Subject: Re: - "Birdies" From: Bob Masta <audio@xxxxxxxx> Date: Wed, 15 Mar 2006 09:30:31 -0500Hi, Maxime. I'm not sure exactly what you are looking for, and I don't have any references to provide. But if you are looking for a perceptual description, here's what I know: "Birdies" are little whistling sounds that are related to the program material, but are not harmonics of it. They used to be a serious problem in sigma-delta converters, which compare the input signal to a reconstruction of the output signal, and generate a "higher than" or "lower than" response on each sample. That 1-bit stream is then used to create the reconstruction for the comparison (and the eventual output). Nowadays, this is all done at very high sample rates and then ultimately converted down to a nominal rate, and the reconstruction processing is very sophisticated. But simple early schemes had interactions between the input signal and the sample frequency that caused "birdies" at sum and/or difference frequencies. The birdies might be only 40 dB down, but even if they were much softer than that they were clearly audible, especially on sparse program material like simple sine waves, flutes, etc, since they appeared in non-harmonic locations and were not masked by the program itself. They also often had the annoying habit of sweeping in the opposite direction to a sweep in the signal frequency, which made them really obvious. Hope that helps! Best regards, Bob Masta -------------------------------------------------- On 14 Mar 2006 at 20:56, Maxime Leroy wrote: > Dear list, > > As i am new to the list i'd like to say hello to everyone, and also ask a question. > > First let me quickly introduce my field of research, i'm currently conducting some research about objective evaluation techniques for low bit-rate coded music samples, but in a different way than PEAQ technique > as i intend not to use the original sample for comparison. > > As i was investigating known coders artifacts i came across one called "birdies effect" and it seems to me > it is quite important, so i researched a bit for litterature, but most of it deals with "birdies effect" from a > coding point of view and i was not abble to find a perceptual approach of the phenonmenon. > > So finally i would be gratefull if anyone could point me toward some good references, some that could > explain the perceptual impact of such a degradation of the spectrum. > > > I hope that was clear enough, many thanks in advance, > > Maxime > > >