Subject: Re: RE : - "Birdies" From: Peter Lennox <P.Lennox@xxxxxxxx> Date: Thu, 16 Mar 2006 14:01:21 +0000the description below was my understanding of birdies. I think there must be documentatino on perceptual effects in AES papers regards ppl Dr. Peter Lennox S.P.A.R.G. Signal Processing Applications Research Group University of Derby http://sparg.derby.ac.uk Int. tel: 1775 >>> Maxime Leroy <m.leroy@xxxxxxxx> 15/03/2006 19:34 >>> Thank you Bob for you comments, "But simple early schemes had interactions between the input signal and the sample frequency that caused "birdies" at sum and/or difference frequencies." I realise now "birdies" might not be exactly what i meant. I will rephrase then, if you ever looked closely at the spectrogram of a sample of music encoded at low bit-rate (20-64Kbps)by either mp3 or AAC codecs, you might have noticed dark spots in some places where it is obvious the energy of the signal is not suppose to be so small. I suppose that artifact is due to the richness of the signal at this precise moment (in comparison with the bit-rate) and therefore bit allocation can not cope with the demand. Then the coder being unable to encode leaves a hole in the spectrogram. If i'm correct with the above assumption, what i'd like to know is if there is any documentation or perceptual intepretation of this problem of coding. Regards, Maxime ________________________________ De: AUDITORY Research in Auditory Perception de la part de Bob Masta Date: mer. 15/03/2006 14:30 À: AUDITORY@xxxxxxxx Objet : Re: - "Birdies" Hi, 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 ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________