Lastly, what is left is not only distortion but noise from the amp. A simple meter will read the combined result of borh. If your noise is at -80dBu and your distortion is at 0.01% they will be indistinguishable by this method. So in a more sophisticated set up you would do an FFT on the output so you could pick out and measure individual harmonics.
Friday November 24, 2006
Norris Turney, Southfield, US
Since we're talking about Distortion, I really would like to know just how do you measure distortion
in equpment, and what type of meter would you use?, and what type of scale?.
Thursday November 23, 2006
Ian Bell, Northrepps, UK
My apologies if I offended, that was not my intention. In Geddes presentation (available at his web site) he says THD and IM distortion are both inadequate methods of relating measurements to perceived distortion. His central premise is that it is the transfer function of the amplifier that is important and he proposes a metric based upon this. He then conducted some listening tests using various transfer functions which demonstrated that perceived distortion was poorly correlated with THD and IM distortion but correlated very strongly with his metric. However, he does not say how to measure this metric or to derive it from other measurements like THD or IM distortion.
Ian
Wednesday November 22, 2006
David Mellor, Record-Producer.com
Now now, settle down please.
The GedLee metric assesses harmonic distortion, not intermodulation distortion.
The article isn't about intermodulation distortion, so intermodulation distortion isn't mentioned.
I don't mind negative comments on articles, or corrections where appropriate. But to say an article is 'half assed' on the basis of something it doesn't even pretend to cover seems a little unreasonable.
Wednesday November 22, 2006
Ian Bell, Northrepps, UK
This article comments only on the distortion caused by a single frequency. I don't see anywhere where it says that intermodulation distortion is important. Sounds like Anton C. is bitter that someone cares enough about this site point out its defects in the hope it might get better.
Ian
Wednesday November 22, 2006
Anton C., Germany
The article is a reasonable comment on the subjective effect of harmonic distortion. I don't see anywhere where it says that intermodulation distortion isn't important too. Sounds like Ian Bell is bitter about something and wants to kill this site that I always found interesting and entertaining.
Wednesday November 22, 2006
Ian Bell, Northrepps, UK
I was refering to the article here, not the one by Geddes et al.
Ian
Tuesday November 21, 2006
Anton C., Germany
Lidia Lee and Earl Geddes have presented their work to the Audio Engineering Society - http://www.aes.org/events/115/papers/SessionF.cfm
Maybe 'half assed' Ian Bell is more clever than they.
Tuesday November 21, 2006
Ian Bell, Northrepps, UK
Oh dear, another half assed technical article. Whatever gives people the idea that measuring harmonic distortion is the only test of sound quality available. For example, for music, intermodulation distortion is far more audible but is it ever specified or measured - no. Back in the 50s and 60s is was a STANDARD measure for prefessional tape recorder and amplifier performance. And then there is square wave performance - a very simply highly revelaing test but rarely mentioned these days.
The problem is not that equipment can measure well yet not sound entirely clean - the problem is people just don't bother to make a range of performance measurements.
Ian
Tuesday November 21, 2006