Re: MD: Listening to compression
Anthony Lalande wrote... While I'm not sure what exactly AGC means, I have heard somewhere that ears are able to focus and lose focus of sounds and noise in a dynamic way. I was once advised to buy full-ear headphones as opposed to ear-buds because sounds hitting the outer part of the ear actually help the inside of the ear adjust to what it's listening to, and can help prevent damage in a way that ear-buds cannot. Does anyone know enough about the working of the human ear to validate or deny this? This sounds like rubbish to me. The tympanic membrane (ear drum) is linked to the ossicular chain (hammer, anvil and stirrup in plain English) and they convert sound energy into mechanical energy before it gets passed to the cochlear in the inner ear to be converted into electrical signals which are processed by the brain. The shape of outer ear (ie the bits stuck to the side of your head) is good for capturing sound and directing it down the ear canal, which acts as a resonating chamber, but the ear itself cannot simply 'adjust' to the sound that 'hits' it. Remember, the pupil of the eye can contract and expand to allow in less / more light, but there is no equivalent function in the ear. If the amplitude of the sound goes above the loudness discomfort level (LDL) for a period of time you may suffer temporary or even permanent hearing loss. This can occur in many ways, commonly in the form of hairline cracks in the tympanic membrane; or hair cells in the cochlear which carry on 'firing' without stimulus (usually resulting in tinnitus). Meanwhile, its the brain's job to process the electrical signals fed to it by the inner ear. Although the cochlear does do an astonishingly complex job, the brain, *not* the ear does the processing, and as I've already written, the ear can't really adjust itself (only *you* can, by putting your hands over your ears) Oh yes, AGC is Automatic Gain Control. This is a technique used on some MiniDisc recorders and hearing aids as a way of 'squashing' peaks in the sound being recorded / processed. However, it is a relatively coarse method of control, and the use of AGC on hearing aids can lead to much distortion of speech. Of course, all this isn't to say that you can't damage your hearing with earbuds. They provide less isolation than full-ear headphones, which makes it tempting to increase the volume in order to overcome extraneous noise. So yes, earbuds may indeed be (potentially) worse for the health of your ears, but not for the reasons you were told. Hope that answers everything. Robin. Robin Landy Manchester University [EMAIL PROTECTED] Mobile: 07968 775304 --- - To stop getting this list send a message containing just the word "unsubscribe" to [EMAIL PROTECTED]
MD: Listening to compression
From: "Eric Woudenberg, Minidisc.org Editor" Hi Howard, Howdy! "Howard Chu" [EMAIL PROTECTED] writes: Use a bit-accurate CD-ROM drive and rip an audio file off a CD onto your PC. This is the original, "perfect" data source. Encode it as an MP3 using any bitrate you care to test, and then decode it back to PCM format. If you invert this file and add it to the original WAV file, the result will be the difference between the two signals, the "error" between the original and the MP3 file. You can listen to this error signal and measure its RMS value, to give both subjective and objective quality measures. As you state, the error signal (or its square) is a valid quantitative measure of the loss incurred by the coder. But even though you can listen to the error as an audio signal, it isn't a valid subjective measure since the error in question is never normally heard standalone, rather only as a deviation in the presence of the full audio signal (and is hence being masked by the signal). I guess that's true, perhaps it's real value is in objective measurement. Certainly it reveals a lot though, such as how much high-frequency signal is thrown away by a particular encoding. It can also highlight artifacts that you can identify once you know they're present, such as the pre-transient noise in the Microsoft Audio format. (Something that MP3 and ATRAC don't suffer from... See http://www.real.com/msaudio/ for the story) Once you know it's there, your subjective perception will be more attuned to the flaws... General aside on subjective evaluation of coders: Automatic measurement of the subjective quality of perceptual encoders is a current research topic and involves some degree of modeling the human auditory system in the measurement phase. I've always thought that this was a bit of a tail-chasing problem, since if you've got a model with superior accuracy in the measurement system, why not use that model in the encoder as well and further reduce the perceptual loss? Heh. You can't develop a high-precision model for human perception; you are inherently forced to approximate since every person is different... But I suppose a decent neural-net could duplicate a lot of the behavior. It works for vision, and the nervous system's basic characteristics are consistent throughout. (It can be said that all five of the human senses operate in "block floating" mode. Overall you can sense a very wide dynamic range of signals, but you only have a small window on that range at any given time. E.g., you can see details in dim light, and you can see well in bright light, but a sudden transition from dim to bright or vice versa leaves you blinded: the input has exceeded your dynamic range window, your visual system has "clipped" ... Hearing is not much different, except that you don't have the equivalent of the AGC that the eyes (pupil, iris) have.) An automatatic system that produced reliable subjective scores for audio and speech coders would be a great boon to developers, since they could save the considerable time and money spent using human subjects. (They could also save wear and tear on their subjects -- at Lucent I would occasionally participate in experimental evaluations of cell-phone speech coder samples (as a favor to colleagues). I found it to be painstaking and frequently frustrating work). Heh. The worst I ever had to do was sit at a newly designed computer desk and give opinions on its ergonomic efficiency... -- Howard - To stop getting this list send a message containing just the word "unsubscribe" to [EMAIL PROTECTED]
Re: MD: Listening to compression
... you can see details in dim light, and you can see well in bright light, but a sudden transition from dim to bright or vice versa leaves you blinded: the input has exceeded your dynamic range window, your visual system has "clipped" ... Hearing is not much different, except that you don't have the equivalent of the AGC that the eyes (pupil, iris) have.) While I'm not sure what exactly AGC means, I have heard somewhere that ears are able to focus and lose focus of sounds and noise in a dynamic way. I was once advised to buy full-ear headphones as opposed to ear-buds because sounds hitting the outer part of the ear actually help the inside of the ear adjust to what it's listening to, and can help prevent damage in a way that ear-buds cannot. Does anyone know enough about the working of the human ear to validate or deny this? - To stop getting this list send a message containing just the word "unsubscribe" to [EMAIL PROTECTED]
